-L_ A new technique for thermal neutron detection using pyroelectric ceramics

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Nuclear Instruments and M~thods in Physics Research A311 ( I q 9 2 ) 558-562 NL~rth41ollimd

. . . i i , ,

NUCLEAR INSTRUMENTS

i METHODS IN PHYSICS RESEARCH

S ~ J ! o n A

i i , llll

A new technique for thermal neutron detection using pyroelectric ceramics

S.B. Crestana ~ and S. Mascarcnhas :

Itlstituto tte Ffsi('a e Qlu'.ttea de S~o ('czr/(x~, IFQSC-USP, CP 369, 13560, S~o Carlos' (SP), Bra:tl

L.P, Geraldo

Instttuto tie Pt'sqlltsas Energt~tlcas e Nu('lean, s, IPEN.CNEN [ SP, CP !10,0, Pitlhetro~, 054¢)t~, S~o Paulo (SP), Brtml

A. De Carvalho

Unl~'er~'lehtde Estadual de S~o Paulo, CP 31, Ilha Solt(,ml (SP), Brazil Received t) August It)91

in this article a new technique tk~r thermal neutn}n detection using pyroelcctric ceramics is described. The detector system is basically constituted of a PZT (lead zirconate titanate) ceramic attached to an uranium disk. The energy released in the uranium tlssion gives rise to an electrical signal in the detector which is amplified by a Iock-m system, The neutron beam impinging on the uranium disk was modulated with a cadmium chopper, Thermal neutron fluxes within the interval of 10 a to 10 6 n / c m : s have been detected using a U~O s pellet with 20% enrichment m :a~O.

1. Introduction

PZT (lead ztrconate titanate) ceramics have bccn successfully used for photothcrmai detection by sevcral authors [1-3]. Thc pyroelcctrlc effect presented by these material allows the detection of thermal fluxes of low intensities at room temperatures [4].

t Present address: COPESP/1PEN, CP 11253, 05593, S~o Paulo (SP), Brazil.

: Present address. EMBRAPA, NPDIA, CP 741, 13560, $5o Carlos (SP), Brazil.

More recently, besides crystals and ceramics, thin films such as polyvmylidenc difluonde ( P V F , ) have also been used as photopyroclcctric detectors [5,6].

A pyroclectric detector is basically constituted of a thin layer of pyroelectric ceramic w~th the electrode normally onented with relation to its polarization vec- tor. Thus, it is a thermal transducer as well as a capacitive element. T h e pyroelectric effect is basically a spontaneous induction of polarization due to a change of temperature in the ceramic. On the other hand, a change of polanzation is equivalent to a source of voltage V, in series with the ceramx capacitance C as

Fig 1

~ SILVER ELECTROOE

. . . i ; i ~ - . . .

~ b - o r

HEAT

I PZT

)

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Electrical configuration of pyroelectnc detectors with area A, side b, thickness d and capacitance C.

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is skctchcd in fig. 1. The signal V,, is very low CpVI and in order to scparatc it from the background, produced mainly by tcmpcrsturc, acoustic and Johnson noise.

the mc;r>urcmcnts have to bc pcrformcd in a \,yn- chmnou\ form. A pha\c modulattc~n tcchmquc cmploy- ing iI I&.-in ,Implificr a11d it chopper jystcm 14 u~a11y cmploycd.

A change in the power lcvcl of the radiation inci- dent on the ceramic produces a change of tcmpcraturc AT, and a charge diffcrcncc of magnitude Q appears across the ceramic clcctrodcs [7] with

Q- PAAT. (1)

whcrd, A is the arca of ant clcctmdc. and P i\ the pyroclcctrlc cocfficicnt in C/m’ K.

If the PZT ceramic I\ conncctcd to a htgh Im~di~~cc load [7]. for example a b&-m ~unphftcr, II HC’IX JI a VOltit& \ourcc F. WI h

I“ = ( P /pc ,oCd ) I$‘/. (‘31

bhcrc rhc \ynihol4 ,ux

P - density of the pymclcctric material, CP - specific heat of the ceramic, w - modulation frcqucncy, C- capacitance of the ceramic.

d - clcctmdc spacing, and w - mndulatcd power.

According to cq. (2). the tcchniquc pmvidcs maxi- mum scnsitlvity for mput power modulated at low frcqucncics.

0 ^A ^BRAPHITL COLLIMATORS

0 B ^Il¶YUTH ^FILTER (20 es I 1 MATERIALS 1

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LLAD COLLIMATORS

ALIJIYIINUN FLAWOE pJ CORCRLT

PRINCIPAL COLLINATORS a WATEB

SECONDAlly CDLLIYATORS @RAPHtTE

CHOPPER POSITION IBlSllUTH

DETECTOR POSITION ifa LEAD

ADDITIOWAL WATER SHIELOIRO (20 am 1 fzJ BORATE0 PARAFFIN

Fig 2 Eqxrlmentai arrangement for neutron beam colhmatron at the TEA-RI reactor.

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560 S.tt. Ctl'$tlltl(I ( ' t t d . / N('l~t~)~l d(,tt'¢tlon IQ$1tlg pynn,lectrk' t'etxOlllt'.~

T H E R M A L N E U T R O N S --~

M E C H A N I C A L C H O P P E R

R E F E R E N C E , , [ ,

I - L O C K - I N A M P L I F I E R

. . . U R A N I U N : " P Z T

[ P Y R O E L E C T R I C

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I X-Y RECORDER

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Fig. 3. Dtagramofthe neutron detectorsystcm.

S A M P L E

This type of detector system has already been used tbr X-ray [8,91, -t-ray [10] and ion b e a m [7] flux measurements, In this present article a similar device was used to detect thermal neutrons via the (n, f) reaction in :'~'~U, An uranium pellet attached to the P Z T ceramic produced the modulated power which is related to the thermal neutron flux by the following relation- ship [11-13]:

W = @ a N . E (3)

where,

@ - thermal neutron flux, cr - fission cross section for -~SU, N - n u m b c r of ~35U nuclei, and

E - energy released m one fission event.

2. Experimental procedures

The irradiations have been carried out at a beam tube (BH-8) of the IEA-R1, 2 MW pool type research reactor. The experimental a r r a n g e m e n t used for neutron b e a m collimation is presented in fig. 2. The thermal neutron flux and the cadmium ratio at the position H have been measured by the activation technique with gold foils and the results obtained were 8.5 × 105 n / c m 2 s and 150 respectwely.

At the end of the collimation assembly (position G) there is a c a d m i u m stopper which allows to interrupt the thermal neutron beam when it is necessary. A mechamcal c h o p p e r installed m front of this beam stopper has the function of modulating the neutron beam. This c h o p p e r has been built with cadmium foil of 1 mm thickness. It is driven by a small motor which has the rotation frequency controlled by a Variac placed outside of the shielding shown m fig. 2. The chopper rotation is monitored by a photodiode system which sends a square wave as a reference signal to the lock-m amplifier.

The detector system is positioned in front of the c h o p p e r by means of a small cart which is moved on an aluminium support through a pulley system. T h e cart basis has been built with a special plastic cushion in o r d e r to isolate the detector against mechanical vibra- tions.

O u r detector is constituted of a square P Z T 4 ceramic 2 em long and 3 m m thick ( E d o n - W e s t e r n Corp.) attached to a uranium (U~O s) pellet with 14 m m diameter and 2 m m thickness. A good coupling between ceramic and u r a n m m has been obtained by using an Apiezon-L type grease. The silver electrodes of the pyroelectrie detector were connected to a lock-in amplifier through coaxial cables as is sketched in fig. 3.

3. Results and discussion

The detector response for a thermal n e u t r o n flux of about 8.5 × 10 ~ n / c m 2 s has been studied employing

I o 0 0 -

~'~ 800.

, . j ~ ~ ~ _ _ 7 5 0 u V

~ - - - ~ 7 ~ - " ~ ' ~ " ~ "730uV 0

o

~

<

z 600 I

w 400

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C> 2 0 0

" . | i I ) | i I ~ | i ! i ! i | i

0 2 4 6 8 10

EXPOSITION TIME (min)

F~g 4. The pyroelect1~c detector response as a function of exposition hme. (1) and (3) detector background signal, (2) with neutron flux = 8 5 × 10 s n/cm 2 s, modulation frequency

= 3.0 Hz.

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S.B. ('re,~tana ct u L / Neutron detectton u~mg pt'rt~'le< tn{- cermnt{ ~ ~ 1

an uranium (U~OM) pellet converter of 1.694 g with 211~ enrichment in -'~U isotope. A~ can be seen in fig.

4 the signal generated by the detector wa,, 7411 + 11) p.V Ior a m o d u l a t , m frequency el 3 Ih,. On the other hand, wroth the thermal qcutron tlux interrupted b~ the c a d m m m stopper, the detector background signal ~a~

only 2 IxV at the same modulation frequency. T h e s e results show that, for monitoration of thermal neutron fluxes with intensities >_ 1 0 ' n/cm-" s, converters of natural uranium could be adequately used. According to the signal fluctuation (_+ 111 p.V) obtained in thi~

experiment, one can say that with the present detector system, a variation of only -- I.Se~ in thermal n e u t n m fluxes would be observed.

Fig. 4 sho~,'s the detector signal recorded during approximately 10 min of observation, Measurements p e r f o r m e d during all the reactor operation period ( --- 8 h) did not show any significant variation of the detector response for a constant n e u t r o n flux, when the P Z T ceramic produced by E d o - W e s t e r n Corp. was u~ed.

However, for P Z T ceramics from other manufacturers a slow and continuous decay of the signal has been observed m some cases during the neutron exposition.

T h e detector response for different thermal neutron fluxes has been experimentally studied and the results are shown in fig. 5. Neutron fluxes of several intensities were obtained using foils (200 ;xm thickness) of Makro- fol-E ( C I ~ , H I 4 0 ~ , p = 1.21 g / c m ~) to attenuate the neutron b e a m before ~mpmging on the detector system. Measurements of neutron fluxc~ ha~c been per- formed x~th gold rods for two particular situahons, one w~thout attenuation and the othcrxv~th 3 mm thickness of MakrofoI-E. The results obtained were 8 5 × 1(} ~ and 4.2 × 10 ~ n / c m -~ s respcctwely which are m excel-

1 0 0 0 -

%-

1 0 0 t . . . I . . . I . . . I . . . ,

0 4 8 12 16

THICKNESS OF MAKROFOL-E (ram) Ftg 5 Pyroelectnc signal amphtude as a function of Makro- fol-E thickness Detector made wtth U120%), at a modulatton

frequency of 3.0 }tz

1000-

800'

600.

~o:

2002

16o' . . . o . , . , & i , . ;do'

1/,, (m.)

Ftg 6 Pyroelectnc ,,ignal amphtude a,, a functmon of inver~¢ of

the chopper modulation frequency

lent a g r e e m e n t with the cxpertmcntal data presented in fig. 5. O n e can also observe in this figurc that the dctcctor response for different thickness of MakrofoI-E follows the exprcssion:

~ = q~oe "' (4)

as expected from the theoretical model pred~ct.ons [14] In thl,, equation, q) is thc ncutron flux observed after the neutron beam ha,', passed through a thtckncs', of a material ~ t h an ab~orpt,m coefficient ~ and ~ o

ts the neutron flux incident.

Mca,,urements have also been made using different modulation frequencies and the results obtained arc shown m fig. 6. As can bc sccn m th~s figure, the experimental data points are in excellent agreement with the theoretical predictions expressed by eq. (2).

According to this equation and theoretical model pro- posed by Liu and Donald [4], and Rosencwaig [3], the s~gnal g e n e r a t e d by a pyroelectric detector should be inversely proportional to the modulation frequency of the chopper.

4. Conclusions

The deiector system developed in this ~ork has shown to be convement and adequate for m o m t o r m g thermal neutron fluxes w~thm the interval of 10 ~ to 10 ~' n / c m 2 s. Much higher neutron fluxes can be measured by changing the modulation frequency., using a converter of natural uramum or by means of beam attenuation w~th appropriate materials.

Th~s technique of thermal n e u t r o n detection may also bc an ~mportant tool for other areas of the nuclear technology,, such as: measurements of Z35U enrichment

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562 S.B. Crestana e~ aL / M'utnm det(,('tion u,~'ing pynJelectric ¢~emmics

in uranium samplgs, m~asurcments of the gnergy released in nuclear fission, study of thermal parameters of nuclc~r fuels, t'~r example thermal diffusivity0 etc.

Besides fission other nuclear reactions may eventually be also studied with the present technique.

Acknowledsements

Th~ authors arc indebted to CNPQ, EMBRAPA, CNEN and COPESP for financial support of this re- :;zarch, They also wish to thank Dr. R, Pugliesi and Dr.

A, Mirage from IPEN and Dr. M,H. dc Pau|a from MS University for many helpful discussions.

References

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[4] S.T. Lia and D. Long, Proc. IEEE 66(1978) p. 14.

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1 i (1984) 73.

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Eng. 89 (1985) 150.

[13] R. Vandenbosch and J.R. Huizenga, Nuclear Fission (Academic Press, New York, 1973). p. 329.

[14] P. Morrison and B.T. Feld, The Neutron - Experimental Nuclear Physics V I1 (Wiley, New York, 1953) p. 279.