TESTING OF SMALL DETECTORS WITH GLASS ROD LIGHT GUIDES

FOR MULTIPLICITY MEASUREMENT PURPOSES

Dina Chernikova1, Imre Pazsit1, Anders Nordlund1, Berit Dahl1, Gudmar Grosshoeg1,Jan Pallon2, Mikael Elfman2

1Chalmers University of Technology, Division of Nuclear Engineering, Gothenburg, Sweden

2Lund University/Lund Institute of Technology, Division of Nuclear Physics, Lund, Sweden

Abstract

In this paper we investigate the applicability of small scintillators with glass rod lightguides to measure both neutrons and gamma rays from the sample. Experimental test ofthese detectors and their suitability for the task can be performed at Lund University usingnatural uranium rods, therefore the MCNPX simulation set-up corresponds to the Lund con-figuration. The long-term goal of this research is to develop a method of joint-multiplicitycounting for fuel evaluation into a technology capable of quantifying plutonium in the fuelpool, where a lot of factors, such as presence of neutron absorbers (boron acid), can affect al-most all parameters, such as multiplication etc. Therefore, part of the present investigationswas devoted to the development of a direct method for determination of the concentrationof boron acid in the fuel pool using scintillation detectors with further correction of mea-surement results. For this purpose we suggest a method which utilizes a relation betweengamma lines with energy of 480 keV and 2.23 MeV for the direct evaluation of concentrationof boron acid in water. Test results of the new method, and an answer to questions regardingthe ability to measure both neutron and gamma rays using small scintillation detectors withglass rod light guides are provided in this paper.

1. Introduction

Knowledge of physical parameters of irradiated and non-irradiated nuclear fuel is one ofthe key elements of international nuclear materials safeguards. There exist several nonde-structive assay techniques for this purpose that are complementary to each other. For fuelassemblies in a hot cell lab, a number of methods are available, including reconstructionof the spatial distribution of various isotopes with gamma tomography methods. In thefuel pool, however, only a few are available such as the Fork Detector [1], the Under WaterNeutron Coincidence Counter (UWCC) [2] and the Cerenkov viewing device [3]. The orig-inal version of the UWCC was just a modification of the Fork detector, where four fissionchambers were substituted by four 3He neutron detectors. However, one significant concernregarding the use of 3He gas proportional detectors is the global shortage of 3He [4]. A recentestimate by GE Reuter Stokes projects that the total 3He demand is ∼65 kliter/y (40-70kliter/y), while the total supply is ∼10-20 kliter/y [5]. Thus, there is a rising and urgentdemand for the development of new methods of detection.

Scintillation based techniques may be considered as candidates for future measurementsystems if a scintillation crystal is optically connected to a photomultiplier tube (PMT) viaa glass rod light guide so that the PMT will not be damaged by high radiation background.

1

Therefore, in the present paper we investigate the applicability of recently developed smalldetectors with glass rod light guides [6] to measure both neutron and gamma radiation.

The long-term goal of this research is to develop a method of joint-multiplicity countingfor fuel evaluation into a technology capable not only of quantifying plutonium (Pu) butalso providing spatial resolution within the fuel assembly in the fuel pool. To achieve thisgoal, investigation of the influence of different parameters is required to make a step towardsindustrial implementation. As it was mentioned in [7] one of the important parameters whichinfluences the result is the impact of neutron absorbers (boron acid) on the characteristicsand performance of measurement system. Thus, one of the goals of the current investigationwas to develop a direct method of determination of the concentration of boron acid in a fuelpool using scintillation detectors with further elimination of this effect from measurementresults.

2. Testing of small detectors with glass rod light guides

In order to test the ability of small detectors with glass rod light guides to measure bothneutrons and photons, first, a number of Monte Carlo simulations of the Lund measurementsetup, consisting of more than 200 fuel rods of natural U (density 18,97 g/cm2, radius 0.77cm) and two small scintillators were performed. Then, as a complementary check, a simpleexperiment was organized to test the detectors and their suitability for the task that can beperformed at Lund University using natural uranium rods.

Figure 1a shows a schematic drawing of the detectors for simulations and Figure 1b showsa photo of one of the detectors. Cylinder shaped scintillation crystals (BC-420, Saint-GobainCrystals) with a diameter and a height of 0.6 cm were optically connected to PMTs via glass(pure SiO2) rods detectors. For physical protection, each detector was sheltered in an Alcylinder.

(a) (b)

Figure 1: a) A schematic drawing of the detectors for simulations, b) Photo of one of the detectors.

2.1. Monte Carlo simulations - Lund set-up

The MCNPX code [8] was used to simulate time-dependent measurements which canbe performed at Lund University using natural uranium rods. Figure 2 shows a schematicdrawing of the configuration for simulations.

2

Figure 2: A schematic drawing of the configuration of Lund set-up for simulations.

The estimation of the number of neutrons with energy higher than 0.3 MeV and thenumber of gamma photons with energy higher than 0.16 MeV which reach the detectors,as well as a gamma spectrum entering the detectors were performed by using the F4 tally1.1·108 histories were run to reduce statistical error to less than few percent. Figure 3 showssimulation results, in which 238U represents more than 99% of the total amount of theuranium in the set-up. The results are normalized to one source neutron per second.2

The rough estimation of the number of spontaneous fissions per second which couldtotally take place in 238U (∼7·103), and results of the performed simulations showed thatthe size of these detectors make it impossible to get a response of the gamma ray radiation,as well as the neutron multiplicity measurements will take a long time, which can become aproblem for underwater measurements in a fuel pool. Hence, more sensitive small detectorsneed to be found for such measurements.

2.2. Experimental results for an Americium-Beryllium (AmBe) source and small detectors

For the measurement we used an AmBe source (1.1·107 particles per second, source ofneutrons and gamma rays) placed at a distance of 15 cm from the detector. The detector wasbiased to -1260V. Signals from the PMTs were connected to the digitizer. The digitizer isan 8 channel, 12 bit 250 MS/s, VX1720E. The digitizer run the preliminary version of DPP-PSD firmware for the digital charge integration and neutron-gamma discrimination. Thebaseline of the signal was calculated by a programmable length mean filter and subtractedfrom the signal, then the result was compared to the trigger threshold and a trigger was

1Flux averaged over a cell.2Based of the previous experiments threshold levels for gamma and neutron detection were

chosen to be 0.16 MeV and 0.3 MeV, respectively.

3

0 20 40 60 80 1000

0.2

0.4

0.6

0.8

1

1.2x 10

−5

Time, ns

Flu

x of

par

ticle

s in

det

ecto

rs, r

el.u

n.

Neutrons with energy high than 0.3 MeVGamma with energy high than 0.16 MeV

0.5 1 1.5 2 2.5 3 3.5 40

0.5

1

1.5

2x 10

−6

Energy of gamma, MeV

Flu

x of

gam

ma

part

icle

s, r

el.u

n.

(a) (b)

Figure 3: a) Time distributions of the number of neutrons with energy higher than 0.3 MeV and the numberof gamma with energy higher than 0.16 MeV which reach the volume of detectors, b) A spectrum of gammaparticles entering detectors.

issued. The PSD (Pulse Shape Discrimination) parameter was calculated according to thefollowing formula:

PSD =QL −QS

QL

(1)

where QL is the charge calculated within the long gate and QS is the charge calculated withinthe short gate. Figure 4 shows the measurement results of ”neutron-gamma” separation inthe small detectors. In comparison with the results (described below, Figure 8) obtained forEJ-309 (D76 x 76 mm) scintillation detectors, the simple conclusion can be drawn that thesize of small detectors with glass rod light guides (D6 x 6 mm) make them gamma-insensitive.Therefore, future studies should be made with bigger size detectors.3

3. Determination of the concentration of boron acid in the fuel pool using scin-

tillation detectors

One of the parameters which has a strong influence on the measurements of the Pucontent in nuclear fuel pool is the presence of neutron absorber, boron acid4. According to[7] boron affects several processes: multiplication, detection efficiency, die-away time andthe doubles/totals ratio in the case of UWCC technology (UWCC based on the detection ofthermal neutrons by 3He counters). Of course, in the case of fast scintillation detectors thisinfluence is not significant but it is still important to develop an algorithm of independentevaluation of the boron concentration in the water of the nuclear fuel pool in order to beable to make required corrections of results while measuring with scintillators.

Therefore, in this paper we investigate a method of evaluation of the boron acid concen-tration in water using two gamma-lines in the gamma spectrum obtained by the scintillationdetector. One of the gamma lines, 480 keV belongs to the thermal neutron capture in boron,

3However, in addition, it is worth to check if the small detectors are suited to pulse shapeanalysis.

4If the storage pool contains the borated water.

4

Figure 4: 2D plot of the counts as a function of the pulse energy (ADC channel) and PSD parameter forsmall detectors.

the other line, 2.23 MeV characterizes the capture of thermal neutron in hydrogen. Thus,the relation between them can reveal the concentration of the boron in the water.

In order to test this concept, first, Monte Carlo simulations were performed, whereaftera simple experiment was performed, as described below. In both the simulation and theexperiments, EJ-309 (D76 x 76 mm) scintillation detectors were used.

3.1. Monte Carlo simulations

The simulation set-up consisted of a 252Cf-source and four EJ-309 detectors which wereplaced at a distance of 25 cm from the source. An MCNPX estimation of time distributionsof the number of neutrons with energy higher than 0.3 MeV and the number of gammaphotons with energy higher than 0.16 MeV which reach the volume of detectors, as well asthe spectrum of gammas entering the detectors were performed by using the F4 tally. 1·108

histories were run to reduce statistical error to less than a few percent. Figure 5 shows aschematic drawing of the set-up for simulations.

Simulations were performed for several cases: with air between the source and detectors,and when detectors and the source are placed in the water (pure water; water with boronacid, concentration 1500 ppm, 2000 ppm, 2500 ppm, 4000 ppm).

Several conclusions can be drawn from these simulation results. First, as Figure 6a shows,

5

Figure 5: A schematic drawing of the set-up for simulations.

the neutron response of the detector5 is not affected by the concentration of boron acid in thewater. On the other hand, when comparing cases with water and air surroundings, Figure6a shows that a significant part of fast neutrons are slowed down in the water and cannotbe directly detected by the fast neutron scintillator.

0 10 20 30 40 50 600

0.2

0.4

0.6

0.8

1

1.2x 10

−4

Time, ns

The

ave

rage

neu

tron

flux

in d

etec

tors

, rel

.un.

AirWaterWater+1500 ppm of boron acidWater+2000 ppm of boron acidWater+2500 ppm of boron acidWater+5000 ppm of boron acid

0 5 10 15 20 25 30 35 40 45 500

0.5

1

1.5

2

2.5x 10

−6

Time, ns

The

ave

rage

pho

ton

flux

in d

etec

tors

, rel

.un.

Air

Water

Water+1500 ppm of boron acid

Water+2000 ppm of boron acid

Water+2500 ppm of boron acid

Water+4000 ppm of boron acid

(a) (b)

Figure 6: a) Time distribution of the number of neutrons with energy higher than 0.3 MeV in the volumeof detectors. b) Time distribution of the number of gammas with energy higher than 0.16 MeV.

The situation regarding gamma detection is different. For the case when the detectorswere placed in air, the time distribution of the gammas has only one maximum (between6-8 ns after the starting neutron). This peak mostly consists of the gamma originating fromtwo sources: first is the radioisotope source itself and the second is the 2.23 MeV gammawhich is produced due to the thermal neutron capture in the hydrogen which is containedin the detector [9]. However, if the case when water (or borated water) is considered, twogamma peaks appear in the time distribution. This can be explained by the fact that thereare already four sources of gamma rays in these cases, two of them are the same as in the

5EJ-309 scintillator is sensitive for fast neutrons.

6

”air case” (they are responsible for the same peak between 6-8 ns in the time distribution)and the other two are the 2.23 MeV and 480 keV gamma lines which are produced due tothe thermal neutron capture in the hydrogen and boron, respectively (they are responsiblefor the peak between 2-3 ns in the time distribution). One conclusion which is relevantto the new method is that the total number of detected gammas does not depend on theconcentration of boron acid in water. Overall these results indicate that the relation betweenthe amount of gamma rays with energy of 480 keV to the amount of gamma rays with energyof 2.23 MeV should characterize the concentration of boron in the water.

To prove this conclusion, the spectrum of gamma rays in the detectors was simulated(Figure 7a) and the relation between the gamma lines was obtained. Figure 7b shows thegood agreement between the obtained relation (blue curve - for various concentration of boronacid) and real change of boron content in the water (red curve). Thus, these preliminary

0 0.5 1 1.5 210

−9

10−8

10−7

10−6

10−5

10−4

10−3

Energy of photons, MeV

Ave

rage

flux

of p

hoto

ns, r

el.u

n.

Air

Water

Water+1500 ppm of boron acid

Water+2200 ppm of boron acid

Water+2500 ppm of boron acid

Water+4000 ppm of boron acid

480 keV2.23 MeV

1500 2000 2500 3000 3500 40001

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

Concentration of boron acid, ppm

Nor

mal

ized

val

ues,

rel

.un.

Concentration of boron acid obtainedfrom gamma lines 480 keV and 2.23 MeVin EJ−309 detectorsReal concentration of boron acid

(a) (b)

Figure 7: a) Spectrum of gamma particles entering the detectors, b) Normalized value of the amount ofgamma rays with energy 480 keV to the amount of gamma rays with energy 2.23 MeV (blue curve) and realchange (red curve) of boron content in the water.

numerical results show that by using the ratio between gamma lines with energy of 480keV and 2.23 MeV, it is possible to directly obtain the concentration of boron acid in thewater of the nuclear fuel pool. However, in reality the practical implementation of thismethod can face some difficulties which are connected, for example, with energy resolutionof the scintillation detectors. Therefore, an important future step will be to test this methodexperimentally. Some of the preliminary experimental studies have already been made inour laboratory in Chalmers (described below).

3.2. Preliminary experimental results for an AmBe source and water container

For the measurement we used an AmBe source (as a source of neutrons and gamma rays)placed at a distance of 15 cm from the detector EJ-309 type (D76 x 76 mm). The detectorwas biased to -1760V. The measurement equipment was the same as in the experimentwith small detectors. The DPP-PSD parameters used for the acquisition were as follows:trigger threshold - 80 ADC counts, short gate width - 52 ns, long gate width - 152 ns, gateanticipation - 40 ns, baseline threshold - 3, mean filter - 16, sensitivity - 40fC/LSB. The

7

rough calibration of the detector was performed with 137Cs (662 keV gamma ray) and 60Co(1.33 MeV and 1.17 MeV gamma rays) sources. Figure 8 shows the experimental set-upand the quality of the neutron-gamma discrimination. The Al tank, first, was filled withpure water for measurements. The influence of measurement errors was not considered. Themeasurement time was equal to ten minutes.

Figure 8: A photo of the experimental set-up and 2D plot of the counts as a function of the pulse energy(ADC channel) and PSD parameter (the Al tank was not filled in with water).

Figure 9 shows a preliminary experimental spectrum (energy histogram) of gammas en-tering the detector when the Al tank was filled with pure water. Two gamma lines withenergy of 480 keV and 2.23 MeV appear in the spectrum, as it was predicted by simulations.The first line with energy of 480 keV corresponds to the thermal neutron capture in boroncontained in the glass of the PMT tube. The second line with energy 2.23 MeV belongsto the thermal neutron capture in hydrogen contained in the water and the scintillator it-self. However, it should be mentioned that the measured gamma spectrum contains a lotof gamma from the original AmBe source, which contaminates the overall picture6. Furtherexperimental studies are planned to be performed for various concentrations of boron in thewater during the summer of 2012.

6252Cf can be considered as a source of neutrons in future measurements.

8

Figure 9: A spectrum of gamma particles entering the detector (energy histogram) - preliminary results.

4. Conclusion

Several conclusions can be drawn from these studies. First, simulation and experimentalresults shows that the size of scintillation crystals in the small detectors with glass rodlight guides is too small to get a response of the gamma radiation, as well as that neutronmultiplicity measurements will take a long time which can become a problem for underwatermeasurements in a fuel pool. Therefore, future studies should be done with detectors ofhigher sensitivity, or lager size.

We have also described a method for determination of the concentration of the boronacid in the fuel pool using scintillation detectors. The suggested method is based on therelation obtained between two gamma lines with energy of 480 keV and 2.23 MeV. UsingMCNPX simulations we showed that, the content of boron in the water can be estimatedusing this relation within few percent. However, in reality the practical implementation ofthis method can face some difficulties which are connected, for example, with the energyresolution of scintillation detectors. Therefore, the experimental set-up was created in orderto test the new method, and first preliminary studies were performed with a water containerbetween the detector and the source. The results of the experiments showed the presenceof two gamma lines with energy of 480 keV (corresponding to the thermal neutron capturein boron contained in the glass of PMT tube) and 2.23 MeV (corresponds to the thermalneutron capture in hydrogen contained in the scintillator and water surroundings), as it waspredicted by simulations. Further experimental studies with various concentration of boronin the water are planned for the summer of 2012.

Acknowledgement

This work was supported by the Swedish Radiation Safety Authority. A research grantfrom AForsk made it possible to acquire the measuring equipment.

9

References

[1] P. M. Rinard and G. E. Bosler, ”Safeguarding LWR Spent Fuel with the Fork Detec-tor”. Los Alamos National Laboratory Report, LA-11096-MS, 1988.

[2] G.W. Eccleston, H.0. Menlove et. al. The Underwater Coincidence Counter for Pluto-nium Measurements in Mixed-Oxide Fuel Assemblies(ISPO-375). UC-706, LA-13574-M, 1999.

[3] J. D. Chen A. F. Gerwing, R. Keeffe, M. Larsson, K. Jansson, L. Hildingsson, B.Lindberg, E. Sundkvist, U. Meijer, M. Thorsell and M. Ohlsson, ”Long-Cooled SpentFuel Verification Using a Digital Cerenkov Viewing Device”, International AtomicEnergy Agency Report, IAEA-SM-367/14/07, 2007.

[4] A. J. Keller, ”An Increasingly Rare Isotope”, Physics 241, Stanford University, Winter2011.

[5] R. T. Kouzes, ”The 3He Supply Problem”. Report No. PNNL-18388, Pacific NorthwestNational Laboratory, 2009.

[6] Erika Netto, Pilot measurements with a small detector based detection system forfissile material, Thesis ISSN 1653-4662, Sweden 2009.

[7] I. Koulikov, J. Fager, Y-G Lee, G. Eccleston, H. Menlove. ”Impact of neutron absorberson the underwater neutron coincidence counter performance”. IAEA-SM-367/14/05.

[8] Monte Carlo N-Particle Transport Code System Including MCNP5-1.60 and MCNPX2.7.0 and Data Libraries.

[9] V. L. Romodanov, V. K. Sakharov, D. N. Chernikova. A method for decreasing theloading on scintillation channels in facilities with pulsed neutron sources. Instrumentsand Experimental Techniques (ISSN 0020-4412), Volume 53, Issue 3, pp 362-367, DOI10.1134/S0020441210030061

10