at SciVerse ScienceDirect

Radiation Measurements 47 (2012) 577e587

Contents lists available

Radiation Measurements

journal homepage: www.elsevier .com/locate/radmeas

Design parameters and technology optimization of 3He-filled proportionalcounters for thermal neutron detection and spectrometry applications

D. Mazed a, S. Mameri b, R. Ciolini a,*aDepartment of Mechanical, Nuclear and Production Engineering (DIMNP), University of Pisa, Largo L. Lazzarino 2, I-56126 Pisa, ItalybCentre de Recherche Nucléaire de Birine (CRNB), Commissariat à l’Energie Atomique, B.P. N. 180, Aïn-Ousséra, 17200 Wilaya de Djelfa, Algeria

h i g h l i g h t s

< Technological aspects and design of 3He neutron proportional counters are reviewed.< Pulse height distribution of the counters were simulated by Monte Carlo code.< Some laboratory developed 3He-based proportional counter prototypes are described.< A set of gas composition fillings were experimentally investigated.

a r t i c l e i n f o

Article history:Received 22 December 2010Received in revised form16 May 2012Accepted 1 June 2012

Keywords:Neutron spectrometryGas detectorsNeutron proportional counters

* Corresponding author. Tel.: þ39 050 2218026; faxE-mail address: r.ciolini@ing.unipi.it (R. Ciolini).

1350-4487/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.radmeas.2012.06.002

a b s t r a c t

Technological aspects and key features related to the development and design optimization of 3He-filledproportional counters for thermal neutron detection and neutron spectrometry applications are presentedin this article. Basic operation features are reviewed in details by focusing on their determinant impactupon the definition of the consistent design parameters. Previous Monte Carlo simulations, where bothwall effect and electron attachment effects due to occlusive electronegative impurities were taken inconsideration, were carried out in 3He-filled proportional counters for the study of the pulse heightdistribution and gamma rays influence. From interpretation of the main quantitative results, basicparameters for design optimization requirements were drawn-up. Some laboratory developed 3He-basedproportional counter prototypes are then described. The characteristics of a set of gas composition fillingsthat were experimentally investigated are then compared to those obtained with a commercial specimenthat was taken as a reference standard. Finally, a brief conclusion underlines some consistent insightstoward the improvement of quality performances during future development tasks of helium-filledproportional counters intended for thermal neutron detection and neutron spectrometry applications.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Gas-filled proportional counters are long established andwidelyused for instrumentation and control of nuclear power plants,nuclear fission and fusion research reactors, and neutron beamexperiments, especially for thermal neutron detection and spec-trometry purposes (Knoll, 2000). However, till now the keyconstruction features and practical issues involved in the tech-nology development programs and qualification processes of suchcounters are still unavailable in the specialized textbooks andremain always an inaccessible lore or art of a few manufacturers,and insufficiently documented in the literature.

: þ39 050 2218065.

All rights reserved.

Neutron sensitive detectors in general, and 3He-filled propor-tional counters in particular, are essential for nuclear reactorcontrol and also in many other experiments around neutron beamfacilities (Convert and Forsyth, 1983). For neutron spectrometrypurposes, the nuclear reactions 3He(n, p)3H, 6Li(n, a)3H, 10B(n, a)7Li,1H(n, n)1H, 12C(n, a)9Be and 28Si(n, a)25Mg are the main candidatesin which the neutron energy is determined by measuring theenergies of the charged reaction products. However, the use of the3He(n, p)3H reaction (Q ¼ 0.765 MeV) remains the leading methodso far adopted in the neutron energy range from thermal to about5 MeV (Weyrauch et al., 1998; Brooks and Klein, 2002; Alevra andThomas, 2003; Vega-Carrillo et al., 2005; Manolopoulou et al.,2006). The efficiency of the helium based proportional counterssuits the energy-dependence of the different involved interactionmode cross-sections of neutrons with 3He (and 4He) in relatedranges, as shown in Fig. 1.

10-6 10-5 10-4 10-3 10-2 10-1 100 10110-1

100

101

102

103

3He(n,p)3H1H(n,n)1H6Li(n, )3H4He(n,n)4He10B(n, )7Li

4

He(n,n)4

He

1

H(n,n)1

H

6

Li(n

,) 3

H

10

B(n

,) 7

Li

Cro

ss sectio

n

[ bar

ns ]

Neutron Energy [ MeV ]

3

He(n

,p) 3

H

σ

α

α

α

α

Fig. 1. Neutron cross sections of commonly used reactions for thermal neutrondetection and spectrometry.

D. Mazed et al. / Radiation Measurements 47 (2012) 577e587578

The Detection and Measurements Laboratory of the Centre deRecherche Nucléaire de Birine (CRNB), belonging to the AlgerianCommissariat à l’Energie Atomique (COMENA), has initiated manyyears ago an ambitious program aimed at the development ofa broad variety of neutron sensitive gas-filled detectors such asboron ion chambers, boron lined proportional counters (Mazed,2007), boron trifluoride proportional counters (Mazed et al.,1997), 3He-based proportional counters (Tahri, 1993; Mazedet al., 1998), and fission counters to suitably accommodatea large range of applications in the nuclear technology field. Theobject of this program consisted primarily in the optimisation ofthe design and construction of neutron sensitive proportionalcounters, and this was conducted over the past decade (Mazed,2001). It gave rise to very encouraging results, although sometechnological issues still remain to be further addressed. One cancite for instance application of appropriate welding and assem-bling techniques when soldering heterogeneous materials alto-gether such as aluminum with ceramics, aluminum withstainless steel, tungsten with aluminum, etc. These problems arenow being solved by making use of adequate welding technologyand alternative material lamination techniques (TungstenInert Gas (TIG) process, cold welding, thermo-mechanicalassembling).

Four prototypes of 3He-based proportional counters wererealized inside this project: it is the purpose of this paper topresent the main achievements obtained in this gas-filled detec-tors technology development. However, a special emphasis will bedevoted here to 3He-filled proportional counters. Indeed, thesedetectors are taken here as a guideline to illustrate the constraintswithin which the appropriate design parameters and gas mixturecomposition should be chosen to meet fundamental operationrequirements for thermal neutron detection and neutronspectrometry applications. The basic parameters of 3He-filledproportional counters, and how these are consistently taken intoaccount within the construction and development process, arepresented. Some useful technology methods aiming at their designoptimisation are also highlighted comparing the operationperformance of the 3He-filled proportional counter developedprototypes to those of a commercial specimen taken as a referencestandard.

2. Basic parameters of 3He proportional counters

2.1. General principle operation

Neutron sensitive proportional counters herein considered aremade of a cylindrical cathode tube in the axis of which is stretcheda thin anode wire. They are filled with a gas mixture whichcomposition and pressure are optimized with respect to the type ofthe secondary charged particles created inside the sensitivevolume. Since the neutrons do not bear an electric charge, they canbe detected only through the charged particles produced by nuclearinteraction with a converter material, presenting sufficiently highneutron absorption cross section, inserted within the counter ina given appropriate form (gaseous, liquid or solid compoundforms). 3He gas is most suitable neutron sensitive gas used formaking detectors because of its favorable properties such as highneutron absorption cross-section, chemical inertness and goodbehavior in proportional counters at high pressures. The 3He(n,p)3H reaction (Q-value ¼ 0.765 MeV) has interesting properties inmany applications including neutron spectrometry. Its cross sectionfor thermal neutrons (v¼ 2200m s�1) is about 5400 b and varies as1/v. In some applications, a neutronmoderator is used to exploit the1/v cross section allowing fast neutrons to be detected with higherefficiency.

The proton and the triton, which are emitted with energy of574 keV and 191 keV in presence of thermal neutrons, respectively,are charged particles and are easily detected in the sensitivevolume of the counter, where they ionize the filling gas as they areslowed down and stop. The created ionization, consisting in elec-tron ion pairs, may be collected by an electric field for pulse signaldevelopment, allowing neutrons events to be detected, processedand counted. The electrons move about 1000 times faster than thepositive ions so the movement of the positive ions can be ignoredover the timescale in which the electrons are collected. Near thecathode the electric field is weak and as a consequence the elec-trons drifting toward the central anode wire do not gain sufficientenergy between collisions to ionize the gas. If the gas containselectronegative impurities such as oxygen, then the electrons canattach themselves to these gas atoms to form slowmoving negativeions and they are effectively lost from the drift process. This is whythe gas purity is so important in proportional counters: in partic-ular, it is essential to keep the concentrations of oxygen and otherelectronegative species below 10 ppm during the counter fillingprocedure.

Unfortunately, because 3He is a noble gas, no solid compoundscan be fabricated with it and the material must be used exclusivelyin a gaseous form. The 3He pressure and dimension of the counterdetermines the neutron detection efficiency at a given neutronenergy. However, even at high pressures, 3He itself is not suffi-ciently dense to stop all the proton and triton particles emergingfrom the detection reaction, so to reduce the wall effect and toachieve better position resolution (Desai and Shaik, 2006). The walleffect issue is discussed in Section 3.1.

2.2. Pulse signal formation and related time parameters

Immediately after the neutron absorption event, the producedproton and triton begin to slow down and ionize the filling gasmixture. The released electrons drift nearer to the anode, where theelectric field of the cylindrical proportional counter becomesstronger (Knoll, 2000). At a radius of about twice or threefold thatof the anode wire the electron gains enough energy betweensuccessive collisions to ionize another gas atom (i.e. a second freeelectron is obtained along with a positive ion). The two electronscontinue to drift toward the anode (while the positive ions goes

Fig. 3. Digitized (Tektronix e 100 MHz oscilloscope) current pulses of two neutron-capture events in 3He proportional counter. The wider pulse had the proton andtriton track nearly perpendicular to the anode wire, and the narrower pulse had thetrack nearly parallel to the anode wire. One can notice also the effect of double-pulsing.

0 1 2 3 4 5 6 7 8 9

0,0

0,2

0,4

0,6

0,8

1,0

1,2

T = 2 .1 μs ; t = 6 .3 ns

Pu

ls

e h

eig

ht, h(t), [

no

rma

lize

d a

t t =

9 μ

s]

Time [ μs ]

extended ion iza tion track (p+ t track ) po in t-like ion ization track (low energy photon absorp tion)

T = 0 .13 μs ; t = 9 .01 ns

Fig. 2. An example of the pulse shape characteristics for point-like ionization track(low energy photon absorption) and extended ionization track (proton and tritontracks). The corresponding time parameters t0 and Td are indicated (see later in thissubsection). The resulting differences in the rise of the pulses enable the pulse shapediscrimination in 3He filled proportional counters.

D. Mazed et al. / Radiation Measurements 47 (2012) 577e587 579

slowly toward the cathode), until each of them gains enoughenergy to ionize other atoms and so on, producing an electronavalanche around the anode wire. The total number of avalancheelectrons per single primary electron is defined as the gas electronmultiplication or gas gain. For neutron counting applicationstypical gas gain values lay in the range 10e100 and rarely valuesexceeding 1000 are allowed, for achieving better pulse height andspatial resolutions and also for optimizing the operation lifetime ofthe device (Brinckmann and Gerber, 1961).

A perusal examination of the induced charge signal at the anodeleads to note that a large number of electrons are produced veryclose to the anode, so they have only to move through a very smallspace before reaching the anode itself. Their fast contribution to theelectric signal is therefore correspondingly small. Considering thepositive ions drifting toward the cathode, most of them movethrough virtually the whole voltage drop and this induces thepreponderant component of the signal on the anode. The initial fastrise is due essentially to the ions moving through the stronger fieldnear the anode. Afterward, as the ions approach the cathode, theyslow down because of the weaker field and they have to travelfurther to produce the same drop effect on the anode potential. Theion collection time is of the order of l ms.

In pulse mode operation, the voltage pulse signal is usuallydifferentiated with a shaping time constant of 1e4 ms to give ananalog pulse signal which is directly proportional to the number ofelectrons formed in the avalanche, which is in turn directlyproportional to the number of primary ions pairs formed by theinteraction of the incident radiation with the counter gas. If veryhigh counting rates are required then shorter time constants mustbe used to utilize the fast electron pulse, but there can be signal/noise problems because the amplitude of this signal is much lessthan that due to the positive ionsmovement. Longer time constantsare also used to improve the rejection of gamma ray events in thecounter. Typically the energy deposition in the gas resulting froma gamma ray interaction is low because the electrons produced arelightly ionizing and can pass across the gas volumewith the loss, atmost, of only a few tens of keV. If neutron spectroscopy is requiredthen longer shaping time constants have to be used in order toobtain a signal that is truly proportional to the total energy release.Thus the count rates that can be achieved in spectroscopymode areconsiderably reduced from those used for total event counting.Unfortunately, the ion-pair charge distribution of the primaryionization is not always point-like, so the orientation and theposition of the primary track have to be taken into account inaccurate spectroscopy works.

Generally, the detector voltage pulse shape depends on theelectron and ion drift velocities. The charge carriers drift velocitiesmay be expressed as a function of the electric field to pressure ratio,E/p, through the corresponding mobility m in the gas mixture(Ricker and Gomes, 1969):

v ¼ mðE=pÞ Ep

(1)

For the proton and triton ionization tracks in 3He-filledproportional counters, the situation is complicated due to theextension of the ionization tracks (the specific ionization is notconstant along the same track), the non uniform distribution oftracks within the counter (because of important neutron absorp-tions near the cathode), and the near isotropic nature of the protonand triton tracks. It is therefore simpler to directly measure thevoltage pulse shape and time characteristics for the counter in therequired experimental configuration following the pulse-matchingmethod, as shown in Fig. 2 (Baaliouamer et al., 1994). On the otherhand, because of the concentration of the Bragg effect at each trackbranch end, the ion tracks perpendicular to the anode wire can

create markedly double-pulse signals (Fig. 3). Therefore the possi-bility of double pulsing due to this track orientation effect requiresfurther careful consideration, especially when short networkshaping time constants are required.

The ion pairs of the primary ionization do not induce anappreciable signal at the electrodes in proportional counters, andelectrons reach the multiplication region around the anode wireafter a drift time tel that could be approximated by (Kowalski,1970):

tel ¼ plnðb=aÞ2Vmel

�r2 � a2

�; (2)

where r is the primary electrons distance to the anode wire (b anda are the counter anode and cathode radius, respectively), mel theelectron mobility and V the applied voltage. To choose an appro-priate mel value in Eq. (2), an average intensity of E/p throughout the

D. Mazed et al. / Radiation Measurements 47 (2012) 577e587580

sensitive volume of the counter can be estimated (Harling, 1965) byweighting with the probability that a primary electron has to bereleased at a distance r from the anode wire (a << b):

�Ep

�ave

¼Zba

Ep

2r�b2 � a2

�dr

¼Zba

Vprlnðb=aÞ

2r�b2 � a2

�dr z2V

pblnðb=aÞ : (3)

The electron mobility mel is typically 103 times greater than thatfor ions and depends on the filling gas composition. Indeed,inelastic collisions lead to a reduction of the agitation energy andconsequently to an increase of the collision time during the elec-tron drift process. Thus, the electron mobility is greatly enhancedby the presence of a small component of a polyatomic gas, char-acterized by dense vibro-rotational energy levels that increase theinelastic collision rate and also the mean electron energy loss(electron cooling effect). Since proportional counters are usuallyoperated at sufficiently high gas electron multiplication value, themotion of the positive ions, drifting from the avalanche region,close to the anode wire surface, is assumed to produce the entireoutput signal pulse, whilst the contribution of electrons is negli-gible. The positive ion collection time tion is given by:

tion ¼ plnðb=aÞ2Vmion

�b2 � a2

�¼�b2

a2� 1

�t0; (4)

where t0 is defined as the charge collection characteristic time:

t0 ¼ a2plnðb=aÞ2Vmion

; (5)

As shown by Eq. (4), the ion collection time depends upon thecounter size, gas pressure, anode diameter, voltage and gascomposition. Positive and negative ions have nearly the samemobility. The mobility differs for gas type and is generally constantwith respect to E/p. In binary gasmixture themobility of the ions mij,ion can be approximately determined with (Harling, 1965):

1mij;ion

¼ 1pi þ pj

pi

mi;ionþ pjmj;ion

!; (6)

where i and j are the indices referred to the two gas types withpartial pressures pi and pj, respectively.

In case of point like ionization tracks and with a producedcharge Q, the output voltage pulse shape h(t) is given by Ramsey(1940) and Wilkinson (1950):

hðtÞ ¼ QC

12lnðb=aÞlnð1þ t=t0Þ; (7)

where Q/C ¼ hmax is the final pulse height which is reached aftera time interval Tion (C is the capacitance of the counter). From Eqs.(4) and (7), one can derive that the output voltage pulse reacheshalf maximum amplitude at a time t1/2 given by:

t1=2 ¼ b� aa

t0 ¼ aaþ b

tionzabtionz10�3tion: (8)

However, 3He proportional counters are characterized by anextended primary ionization tracks (the proton and triton tracks).Therefore, one should take into account the electron delay time Td,that is the time period spanning between the arrival of the first andthe last primary electrons, originating from the same ionizationtrack, at the multiplication region (Townsend region) near the

anode wire. In this case, the output pulse shape Eq. (7) should besubstituted by a more general expression (Baaliouamer et al., 1994)which can be defined by two parts, one for t � Td (i.e. during thearrival of the primary electrons to the multiplication region):

hðtÞ ¼ QC

12lnðb=aÞ

t þ t0Td

�ln�1þ t

t0

�� 11þ t0=t

; (9a)

and the other for t � Td (i.e., after all the primary electrons havereached the multiplication region):

hðtÞ ¼QC

12lnðb=aÞ

t þ t0Td

�ln�1þ t � Td

t0

�� 1� t þ t0

Td

ln�1� Td

t þ t0

�: (9b)

For ionization tracks nearly parallel to the anode wire (Td/0),the pulse shape described by both Eqs. (9a, b) reduce simply to thatgiven by Eq. (7) related to the point-like ionization track case, sincethe primary electrons reach the gas electron multiplication regionat almost the same time.

Fitting the Eqs. (9a, b) to experimental pulse shape signalscharacteristics, it is possible to deduce pulse shape and time char-acteristics, providing also the determination of the correspondingvalue of mion, the ionmobility of the positive charge carriers, makinguse of the Eq. (5). If rise times are needed, then in most casespractical measurement is the easiest method and it will be moreaccurate than calculation from first principles. Rise times increasewith cathode radius, while reduces with shorter initial track lengthobtained by increasing the quench or total gas pressure.

2.3. Gas electron multiplication

The gas multiplication factor (gas gain) A in a cylindricalproportional counter is important if the determination of theabsolute values of the incident particle energy (e.g., in spectros-copy) or w (mean energy for the creation of an ion pair) arerequired. It is also of interest during the design and in operation inorder to ensure that a suitable operating point can be established.For example, it is useful to understand the factors controlling gasgain if batches of counters are to be connected in parallel or are tobe operated from the same voltage supply. There is a considerablevolume of literature on this subject and a critical review of thesecan be found inWilkinson (1950). Awell founded theoretical modelof gas electron multiplication have been developed previously tobest fit the experimental gas gain data obtained in binary gasmixtures-filled proportional counters intended for thermal neutrondetection and fast neutron spectrometry applications (Mazed andBaaliouamer, 1999; Mazed, 2002). According to this model,a general semi-microscopic gas electron multiplication formulawas derived in the form:

lnApaSa

¼ K

"ð1þmÞln Sa

S0þXni¼1

ð�1Þiþ1

i$i!

�S0Sa

�ið1þmÞ#� L; (10)

where p is the total fill pressure, Sa stands for the electric fieldstrength to pressure ratio at the anode wire surface of radius a,while the remaining constants S0, K and L are determined only bythe specific nature of the gas mixture, and m is the electronmoderation parameter, valued in the range 0 � m � 1 (its valuedepends fairly on the nature of the quench gas and increases withincreasing quench gas fraction in the mixture). The exact physicalsignificance of these gas constants was already extensively dis-cussed (Mazed and Baaliouamer, 1999; Mazed, 2002; Mazed andAmokrane, 2003).

0 200 400 600 800 1000

0

500

1000

1500

2000

2500

3000

ResolutionR = 4.5 %

Gamma pile-up background

Full energy peakEp+t

= 765 keV

Triton edgeEt = 191 keVC

ou

nts

[arb

itrar

y]

Deposited Energy [ keV ]

Monte Carlo simulation Measured

Proton edgeEp = 574 keV

LND252 3He PCHV = 1050 VAmpli. Gain = 100Shap.Time = 4.5 sμ

Fig. 4. Computed and measured pulse height spectra from LND 252 3He proportionalcounter, where the spectrum key features are shown (proton and triton edges and thefull energy peak).

D. Mazed et al. / Radiation Measurements 47 (2012) 577e587 581

Restricting the expansion of the control variable to the zeroorder term, i.e., keeping only the natural logarithm term,a generalized form of the well-known Diethorn formula with m inthe range 0 � m � 1 is obtained from Eq. (10). When m is zero,this simply reduces to the original Diethorn formula (Diethorn,1956). When extending the expansion to the first order termapproximation, the generalized Zastawny formula for m betweenzero and unity is obtained. In the particular case of m ¼ 0, thisfirst order approximation reduces simply to the classical Zas-tawny formula (Zastawny, 1966). These two approximations aresometimes found sufficient to describe the experimental gas gaindata in helium-filled proportional counters (Baaliouamer et al.,1996; Desai and Shaik, 2006; Ravazzani et al., 2006). Thesecond and third order approximations of the Eq. (10) arehowever required when accurate parameterization is desired overan extended range of working variables, particularly in the lowgas gain range.

As the gas gain in a counter is increased, a point at which spacecharge effects start to become important is reached. These effectslead to variations in gas gain between different avalanches, i.e. non-linearity in the proportionality of the counter, and are particularlysevere for detectors designed for low energy X-rays (or other highspecific ionizing particles, e.g. alphas) operated at very high gasgains (higher than 105). With neutrons, because of the long (protonand triton) track lengths in 3He counters, the avalanche is spreadout and space charges effects are less likely to be a problem (exceptfor ionizing tracks perpendicular to the anode wire). Because therelatively large signal does not require the use of high gas gains, it isrecommended that they are kept well below 103, to limit the totalnumber of ion pairs to 108, avoiding thus themanifestation of spacecharge distortion effects, and also to extend the lifetime of thecounter. Indeed, according to Brinckmann and Gerber (1961), thelifetime is inversely proportional to the operating gas gain value, asit will be discussed below.

2.4. 3He proportional counter lifetime

The usefulness lifetime of a 3He proportional counter dependson the maintenance of the vacuum tight of the counter body andthe preservation of the 3He and quench gases purity and quantitywithin it. The preparation of the counter body is a crucial factorregarding to this. The body must be designed so that there isa minimum of trapped volumes, which trap gas impurities that arethen slowly exchanged with the counter gases after filling. For thisreason the components of the body must be processed (walldesorption) at high temperature, under vacuum, to remove tracesof adsorbed gases, especially water vapor and oxygen. After thistreatment the counter should be assembled and filled as soon aspossible. Counters that are manufactured and then filled at a laterstage should be filled with a clean purified gas, preferably thequench gas, during storage.

In operation the counter will deteriorate due to two mainfactors. The first is the burn up of the quench gas: each ion pair thatis created in the counter will ionize the constituent gases and thepositive ions formed will move to the cathode where they may beadsorbed. Therefore, the operating gas gain value should be set attheminimum required, since subsequent electronic amplification isprovided by the associated pulse amplifier. Reactor studies ofionization chambers using hydrocarbons as quench gases and BF3proportional counters have shown that they are degraded muchfaster by gamma irradiation. So the counter performance willchange as the quench gas is burned up, eventually becomingunstable. Within these conditions, CO2 is therefore the suitablequench gas for counters that are to be used at high neutron fluencesand/or high gamma radiation levels.

The second factor is the formation of tritium: each neutroninteraction with a 3He nucleus leads to the formation of a tritiumatom. This increases the number of beta decays in the counter andleads to a build up of noise and a shortening of the counter plateau.This is only serious for counters operating in high neutron fluxes forlong periods (e.g., within nuclear reactors), for out of pile applica-tions this effect is negligible. A high tritium decay rate will alsoburn up the quench gas.

3. Technology and design optimization criteria

3.1. Wall effect

When the length of the primary ionization track is a significantfraction of the counter diameter, there will be a number of events inpart of the track which are lost in the counter wall. This leads toa reduction of the signal output and aworsening in the shape of thecounters characteristic high-voltage plateau. It has been demon-strated geometrically that the fraction of events that interact withthe counter wall for a primary ionization track length R isapproximately R/D, if D is the counter internal diameter assumed tobe larger than R (Shalev et al., 1969). If R exceeds D the approxi-mation fails: almost all of the events (apart from those that producetracks at a small angle with respect to the anode wire and that canevolve along the length of the tube) are affected in such a way bythe wall effect, which is near 100%. The previous statementassumes the active length of the counter is much longer than itsdiameter, which is usually fulfilled in practice, and the 3He inter-actions occur uniformly throughout the gas volume. The latterassumption however is not always valid: for example, for high-pressure gas fillings the outer regions of the counter shield theinner ones and, as a consequence, a higher number of neutrons aredetected close to the wall, increasing the effect.

A typical 3He proportional counter pulse height spectrum isshown in Fig. 4 (Tahri, 1993; Mazed et al., 1998). When the 3He(n,p)3H reaction takes place close to the cathode wall, it is possible forthe proton to go toward the wall effectively without depositingenergy in the gas. However, because the proton and triton areemitted in opposite directions, such events correspond to the tritonmoving away from the cathode into the bulk gas. Thus, theminimum energy deposited in the counter is 191 keV, the kineticenergy of the triton. The triton edge is the first characteristic featurethat appears in the pulse height spectrum of Fig. 4. If an energy

D. Mazed et al. / Radiation Measurements 47 (2012) 577e587582

threshold is set in the region below the triton edge, it is clear fromFig. 4 that essentially all the 3He(n, p)3H events can be counted withconsistent discrimination of the gamma background.

In the opposite case, when the triton strikes the cathode wallwithout depositing energy in the gas the proton, heading in theopposite direction, can deposit 574 keV. This explains the secondprominent edge characteristic in the spectrum (proton edge).Between these two extremes the energy deposited in the gasincludes a fraction of either the proton or triton edges.

The full energy peak itself is further broadened (about 35 keVFWHM) by finite resolution effects which include: statistical fluc-tuations in the number of primary ion pairs produced per unitenergy deposited, possible electronegative molecule gas contami-nation, variation in the gas amplification process due to anodeimperfections, gas dynamic effects such as recombination, lowamplitude gamma ray pile-up noise, and electronic effects.

In practice, a 20% wall effect (i.e. R/D ¼ 0.2) is considered forgeneral purpose neutron counting (thermal neutron flux moni-toring). But if counters are required to operate in high gammafields, then it may be necessary to allow the wall effect to increaseby minimizing the quantity of high atomic number quench gasadded to the 3He. For thermal neutron flux monitoring, we haveoperated satisfactorily with wall effects as high as 70%. Althoughthe wall effect is reduced by the addition of heavy quench gas, thisalso increases the number of ion pairs produced by gamma photoninteractions, leading an energy resolution worsening. In designprocess, a best tradeoff to meet the dedicated application require-ments is required.

The wall effect can be experimentally determined. Indeed, theratio of the area under the proton and triton edges (i.e., the area tothe left of the main full amplitude peak) to the total area can beapproximately calculated according to the ratio (Rt þ Rp)/D, whereRt and Rp are the ranges of the triton and proton, respectively. Forthe spectrum shown in Fig. 4, the wall effect is about 63%. Theranges of the proton and triton in pure 3He at 20 �C and 1 bar areapproximately 71 mm (Desai and Shaik, 2006). The range in 3Healone decreases in inverse proportion with pressure, so the walleffect in a 50 mm diameter counter never exceeds 50% for 3Hepressures over 2.2 bar. However, it is not possible to operate a 3Hecounter without a small amount of quench gas. This is becausehelium, which is a noble gas, produces metastable states withlifetimes of 10 ms or longer after radiation interactions, triggeringunavoidable breakdowns by photon processes (photoionizationand photoelectric effect on the cathode inner wall of the counter).The quench gas deexcites these states, discarding thus the occur-rence of breakdowns.

3.2. Pulse height analysis

The pulse height distribution is an important characteristic of3He proportional counters, especially when dealing with spec-trometry applications. The exact theoretical prediction is quitedifficult to derive, given the complexity of the various microscopicphenomena involved. However, two dominant effects are of majorimportance: the wall effect and the electron attachment effect.

Using the Monte Carlo simulation method, the spectrum iscomputed taking into account the three major cases (Tahri, 1993;Mazed et al., 1998): only the triton suffers the wall effect, only theproton suffers the wall effect, and both the emitted particles arestopped within the sensitive gas without experiencing the walleffect. The electron attachment effect which likely induces a severedegradation on the energy resolution is also taken into account byassuming a parameterized contamination by oxygen (up to 10 ppm)in the filling gas mixture. In practice, the resulting effect is modeledby computing the effective rate of the attached primary electrons

when drifting toward the avalanche region. Major details on theassessment of electron attachment effect due to oxygen contami-nation in proportional counters can be found in Curzio et al. (2005)and in Mazed et al. (2007).

The spectrum has been computed using the LND 252 3Heproportional geometrical parameters and compared to experi-mental spectrum obtained with the same counter taken as a stan-dard reference (Fig. 4). An acceptable agreement is obtained,nevertheless some differences are evident, and they are likely dueto experimental inconsistencies (electronic noise, epithermalneutron component and so on). Hence, this leads us to concludethat assuming the wall effect and the electron attachment effect asthe main phenomena is an acceptable approximation to designsuch detectors.

3.3. Quench gas selection and counter filling procedure

As explained before, the 3He counter requires a small amountof quench gas to correctly work. Up to now the adopted quenchgas has usually been CO2. However, hydrocarbons gases such asisobutane (iC4H10) have demonstrated very interesting charac-teristics (Baaliouamer et al., 1996). Indeed, Desai and Shaik(2006) have suggested that 3He-iC4H10 (80%, 20%) basedneutron sensitive counter (filled up to 3 bars) could be anattractive alternative, since isobutane is almost transparent tophotons, and is also characterized by a good stopping power forcharged particles. The quench gas contribution to gammasensitivity of the counter is not a concern because of its verysmall contribution compared to that due to the counter walls(see also the 3.6 subsection). For this reason, AreCH4 (10%),AreCO2 (5%), KreCO2 (5%), can also be adopted without reducingtoo much the neutron efficiency performance, especially if it isnot easy to obtain pure alternative quench gases. CF4 is anotherchallenging quench gas with interesting fast drift properties.However, to our knowledge there is a little operational experi-ence with the 3HeeCF4 mixture.

The main advantage of Penning mixtures (in which an addi-tional gas with a lower ionization potential is added so that furtherions are createdwhen the different species collide) is that they havea lower value of mean ionization energy w. This translates intoa larger signal for a given gas gain, and it is particularly valuablewhen detecting low energy X-rays which poses severe signal/noiseproblems. This is not particularly important for neutron applica-tions because of the large energy release per neutron detected.Thus, for 3He counters the optimum amount of quench gas isa compromise between minimizing wall effect, curtailing gammaresponse, achieving the desired counting rate capability andmatching working voltage limitations. Modern high voltage powersupplies are very stable and can be reproducibly set, but excessiveoperating voltages may require additional attention to be given tothe ceramic/metal seal connecting the anode wire inside thecounter to the outside, and the connectors and cabling to preventbreakdown, especially if the counter has to be operated withinconditions of high relative humidity.

Ideally, the fastest possible gas mixture (i.e., a high electron driftvelocity at the low electric field which exists near the cathode) isdesired, so that the pulse rise-time is as short as possible. Fast gasesallow shorter amplifier time constants to be used giving higherneutron count rate capability together with maximum rejection ofgamma pile-up events. A gas mixture with a high ionic mobility toreduce the dead time is also preferred. Moreover, a gas that doesnot easily dissociate after being exposed to high radiation doses isrequired. Carbon dioxide is particularly good with respect to thislatter requirement and it also has a low response to gamma radi-ation, because of its low effective atomic number.

Table 1Density, w value and minimal proton stopping power (at 20 �C and 1 bar) forcommonly used pure gases (Tahri, 1993 and references therein).

Gas r (mg cm�3) w (eV) (dE/dx)0

(MeV cm2 g�1) (keV cm�1)

H2 0.0838 37 4.03 0.34He 0.166 41 1.94 0.32N2 1.17 35 1.68 1.96O2 1.33 31 1.69 2.26Ne 0.839 36 1.68 1.41Ar 1.66 26 1.47 2.44Kr 3.49 24 1.32 4.60Xe 5.49 22 1.23 6.76CO2 1.86 33 1.62 3.01CH4 0.67 28 2.21 1.48C4H10 2.42 23 1.86 4.50

D. Mazed et al. / Radiation Measurements 47 (2012) 577e587 583

A value of 0.02 bar of quench gas is generally a practicalminimum quantity to add to counters that contain between 0.5 and10 bar of 3He. From the practical point of view, putting a smallamount of quench gas into a counter must be done before addingthe other components, otherwise it is very difficult to measure andcontrol the quench gas quantity (for example, 0.02 bar of quenchgas added to a 6 bar filling gas corresponds to about 1 �C of gastemperature variation at the filling time). A very small quantity ofquench gas can considerably limit the counter lifetime.

During the choice of the quench gas, one shall keep in mind thatit is always difficult to get the desired quench gas purity at highpressures, because the fractional distillation technique in thepurification process gives an increased risk of gas impurities beingcarried over as the quench gas pressure is raised. If high pressure ofquench gas is required, it is necessary to purify and collect it at lowpressure in a large container and then freeze it into a smallcontainer before allowing evaporation at a higher pressure. Notethat it is vital to purify all the gases during the filling process toremove oxygen and water vapor, and the low energy beta emitter,tritium, from 3He, adopting the fractional distillation process usingliquid nitrogen. When a small amount of quench gas is put into thecounter and then 3He is added, there is a piston effect, i.e. the 3Hepushes the quench gas from the filling pipe into the counter thusincreasing the quench gas effective quantity (and pressure) in thecounter. If a batch of counters is to be filled on a multiple outletmanifold, there may be large differences in the quantity of quenchgas in the filled counters nearest the helium inlet, so that it may bebetter to increase the initial pressure of the quench gas. Otherwisecounter reproducibility can be obtained by replicating the fillingconditions exactly. Another method is to close off the taps on all thecounters in a batch after the quench gas filling has been completed,then pump out the manifold and fill it with the same pressure of3He as the quench gas filling before opening the taps again andcontinuing the 3He fill. There also will be a small piston effect fromgas in the filling tube and tap, but the counter fillings will be likelyreproducible.

3.4. The w-values in 3He proportional counters

Apart from the most standard mixture compositions, in theliterature there is a lack of w values data and the measurementprocedure is quite difficult. The w-value increases as the speed ofthe charged particle reduces (for further discussion see Jesse andSadauskis (1952), Bortner and Hurst (1954), ICRU (1979)). In thepresence of a binary gas mixture, assuming that one of the gasesdoes not contribute to ionization of the other, the resulting wmq

value can be calculated with (m represents the main componentand q the quench component):

1wmq

¼ 1wm

pmpm þ xmqpq

þ 1wq

xmqpqpm þ xmqpq

; (11)

with xmq ¼ sm/sq, where wm and wq, pm and pq, sm and sq are,respectively, the w-values, the partial pressures and the stoppingpowers of the pure gas components m and q. However, if too smallamounts of particular gases are added to noble gases, a lower valueof w than predicted by Eq. (11) is observed: this effect is namedJesse or Penning effect (Knoll, 2000). In this case, the energytransfer from the noble gas to the admixed gas generates the extraionization which enhances the energy resolution of the counter.

In Table 1 thew-values and theminimal proton stopping powersof some pure gases commonly used for filling radiation detectorsare summarized. In 3He counters the ratio of the value of w forprotons and tritons can be obtained from the ratio of the measuredspectrum positions of the proton and triton edges (573 and 191 keV,

respectively). In order to obtain an estimate of thew-value from theobserved pulse height spectrum it is required a good knowledge ofthe counter gas gain. The w-value is not important in neutroncounting work, but only in spectrometry applications.

3.5. The anode wire and the cathode tube

To preserve the signal amplitude recording the detected eventwithin a designed 3He proportional counter, it is necessary to useappropriate technology for insulator feed-through such asaluminum oxide (Al2O3) or ceramic/metal seals. The anode of 3Heproportional counters consists of tungsten, gold or stainless steelwire, typically with 50e100 mm diameters. For position sensitiveproportional counters, very thin resistive carbon wire allowing thelocalization of the detected neutron along the counter are used. Theanode is stretched between two guard rings to protect the signalfrom the end effects by electrically limiting the length of thecounter to the area where the electric field is not disturbed. Thecathode is generally a cylindrical copper, stainless steel oraluminum tube with a thickness of 0.5 mm and an internal diam-eter ranging from1 to 5 cm, depending on the applications towhichthe counter is intended.

The anode wire plays a key role in the performance of thecounter (Kadyk, 1991). If the wire is not exactly circular or itsdiameter varies systematically along its length (but this lattercondition is unlikely verified), the gas gain can vary around thewireso that ionizing events at different positions around it will havea different gas multiplication. Burrs or score marks on the surface ofthe anode can act as electric field raisers and can be particulardamaging to the pulse height resolution. For this reason it is rec-ommended to use fresh dies when anode wires are manufactured.

Inconel wires have been found to give good performance.Tungsten wires have been used in many applications because oftheir availability and mechanical strength. They are often heated to1000 �C in vacuum, e.g. by passing a current through them toeliminate the Malter effect (Benjamin et al., 1968).

Guard tubes (i.e., a tube that is part of the support at the end ofthe anode wire and electrically connected to the anode) are used todefine the active length of the anodewire (Mazed, 2002) and createa region in the counter with much lower electric fields to bettercontrol the transition from the anode, a high field region, to theends of the counter avoiding the corona effect. Note that with guardtubes the gas gain will reach its full value approximately onecounter radius from the end of the tube, so that the active length isapproximately equal to the distance between the ends of the tubeminus one counter internal diameter (Takeda and Kudo, 1994), butby convention the active length of a counter is taken to be thedistance between the ends of the guard tubes.

D. Mazed et al. / Radiation Measurements 47 (2012) 577e587584

The best method of defining the counting volume is to thickenthe anode wires at each end, for example by slipping very thintubes (e.g., hypodermic needles) over the wire itself (Wilkinson,1950). These field tubes are similar to guard tubes, but they areconnected to a separate high voltage supply, that can be derivedfrom the anode supply voltage through passive voltage dividers.The bias applied to the field tubes is set so that they are maintainedat the voltage that would exist at their corresponding radial posi-tion in the counter. This eliminates the region of variable gas gainand, in principle, gives constant gain for the full distance betweenthe field tubes. The signal is taken solely from the anode. As forguard tubes, sharp edges, that could give rise to corona discharges,must be avoided. Field tubes create complications in counterconstruction and their use should only be considered for special-ized counting applications (e.g., spectroscopy or absolute activitycalibration measurements).

3.6. Gamma influence concerns

In a mixed neutron-gamma field, the gamma rays interact in the3He proportional counter by producing secondary electrons fromthe inner wall of the cathode tube, which produce small avalancheswithin the counter volume. As the gamma-ray dose increases, theavalanches begin to pile-up and produce pulses that are largeenough to exceed the preamplifier energy discriminator thresholdfor neutron events. Fig. 5 depicts this phenomenon as an idealizedpulse-height distribution in a typical 3He tube. These pulses arethen erroneously counted as neutrons.

The key issue of the influence of gamma rays need to beaddressed with major attention during the design stage of 3Heproportional counters if these devices have to be used for neutronanalyses in locations where high gamma background is present.Aluminum can be used as a counter body because of its lower valueof photon linear attenuation coefficient, and for reducing long livedgamma emitters produced by neutron activation process. However,the mechanical properties of aluminum are such that high pressurecounters would require a thicker wall (which is not in itselfa problem except that it reduces the quantity of gas and thuscounter efficiency). The main disadvantage of manufacturingaluminum counter body is the serious problem of outgassing. Themelting point of aluminum is much lower than that of stainless

Fig. 5. Measured pulse-height distribution for a LND 252 3He counter in a neutron andgamma-ray field.

steel and this limits the temperature at which they can be pro-cessed. Previous work has demonstrated the difficulty of removingwater vapor from aluminum, even when the counters are baked attemperatures so close to the melting point that the body has to bephysically restrained to prevent it collapsing. A solution is to usea graphite lining to remove gas impurities (Beddingfield et al.,2000).

A unique conclusion, drawn up from analysis of results obtainedthrough Monte Carlo simulations (Tahri, 1993; Mazed et al., 1998)shows that the direct effect of gamma rays on the counter gas canbe neglected because the gamma ray interactions in a counter aredominated by events in the counter wall (Glesius and Kniss, 1988).

The counter diameter and gas density are the key parameters inthe design of a low gamma response counter. However, theseparameters cannot be reduced without increasing the wall effect,thus it is necessary to seek for a best trade-off between these twoitems. For example, if the counter diameter is halved, then theenergy loss per electron is halved, but also the number of photoninteractions is halved (because the cross surface of the counter ishalved), giving rise to a total reduction of a four factor in the energydeposited by the electrons in the counter. This means that thesignal/noise for a single neutron event has been improved andexplains why a counter made of a number of small diameter unitsoperating in parallel in a single body has a better gamma rayrejection performance than a simple counter with the samequantity of 3He gas. If cost is not a deciding factor, one can also useseveral shorter tubes in place of a long tube.

Some useful suggestions to reduce the gamma backgroundduring neutron measurements with 3He proportional counter arethe following: use small diameter counters each with its ownamplifier/discriminator connected as close as possible to thecounter; vary the quench gas to reduce the density of the gas fillingas far as possible while still being able to count neutrons witha reasonable efficiency; choose a gas mixture which has fast elec-tron and positive ion drift velocities and good resistance to radia-tion damage; increase the anode diameter to shorten the collectiontime, even if this require to raise the operating voltage; operatewith a gain as low as possible; use very small differentiating/inte-grating time constants with a low high voltage resistor connectingto the high voltage supply so that the fastest part of the neutronpulse is used (this will reduce the signal/noise and hence theplateau for low gamma operation but will help reject gammaevents at high gamma dose rates). Some of the above indicationsare mutually incompatible and a compromise needs to be found.

Finally, it is necessary to experimentally check the counteroperation at the highest gamma dose rate expected before it entersinto service. A plateau measurement taken in a suitable gammafield with and without additional neutrons is a good diagnostictool.

4. 3He proportional counter prototypes and results

After having overviewed the fundamental properties charac-terizing 3He proportional counters, some developed prototypemodels are described. The operation characteristics will besummarized and compared to a commercial model LND 252(supplied by LND Inc., USA) taken as a reference standard specimen,filled with 4 bar 3HeeCO2 mixture (Table 2). The consideredprototype 3He proportional counters are the following: a multi-purpose unsealed counter named DIC 200 (Baaliouamer, 1990) anda versatile counter named CPV 100 (Baaliouamer and Matatagui,1988).

The proportional counter DIC 200 has a cathode diameter of5.08 cm and it is built with the following technology elements:ceramic/metal seal type, alumina insulators, nickel anode, copper

Fig. 6. A photograph of the Versatile Proportional Counter (CPV 100), beside are shownthe aluminum and stainless steel cathode sets.

Table 2General design parameters of the proportional counters used.

Specifications LND 252 DIC 200 CPV 100

Maximum diameter (cm) 2.54 5.08 2.54Internal diameter (cm) 2.3 4.8 2.3Cathode Material 1100 Al Cu (OFHC) Stainless steelTotal length (cm) 25 35 35Effective length (cm) 10 30 20Anode diameter (nature) 50 mm (W) 100 mm (W, Au-plated) 50 mm (W, Au-plated)Insulation material Al2O3 þ PTFE Al2O3 Al2O3 þ PTFEElectrical capacitance (pF) 8 10.1 15.4Temperature range (�C) �50 up to 100 �50 up to 100 �50 up to 100Connector type HN HN HN

D. Mazed et al. / Radiation Measurements 47 (2012) 577e587 585

cathode tube and brass trap supporting the insulator (witha spring-keeping). The support seal-cathode is soldered by silver,while the TIG welding process is used to fix the seal to the exten-sion of the anode and also between the tube bottom cap and thecathode tube. Point welding process is uses to fix the tungstenanode wire to the hypodermic needle thin tube, and also to solderthe latter with the nickel filament.

The CPV 100, shown in Fig. 6, is a versatile proportional counterwhere many geometrical and mechanical parameters could bereadily changed. It is filled with a mixture of 0.5 bar 3He and 1.5 barKreCO2 (5%), with a cathode diameter of 2.54 cm. It was designedand built to specifically serve as an experimental test tool ratherthan a monitor device. Indeed, it was usually used to characterizeany non-standard gas mixture (different gas compositions and fillpressures) such as those concerned here for neutron detection andspectrometry. Moreover, since it was also provided by a set of

Table 3Experimental observations and characterization results of the examined 3He filled propo

Counter LND 252 DIC 200

Fill gas and pressure 4 bar 3He 0.5 baCalculated sensitivity (cm2) 33.2 32.2Measured sensitivity (cm2) 18.0 16.8Spectrum Normal DistorPlateau length (V) 150 200Plateau slope (% /100 V) 1.14 7.45Operating voltage (V) 1275 2300Best resolution (%) 5Discrimination plateau length (mV) 6400 3400Discrimination plateau length (%/V) 11 41Gas gain characteristics Normal Norma

removable cathodes consisting in different material and differentdiameters, it was found very useful for performing nuclear essayqualification of 10B and 235U lining procedures (Mazed, 2001) whendealing with the development of fission counters, boron ionchambers, and boron lined proportional counters intended fornuclear reactor control.

Four 3He-based gas mixture compositions have been examined,three of them for the DIC 200 proportional counter (the percentagerefers to the atomic or pressure fraction):

- 0.5 bar 3He with 1.5 bar AreCO2 (5%);- 0.5 bar 3He with 3.5 bar AreCO2 (5%);- 0.5 bar 3He with 3.5 bar KreCO2 (5%);

and the last one for the CPV 100 counter:

- 0.5 bar 3He with 1.5 bar KreCO2 (5%).

The first three gas mixtures were planned in order to assess theparticular effect of both the nature of the admixed heavy gas (Argonand Krypton) and also the total fill pressure on the characteristics ofthe 3He proportional counter. The ultimate gas mixture composi-tion, using the CPV 100, was examined to compare the two counterbodies between them. The filling procedure adopted was stan-dardized for all mixtures and required a degassing of the counterbody under constant heating for at least 6 h maintaining a vacuumof less than 6e10 torr inside it, to remove all traces of electroneg-ative gases (water vapor, oxygen, etc.) that could be adsorbed onthe counter inner walls. For the optimized gas mixture, a set ofmeasurements is performed and the results are summarized inTable 3. A block diagram of the experimental setup used is shown inFig. 7. The neutron flux is provided by two cylindrical 239PueBeisotopic neutron sources located within a specially designedparaffinmoderated neutron irradiator. The irradiator is provided bya number of different diameter vertical irradiation channels, wherethe neutron flux presents a nearly flat axial profile, as we checked

rtional counters.

CPV 100

r 3He þ 1.5 bar (Ar þ 5%CO2) 0.5 bar 3He þ 1.5 bar (Kr þ 5%CO2)10.05.9

ted Normal2505.00137511300018

l Normal

Fig. 7. Block diagram of the experimental setup used for the measurement (PA: preamplifier; SCA: single channel analyzer; MCA: multi channel analyzer; E: energy; T: time; S:signal). The pulse spectrometry channel is shown in solid line connections and the current channel is represented in dashed lines. The cadmium covers are used to screen the lowgas gain regions of the counter, suppressing thus the end-effects (see Section 3.5).

00

D. Mazed et al. / Radiation Measurements 47 (2012) 577e587586

using neutron activation detectors. Due to the paraffin moderatoreffect, the resulting neutron energy distribution along the irradia-tion channel is mostly thermal (about 90%). The associatedinstrumentation chain, located in an adjacent room, is consists intwo classical channels: the pulse spectrometry channel and thecurrent channel, as schematized in Fig. 7.

The gas electron multiplication characteristics obtained withthe LND 252 proportional counter along with that obtained withthe CPV 100 proportional counter are shown in Fig. 8. The CPV1shows less gainmainly due to design differences with respect to theLND 252 counter. Other characteristics that can be determined arethe plateau characteristic curve and the bias curve. The formerspecifies the suitable high voltage operating range (the mostconvenient high voltage value for work should be chosen takinginto account the desired gas gain operation, according the gaselectron multiplication characteristic), the latter allows to separatethe neutron signal from the noise and the concomitant gammabackground signal. Fig. 9 shows two standard bias curves, oneobtained using the CPV 100 and the other with LND 252.

Fig. 8. Gas gain characteristics of LND 252 and CPV 100 proportional counters.

Fig. 9. Bias curves of LND 252 and CPV 1 proportional counters (the normalization isreferred to the maximum threshold value).

Further parameters than can be determined, for example thepulse height spectrum as shown in Figs. 4 and 5 for the LND 252counter. It is evident that the CPV 100 has characteristics very similarto those obtained using the commercial LND 252, while the otherthree tests conducted using the DIC 200 presents distorted spectradue to electric sparks between the anode and the insulator insidethe counter, likely as the results of an improper welding betweenthe electrodes.

5. Conclusions

3He filled proportional counters have been used successfully forseveral decades. These counters were used either for countingneutrons or to characterize their energy distributions in spec-trometry applications in many nuclear engineering fields. This hasincreased the number of potential users and decision makers whoneed to be aware of the capabilities and constraints of theseneutron sensors. In this paper we share some of our manufacturing

D. Mazed et al. / Radiation Measurements 47 (2012) 577e587 587

experiences and aim to provide an accessible bridge to the researchliterature.

Many of the design practices of 3He filled proportional countersare part of the intellectual property of a few manufacturers. Simi-larly many of the pragmatic and practical considerations of usersand instrument designers are locked in the minds of a few expe-rienced personnel working hardly withinmany laboratories aroundthe world. This paper provides guidance in a few crucial areas toa new generation of users in the field by drawing on our experi-ences in the construction of 3He-filled proportional counters, anddesigner and operator of other types of such instruments. A majorpart of this work was devoted to the study of the distribution ofamplitude pulses for the 3He proportional counters. It has emergeda good agreement with experimental results despite the largenumber of assumptions made. This study can be developed andrefined, taking into account all the phenomena brought into playand exploiting a source of recent and reliable data on the losses ofenergy particles in gases including 3He, Ar, Kr. Proportionalcounters with 3He are becoming of a systematic use in experimentsaround neutron diffraction beams, whether for monitoring the flowor neutron spectroscopy itself.

References

Alevra, A.V., Thomas, D.J., 2003. Neutron spectrometry in mixed fields: multispherespectrometers. Radiat. Prot. Dosimetry. 107, 37e72.

Baaliouamer, M., 1990. Internal Report DIC/CDSE, December 1990 (unpublished).Baaliouamer, M., Belaragueb, C., Mazed, D., 1994. La Technique impulsionnelle pour

la caractérisation des détecteurs à gaz. CEA Report, CEA-N-2756, 47e53 (inFrench).

Baaliouamer, M., Belaragueb, C., Mazed, D., 1996. Gas gain measurement in helium-isobutane filled proportional counters. Nucl. Instr. Meth. A 382, 490e494.

Baaliouamer, M., Matatagui, E., 1988. INVAP SE Report, 1988 (unpublished).Beddingfield, D.H., Johnson, N.H., Menlove, H.O., 2000. 3He neutron proportional

counter performance in high gamma-ray dose environments. Nucl. Instr. Meth.A 455, 670e682.

Benjamin, P.W., Kemshall, C.D., Redfearn, J., 1968. A high resolution sphericalproportional counter. Nucl. Instr. Meth. 59, 77e85.

Bortner, T.E., Hurst, G.S., 1954. Ionisation of pure gases and mixtures of gases by 5MeV alpha particles. Physiol. Rev. 93, 1236e1241.

Brinckmann, H.F., Gerber, D., 1961. Investigation of BF3 Proportional Counters.A.E.R.E Report Trans., 859.

Brooks, F.D., Klein, H., 2002. Neutron spectrometry d historical review and presentstatus. Nucl. Instr. Meth. A 476, 1e11.

Convert, P., Forsyth, J.B. (Eds.), 1983. Position Sensitive Detection of ThermalNeutrons. Academic Press, London, United Kingdom.

Curzio, G., Mazed, D., Ciolini, R., Del Gratta, A., Gentili, R., 2005. Effect of air on gasamplification characteristics in Argon-propane(1%)-based proportionalcounters. Nucl. Instr. Meth. A 537, 672e682.

Desai, S.S., Shaik, A.M., 2006. On studies of 3He and isobutane mixture as neutronproportional counter gas. Nucl. Instr. Meth. A 557, 607e614.

Diethorn, W., 1956. A Methane Proportional Counter System for Natural Radio-carbon Measurements. USAEC Report NYO.6628.

Glesius, F., Kniss, T., 1988. 3He neutron detectors for hostile environments. IEEETrans. Nucl. Sci. 35, 867e871.

Harling, O.K., 1965. Detector time jitter in 3He and BF3 proportional counters. Nucl.Instr. Meth. 34, 141e145.

ICRU, International Commission on Radiation Units and Measurements, 1979.Average Energy Required to Produce an Ion Pair. ICRU Report 31, Washington,USA.

Jesse, W.P., Sadauskis, J., 1952. Alpha-particle ionisation in mixtures of the noblegases. Physiol. Rev. 88, 417e418.

Kadyk, J.A., 1991. Wire chamber aging. Nucl. Instr. Meth. A 300, 436e479.Knoll, G.F., 2000. Radiation Detection and Measurement, third ed. John Wiley &

Sons, Inc., New York, USA.Kowalski, E., 1970. Nuclear Electronics. Springer, Heidelberg, Germany.Manolopoulou, M., Fragopoulou, M., Stoulos, S., Koukorava, C., Spyrou, A.,

Perdikakis, G., Hashemi-Nezhad, S.R., Zamani, M., 2006. Studies on the responseof 3He and 4He proportional counters to monoenergetic fast neutrons. Nucl.Instr. Meth. A 562, 371e379.

Mazed, D., Baaliouamer, M., Mameri, S., Negara, K., Belaragueb, C., Zitouni, H., 1997.Etude expérimentale et paramétrisation du premier coefficient de Townsenddans les compteurs proportionnels au BF3. In: Proceedings of the 2nd NationalConference on High Voltage, CNHT’97, Tizi-Ouzou, Algeria, 17e19 November1997, pp. 117e121 (in French).

Mazed, D., Baaliouamer, M., Tahri, A., 1998. Développement et technologie descompteurs proportionels à helium 3. In: Proceedings of the 4th MagrebianConference on Electronics and Instrumentation, COMAEI’1998, 8e9 December1998, Bejaïa, Algeria, pp. 123e127 (in French).

Mazed, D., Baaliouamer, M., 1999. A semi-microscopic derivation of gas gainformula for proportional counters. Nucl. Instr. Meth. A 437, 381e392.

Mazed, D., 2001. Construction de détecteurs nucléaires en Algérie: opportunitésprésentes et perspectives futures. In: Proceedings of the 5th InternationalMeeting of Studies on Naval Applications and Sciences, J’NESMA’2001, 28e30May 2001, Algiers, Algeria (in French).

Mazed, D., 2002. Contribution to the study of the charge amplification in propor-tional counters. Proposition of a new semi-microscopic model for the predic-tion of the gas amplification factor, Magister thesis. No. 13/2002/MPH,University of Sciences and Technology, Algiers.

Mazed, D., Amokrane, A., 2003. Nouvelle formulation du facteur de multiplicationdes electrons dans les compteurs proportionnels. Bull. du Bur. Natl. de Métro-logie, BNM 123, 63e68 (in French).

Mazed, D., 2007. Experimental gas amplification study in boron-lined proportionalcounters for thermal neutron detection. Radiat. Meas. 42, 245e250.

Mazed, D., Ciolini, R., Curzio, G., Del Gratta, A., 2007. A new active method forcontinuous radon measurements based on a multiple cell proportional counter.Nucl. Instr. Meth. A 582, 535e545.

Ramsey, W.E., 1940. Measurement of discharge characteristics of Geiger-MuellerCounters. Physiol. Rev. 57, 1022e1029.

Ravazzani, A., Foglio Para, A., Jaime, R., Looman, M., Marín Ferrer, M., Peerani, P.,Schillebeeckx, P., 2006. Characterisation of 3He proportional counters. Radiat.Meas. 41, 582e593.

Ricker, G.R., Gomes, J.J., 1969. Pulse rise-times in proportional counters. Rev. Sci.Instrum. 40, 227e233.

Shalev, S., Fishelson, Z., Cuttler, J.M., 1969. The wall effect in 3He counters. Nucl.Instr. Meth. 71, 292e296.

Tahri, A., 1993. Contribution à l’étude, la réalisation et la caractérisation de comp-teurs proportionnels à 3He et au BF3, DPGS thesis. CRNB/COMENA, November1993 (in French, unpublished).

Takeda, N., Kudo, K., 1994. Neutron response functions improved by taking intoconsideration measured edge effect of 3He proportional counter. IEEE Trans.Nucl. Sci. 41, 880e883.

Vega-Carrillo, H.R., Manzanares-Acuña, E., Martín Hernández-Dávila, V., MercadoSánchez, G.A., 2005. Response matrix of a multisphere neutron spectrometerwith an 3He proportional counter. Rev. Mexic. Fisica 51 (1), 47e52.

Weyrauch, M., Casnati, A., Schillebeeckx, P., Clapham, M., 1998. Use of 4He-filledproportional counters as neutron spectrometers. Nucl. Instr. Meth. A 403,442e454.

Wilkinson, D.H., 1950. Ionization Chambers and Counters. Cambridge UniversityPress, New York, USA.

Zastawny, A., 1966. Gas amplification in a proportional counter with carbon dioxide.J. Sci. Instrum. 43, 179e181.