COLLÈGE DE FRANCELaboratoire de Physique Corpusculaire

LPC 89 35

SUPERCONDUCTING SUPERHEATED GRAINS IRRADIATEDWITH AN ELECTRON BEAM

A. de Bellefon, D. Broszkiewicz, P. Esplgtt

COLLEGE PE FIANCE, IN2P3-CN1S, Laboratoire de Physique Ceiptsciiïiiire,11 Knee M«win lertwte, F-75231 Puris Crfex 05,

SUPERCONDUCTING SUPERHEATED GRAINSIRRADIATED WITH AN ELECTRON BEAM.

A.de Bellefon.D.BroszkiewiCz.P.Espigat.

Collège de France, IN2P3-CNRS . Laboratoire cfc -'.y-iq-je Corpusculaire11 place Marcelin Berthelot , F-75231 Par:'- Cedf.v 05 .France

Abstract: Since one year a conventional raad-out electronicchain has been realized and tested in order to perform real-time read-out of the transition for grains as small as 10um .Now a convincing irradiation of superheated ruperconductinggrains with an electron beam has been realized.Individual signals of transiting grains are signed and shownto be caused by minimum ionizina electron tfeoosited energy( kinetic energy of the electrons is 2.2 MeV ).

Résumé: Après la réalisation d'une élecrorvque de lecturecapable de détecter des signaux de micrcsphere de 10um dediamètre une irradiation convaincante avec v.n faisceaud'électrons a été effectuée.A l'aide d'un dispositif de coïncidence nous avons pu mettreen évidence la transition de grain due au passage d'unélectron de 2.2Mev.

Keywords: Neutrino detection , cryogenics , grain size ,irradiation , beams

Talk given to third international workshop on LowTemperature Detectors for Neutrino and Dark Matter .( Gran Sasso 22-24 / 9 / 1989 )

1. INTRODUCTION

ft is now an evidence that cryogenic detectors are potentiallydetectors for low energy neutrinos or/and dark matter candidates.But in the particular case of superconducting superheated grains,

( hereafter called SSG) as it has already been mentioned "0,several basic questions had to be answered before speaking ofa real detector.One of these was the net effect of a minimum ionizing particle on acollection of grains with a given size.As a matter of fact , up to now only an integrated effect has beenexhibited 2 ) . That means no experiment was able to isolate theeffect of one m.i.p , in particular to separate the heatingcontribution of the beam or anything else from the heating due toan energy deposition.In this short communication , the procedure we used , to perform adecisive irradiation experiment ,is explained and results arediscussed within the limit of what we know.

2.

For our purpose we used a Van de Graaf electrostatic acceleratorwhich accelerates electrons up to 3 MeV .This accelerator is located in the basement of University Paris 7.In order to perform a convincing irradiation with a minimumionizing particle the idea was to show up grains flipped down bythe energy deposition of a single electron going through.

2.1

The experimental set up is shown on fig.1. A small helium 4cryostat in the bottom of which a pick-up coil is kept down to 1.5KThe magnetic field is parallel to the beam.The beam has to go through a mylar window and then through astainless steel wall which surrounds a small volume containing theliquid helium and the coil.The electron energy loss through the beam window , cryostatwindow and steel wall , ahead of the coi! ,amonts to about 200keV .

amplif ier

LiquidHelium

LiquidNitrogen

e" beam

Acquisit.on

•

fig 1

The electronics is designed to read a single c-ivn signal andhas been already extensively described in othor publications '-'Our data acquisition system was a Mac-lntosh micro-ccnpjv--connected to the analyser via a Camac system

2.3 SâJUaiêSIn practice specimens of Sn grains have been prepared by ar- 'sonic method .Tin grains thus obtained were dried and sorted according tosize filtration through meshes.Typically three 5-arrples navr- h o

kept and embedded in paraffin wax. These sample? have beenscanned with an electronic microscope 4) . They showed anextremely good reproducibility ,through many magnetic cycle?the same superheating curves were reproduced within a tewpercent.

2.4___âSi jn j2 lsOn one hand , bath temperature and number of electrons throt.g-> thetarget have been determined at a given applied field . On the otnerhand , the number of superconducting grains performing atransition to the normal state under magnetic field on electronshave also been determined.

MâCUMM

Cl f tS ÏÉ Î CiiSS SECTIiN

fig 2

One of the main thing we have to take erne of is the input powerin the target due to the beam which provides an increase oftemperature AT. A beam flux of 10000 e" /s on diode has beenmeasured and is equivalent to a 1mK increase of the bathtemperature .in practice ,we have been able to decrease the beam intensity downto 1000 e" per second on diode which means a negligible heatingeffect.The description of our coil is given on fig 2., a diode is stuckon the back of the coil in order to get a signal from any electrongoing through the solenoidai coil.The surface ratio between the diode and the coil is of the order of150 which means that for 1000 e~/s we are at the level of a fewelectrons per second through the coil.Before we decided to install a diode in helium we did some tests toknow how it works when it is cooled down to 2K.

to

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Dfoete amplitude ( a.u.)fig 3

Diode amplitude ( a.u.)

Results of our tests are shown on fig S.The diode is clearly lessefficient at low temperature but still enough for our purpose.The applied voltage has been optimized for us and the efficiencywas estimated to be around 50%.

Principle of operation :The principle of the data acquisition is shown on fig 4. The diodegives a pulse with a slow rise time (2JJS) when an electron isgoing through, then this amplified pulse goes to a coincidence unitwith the signal coming from the grains . delayed to be in time .

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fig 4

A 2.2 MeV electron is still around 2 MeV when it reaches thesolenoidal coil after crossing windows and wall.So it crosses the coil (as a m.i.p) and in general will stop in thediode.

Procedure to irradiate :

First.the sample is characterized by sweeping the magnetic fieldfrom 0 to 400 gauss ( above the value of superheating field Hsh)and the individual grain transitions as well as the derivative of thesuperheating curve are recorded.Next , the field is raised from 0 to HQ and stabilized at Ho(irradiation magnetic field).Then the electron beam at minimal intensity is switched on duringa time interval At ( 10 to 300 s ) and switched off.

The cycle is then completed by sweeping the magnetic field fromHo to 400 gauss . On the final curve , above Ho , there are Ngmissing grains corresponding to the grains flipped by the beam asit is shown on fig.6

Three sample of grains have been irradiated :Sample A diameter is less than 8|im. (filling factor: 12% )Sample B diameter is around 20pm. (filling factor: 18% )Sample C diameter is around 4Qjim. (filling factor: 12% )

The first net effect is that we have observed signals from grainsin coincidence with a diode pulse. And most of them are truecoincidences due to an electron and a grain flipped .With a very lowintensity beam ( 1000 to 10000 electrons /s ) the randomcoincidence rate was found to be of the order of a few percent.For the first time the effect of a minimum ionising particle hasbeen directly observed.(fig 5.)

Upper signal : Diode , 10 mV./DivmV./Divlower signa! : grain from sample A ,

5mV./Div 1 n

Upper signal : Coincidence , 500

lower signa! : grain from sample A ,5mV./Div .5 fis./Div.

fig 5

Different parameters have been varied , such as duration of theexposure to the beam , beam intensity, grains sample (figures 6and 7) . Main results are:

- Unmistakable coincidences have been obtained , with a verytow intensity beam.

- The number of missing grains (Ng) in the superheating curveirradiation is greater than the number of signais (N$)recorded on oscilloscope during the time of the irradiation.

- The flipping efficiency is much greater than that expectedand evaluated by Monte Carlo simulation in the hypothesisof a global heating • model.- The amplitudes are smaller than expected and smaller thanthose obtained by magnetic nucieation.

N t10 .

gauss

fig 6Blank histogram corresponds to thesuperheating curve for sample A.Shaded one corresponds to an

irradiation wish 1000 e"/s during 300sDotted one is for same intensity and 50s.

250 260

fig?Histogram corresponds to anincrease of AT=. 12Kat a fixed value Ho=258 G.

of th® field

270 gauss

Up to now we had only evidence for an integrated effect of ionisingparticle on grains 5).what we have successfully done here is toclearly isolate the effect of an electron crossing a grain , bymaking a coincidence between pulses from the diode { electron)and from the pick-up coil ( grain transition ).In order to be sure that grain transition is only due to energydeposition (AE) some checks have been done :

- The sample C has been exposed to high intensity beam.Only1% of the grains in the coil are found to be flipped , which meansthat in sample A , where 10% of the grains were flipped, at most1% could be due to a heating effect and at least 9% to true energydeposition.

8

We are now in a good position to compare the three different waysof making one grain transition, temperature increase (AT),magnetic field sweeping (AH) , or energy deposition (AE).

At a magnetic field fixed value HQ from TQ , the bath temperaturehas been slowly increased and the signals have been recorded .

After return at TQ , and then from the same value HQ the cycle hasbeen finished by sweeping the magnet and signals recorded as well( fig 7 ).The amplitudes are of the same size (fig 8). in other termsmagnetic and thermal oucleation give the same signal for asingle grain which means that the mechanism could be the sameand we refer to our study of the signal 5 ) .The number of missing grains Ng greater than the number of signalsrecorded might be a threshold effect . in some cases there is only apart of the grain transiting so signals coming out could b© toosmall to be recorded .Coincidence between several grains for an electron are ruled outbecause of low intensity beam and filling factor . As a matter offact in average one grain is seen by an electron .For the same reason efficiency is found to be greater than expectedbecause for a partial transition to occur , only a fraction ofavailable energy is needed.

A H

20ûmV./Dïv.SOOnsiDiv.

Fig.8

5.

An irradiation with a minimum ionizing particle has beenperformed with success.This experiment gave results which are still puzzling us:- The efficiency is not clearly understood.- The real transition mechanism for one grain may depend upon theway this transition is induced.- The size of the signal amplitude clearly depends on the twodifferent regimes on one hand temperature increase or magneticfield increase and on the other hand energy deposition .Combining these results with the results obtained by other typeof grains and source irradiation (alpha particles, gammas ^)) itis now possible to look for a more complete test with muonsand at least a 10g detector.

10

BEEEBEMCES

(1) A de Beliefon.P. Espigât,G.Waysand in Solar neutrinos andneutrino Astronomy, A.I.P Conf Proceedings n 126. (1984)Edited by M.L Cherry , W.A Fowler and K.Lande .

(2) A de Bellefon,P.Espigat,D.Broszkiewicz,R Bruère-Dawson in" Low temperature Detectors for neutrinos and dark matter " 'Edited by K.Pretzl.N Schmitz , L Stodolsky Springer-Verlag (1987).

P Blot , Y PeIIan Journal of Applied Physics V45 n3 (1974).

(3) A de Bellefon,P Espigat , D Broszkiewicz , R Bruère-DawsonN.I.M A277 (1989) 211-216

(4) Electronic Microscopy has been done by M r s Sauvinet inLaboratoire de Physique des Liquides et Electrochimie( University Paris 6 )

(5) Other contribution at this Conference.

(6) J.Boniface et al. in Low Temperature Detectors forNeutrinos and dark Matter 2 edited by Luis Gonzalez-Mestresand Denis Perret-Gallix.

For this work the team of the Van de Graaf accelerator is greatlyacknowledged and in particular Mr. Vida!.