The time-energy response of NaI and CsI crystals to 14 MeV neutrons

NUCLEAR INSTRUMENTS AND METHODS 32

(1965) 77--85 ; CCQ NORTH-HOLLAND PUBLISHING CO.

THE Tl[1VIE-ENERGY RESPONSE OF Nal AND CsI CRYSTALS TO 14

eV NEUTRONS

G. D. HICKMAN* and F. HEGEDÜS Swiss Federal Institute for Reactor Research (EIR) Würenlingen, Switzerland Received 7 July 1964 A series of pulsed neutron and activation measurements were performed on bismuth and lead shielded Nal and CsI crystals . The major neutron induced reactions in the shields and crystals have been identified and analyzed . Intensity ratio factors for the various long term (1 sec-4 hour) activities, as a function of the counting energy interval, have been obtained . An expression is given whereby these ratio factors can be used to calculate the

build up ofthese activities in a pulsed experiment. Thevalues wb*'Ch were obtained for the ratio of aNa23 (n,oc) to aNa23 ( n ,p) ranged between 2.1 ± 0.4 and 4.4 ± 0.9, depending on experimental conditions . These values are in fair agreement with the value of 3.3 ± 0.8 which was obtained by Bormann et al .4) . :Pulsed measurements on bismuth and lead indicate tire possibility of constructing directional fast neutron bismuth and lead counters .

1. Introduction A large number ofreaction and cross section measurements are performed which utilize a scintillation crystal such as Nal to detect gamma rays t °om the reaction. In many of these measurements the crystal is located in a field of large thermal and fast neutron fluxes. Several investigators' - 5 .13) have looked at various aspects of the response of Nal and CsI to fast and thermal neutrons, but no complete investigation of this problem has been reported . Therefore, a program was initiated at EIR. to investigate the time-energy response of various crystals and shield assemblies to the action of both fast and thermal neutrons . This reporl') describes a series of activation experiments which were performed on Nal and Csl crystals and. crystal shieldings, to determine the following : 1) The long terns activities (1 sec-4 hour) in varioas crystal-shield assemblies . This is in effect a determination of the build-up activity in the crystalshield assembly which is present in any pulsed experiment. 2) Intensity ratio factors between various activities as a function of counting energy interval. This information allows one to calculate the build-up activities for these crystals on a semi-empirical basis. 3) The best neutron-gamma shield for the crystal for a particular time interval .

counted in the crystal. The paraffin moderator was used to control the ratio of fast to thermal neutrons incident on the crystal. The general procedure was to activate for a definite

2. Experimental arrangement The source of fast neutrons was a Philips Neutron Cenerato t , of which a schematic diagram ofthe experimental arrangement is given in fig. 1. The bismuth or lead shielding around the crystal was required to prevent the gamma rays which were produced by fast neutron reactions in the neutron tribe itself from being Permanent address : Knolls Atomic Power Laboratory, General Electric Company, Schenectady, New York. ~' Loaned to the Institute by the Philips Cvà-ipany .

Kv

Fig . 1 . Schematic diagram for continuous-pulsed operation . XTAL : Nai(Tl) : 3/2" x 3/2", 3" x 3". Csl(Tl) : 3/2" x 1". d(Xtal target distance) : 10 ; 18.4; 36.5 cm. x(cm) : 0; 3,85 ; 7.70 ; 11 .55 ; 15 .40 ; 23.10.

period of time (usually 100 sec), extinguish the neutron generator and count the residual activity of the crystalshield assembly. The counts were recorded with an RCL 512 multichannel analyzer employ.ng the scale, time mode. Normally the 512 time channels were divided into two sections ; the first 256 channels at 0.5 sec/channel and the second 256 channels at 60 sec/channel . For certain measurements all 512 channels were set at 0,5 sec/ channel . A series of pulsed measurements were also made on the, Nal crystal-shield assemblies, as indicated in fig. 1 . Reference to the resull:s of these experiments will be made at appropriate places in this report . The activation experim nts were performed on two 3" x 3") and a CsI crystal (-" x I") Nal crystals (z" x in both lead and bismuth shielding. These experiments were also performed as a function of the energy interval 77

MeV, the the average The been neutrons E, Ec approximately 2Eby was were neutron neutrons scattering Lead = of experiments but Reifenschweiler') or fairl5which energy or intervals with 0measured 4 MeV) different MeV Cer" employed lead incident calculate neutron and differed awidth to 3"thin M*V MeV ") intensity which were shield xbe distribution sodium and where (4 and 3"all have layers from substantially S3Nal on energy by this Na' msecc, in this MeV incident for had slowing arose MeV), While the activities crystal de their for shown this ismodified of iscounting 8an (2 co~intin crystal Vries the in greater from lead rr the of spectrum, Cs'3' no see upper law on the down Comments E, spectrum the different as neutron MeV) attempt room It the and present = ashield, energy than was spectrum, (0 the energy 14 2cutoff properties function 1crystal MeV were Udoe) scattering being estimated D activity MoV 10" generator after was rep MeV), MeV of uncorrected experiments cutoff, HICKMAN 6spectra energy neutrons/ mst neutrons were of used modified rate made passage various In of paraffin X060 ,StsThe The that ithe for untime of c/s as to AND of metastable the MeV Fpeaks 3Bi-Pb MeV reactions be HEGEDUS Bismuth 1identification ul gamma reactions This with aray between are reactions created to are Sliver The short lead Bismuth state pul the of and state 3/, 1absorption seen halflife on which the rays relatively cross xis when metastable lead as That in the and lead from 1, try in 7experimental ofmany the bismuth Csl like which reactions ndisappear have lead neutron 4m~ 3sections 0 MeV these bombarded fig crystal result both lead, long and is Nrecy barn 2, originate ofthe"possible" and state peaks evidenced counting isalso source nhalf shows of C"+ pb`(n,2n)Pb'1' neutron as very The for simihir for identified decay respectively the ~obtainrd of paraffin rft exhibits are life with the these action short ihg threshold interval the and from the of curvt, bar (n,n'y) with slowing +xlead fast 0inmoderator result are the a?of pu the 3short This reactions raäs regard A neutrons and shield fact fast has rlisted for 0pulsed of which 55 down star lived neuw 1and that fast the c/s, in at to iis

0.

78 which counting 5 L,, .:~, ;j .14 a) b) .66 Sources (1 .17-1 .33 calibration . The given . has present which for neutrons doubtedly by bismuth naeasure investizAtorsg fast fVough ;j the was

.

. . .e.

c) d) .E,

.14

.`15

.17 .5 .6±6

.

F . ...

.

Fia. . operation neutron

. .



ure

*

width

,

t.

.

~uta :

r,

3. Analyses try The table . .

.

.

Normal exhibits can pb1a'(n,n'y)pb1o"' .7 (n,2n) 1 .6 . 14 .01 .16 measurement . . MeV . neutron the inserted Bismuth Fig. . moderator neutron pulse

. delay

.

: interval

.

.,

gamma properties . metastable trans . contrast

. . . .57

tion dons .8

.06

.

.,

.7 auusec . n . see

THE

IMF-ENERGY RESPONSE OF

Nal

AND

Csl

79

CRYSTALS

is characteristic oflead . The gamma rays which originate from this state are 0.51 and 0.92 MeV and probably also 0.64 MeV. A series of pulsed measurements, differing only in the time delay between the pulse and. counting, fig. 3, show these gamma. lines to decay with the half life characteristic of the 2.7 cosec state in bismuth. Measurements were also made with various thickness ofthe paraffin moderator . The results of these experiments given in figs 4 and 5 show that the gamma rays are the result of fast neutron reactions in bismuth. The results displayed in fig. 4 were obtained with the CsI crystal while those of fig. 5 were obtained with the large Nal crystal. The peak at 0.53 MeV which remains after the two bismuth peaks have disappeared is unexplained . A half life measurement on the line at 0 .53 MeV showed it to be less than 5 cosec. It is also interesting to compare the effect of various thicknesses of paraffin moderator on the magnitude of the gamma rays which originate from the fast reactions in lead and bismuth . The result of this comparison is presented in table 2. It is seen that the fast reactions in lead and bisTABLE 2 Attenuation factors of gamma intensities in lead and bismuth due to paraffin moderator v Paraffin Î thickness

Attenuation factors (gamma intensity at peak value minus background) Pb 1

3.8

0

7 .7 11 .5 15 .4 23 .1

0 .65±0 .10 0 .52t0.081 0 .48±0.08 ** '*

0 .70±0 .10 0.58±0 .08 0.52±0 .07 0 .36±0.06 0.13 ± 0.05

0 .54 ± 0 .05 j 0.54 ± 0.05 0.31 ± 0 .04 0.21 .05

0.31 ± 0.04 0.19 ± 0.05

* Normalized to zero paraffin thickness . ** Measurements were performed with Cd covered paraffin and large Nal crystal . The unexplained activity at 0.53 MeV prevented a determination of the 0.51 MeV Bi-activity .

muth are effected to the same degree by the paraffin moderator . This is probably an indication that the (n,2n) cross section curve for the production of V°sm is quite similar to the (n,2n) cross section curve for the production of Pb2°' m . Ruby and. Rechen12 ) used the fast neutron reaction in lead as the basis for a fast neutron counter. It appears that one could also construct a "bismuth fast neutron counter'", which in some instances might prove more valuable than the iead detector. The behavior of the intensity of the gamma rays which originate from the

t~ . Di, I~'I C YC ÂâA,I~ AN i~ ~. t~ ~Ci

CW

f~st ncdlro>?t rea~t ona, n le and I~is uth~ in~ d ~tte~ tt t it mm t , ~°ittb tho aid af Ptitrstftn ttticictae55 moderato~ such-a i n, tö cxan tr t +~i ~ nar fast o . neUtr~ l CA~Dt r . ~n eatperï ~t v~ ~crfot n~~i t~ t`~I~a~~31t th~ aet vities ~bse ~~ ~;r~~st~cl l t ~~~-~* I ur ~ ~I~ic:~~ ~~-:~ sä~eld®~ r~r th 1 tö aant: thr~t ~°c~~ ~lrieldcd w i~h i7crra ~~ti . i`h s muth ct~ntparls~~ is ~~n ica . . the ~ ~ ~s ~ c~i~) A,lx'j ~~, iu) aoti~~ti 0:92 R . o in bt~ h rt 1â , ~it the order ns4 1~. \ -~ rl~ ~ of the found ö.~ ~ t aetlfv~t~ ~ i>rt the Ihiarnuth n . ~t r~as thereft~re dee~~ded to +~ l~y bi nnuth as the shied ~ater~al %~~the ' tiou e ~irne ~+~ ~~ n~ fi Î~~~~) ',r,~ït. . ;It ~u~t ex+~rc~i he~ u ln ïther I-+ead or ~~ rr ~r~;~ or nnuth itt putx d sctren~cnt . erc n~pl bis tl utih 1`' `iI hen shied n fely u : th+~ d+~la~ t the 'i1 and is neutrcan ~ul u:n1,i lö ru . r~e ~r, it would be tter to u: 1 d ian e~ses ~ t and t is l ö ~s e~rr t he c°abser unt ror the linear build up of the I act~~ tlr, ~: :5i

3 $501°3

ii

c U

>,

.~~r

:5t~m

;

ii i ~S -

~ .~, A ~~t~~~tö~ xr~~ ~~~ i''he arnplitud~s of th+~ ~~ .u . aud srctur tiou ~F~ 4 . bismuth activity as ~ function of para~ttt moâet~tor wüh s ~, rtctivït es c~f ~~~ rn~, + ~ ~~~ ) and là~`' ~ l l ,~ ~~1~~ x t~ Csi crystal. Comments ; rep. rate ~S c%, neutron pules vvh c~h r~ere +~bt~rirt d ftn tti~~ ,~a~i~~° ti~ar~ e~l t~11^u~r~iw width ?+ ursec, cauntin~ interval 3 tits , time delay 1 m~:! . are p~° r't iu: tahl aut~ , . t~~tï~~ ~f t is a+rnlalitucle of ~° ~;ca 1Ve~ ats+~ ~iv~n itx t ~r~t~ tatbl T'he result c~f the varicau e~claeri tnertt~ ha~° n d, r co~pa ~ith t:he fcalltawwin ul~s ; n 1 . As the para thickne is inc~exrsed lr~s t'a~ na) a ti~i~ al is inore ~~~r~d, a) the ther b) the fast l~~ ° ~11 s) ; :rtd 1\le~~` ~~ a~tivritics are s) d rea. ed, c) the rati+~ of ~~~ to N~~a rernains a ec~r~stant . ~. As the lr~w cuto~ endr i ï a) the ~' a~ activït~ ~ deere .seed and is ern m~t 2.~ ï1~e~, b) the ratios of ~ ~~~1 ~:~s and Ne~~~l ~ ~~ ace i.nerea d, c) the ratio of F~c~~~ea .r is ïncreased . All of these results are in a ree~erit wïth tho that a.re e~cpeeteä on the si c caf the düfereni threshcaid ener~ies for the vaxious reactions, and an the ener i~s ~hich nre aharacteristïr of their decF~ys . ~e c~ar~w sïderations are ~ivcn heic~w . As the thickness of iaara cn is inc~reasc t~tc i`~ , hich is ~raducecl b~a as-~ ~ta,~~ç~ rcactic~r~ is ~~~~ ~t~}~i t~:~ increasc . Alsa, an ïncre sc irt thc t, ~err~~r~i u? n~t rtccn ~ar~ïed hy a dc4rcrcs~ in thc fi~.st tr . "T is r~tt~nns ti~~t the rtctivit.ies ~~af 1®~ö rt.r~d Ne~~`, hath r~~~: :.rc ~ i~~ ~n~rgyiNieV ; 1°nst r~ctr~rons, sh~~taid dect°crt~c~. .~~ïsn, it i ;~ n~~t ~~~r~~risin ~ . i-~r~~utt~ ~,~~d s~ur~c~us ~.ctivities as a fua~cti~n o~ ~~ra n t~.3 scc the rntia c)$ tt~e ttap~~ ~~cst re ct.ïc~n:~t =~;:~ ~~~~ .~ :, ~~4 ra~ar ~~t~~ ;~dr~ ~m ~°~?~rer~d ~" x htaX c°.ry°st~t . ~"z~~~~~ aa

0

as

~nc~rgY~:M~V)

N

~=ig_

~Y

!`t~cr~t5: re~~ . °~::âe .~0 c/~, ~eettr~s~ p~slse ~âcltl~

inter~~~ f ~

3 atasec,, c~urmt~~a ~e~~, ti~t~ dyls.y l ~ns~c. .

crrt: a~n~on~ thc in~fcsti b~aa~, s ar~ t~~~~ ..l~,sc~~t~t~ ~ t°~+~w

ENERGY 1t

PONSE CF

Nal AND CSI CRYSTALS

81

mush shield for neutrons, or by a change in the direct and reflected components of the neutron flux which is incident on the crystal . The activity which is observed in the crystal-shield assembly can be expressed as where 0 = ai = Ni = 1i =

tivitk*o which are observed tidt ed Na crystal,

in a

tiona f r these reactions, it appears that the energy tden (if the cross wtions for these reactions is pro ble wise similar. If this were not the case one ld expect this ratio to change as the paraffin thic nes: i ch n ire the mA imurn d y energy in 1 °" is 2.12 PAeV, the 111 s activity should dcctea~~e to a much greater Rt at than that of F11) (E1 . - .4 McV) and Ne 23 (3) 4,4 WV) as t : cutoff` energy is increased . In ition, it should not bf- possible to observe I"" with a cutoff ctrergy of 2.5 MON . Sirttc tire Maximum energy o Ne" ( .4 Me-V). is lower than that of F 21' (5.4 McV), . the 1' .4tio of l ''"/Ne 33 should increase its the cutoff energy is irrcrc ~.~:d . Fig. 7 shows the KMh .vior of a Nal cry~,t~ l tar varipu9 values of the cutoff energy. The d Yea of both the Ne 23 and 1" activities, in relation to the Fl" activity as the cutoff energy is increased, is clearly displayed . The curves have been normalized to the total ial activity at t = 0. The ratio of F"fNe 23 for a cutoff of 0.14 MeV, no rafbn moderator, and 5.5 errs of bismuth shielding is found to 2.2 0.4 for the large crystal at a distance of 16.4 cm. This same ratio for the small crystal is determined as 2.1 ± 0.4, 4.4 0.9 and 2.8 ± 0.9 at distances of 10, 18.4 and 36.5 cm respectively . Therefore, the ratio of F°/Ne23 is seen to fairly indel ndent of the distance tw(. n the neutron source and crystal. The only exception to this conclusion is the result at 18 .4 cm for the small crystal. The exact nature of thi.s disagreement is unknown . However, such it behavior could cxplairted by a change in the neus ctrunr as a function of the geometrical arrangernent, coupled %with different cross section curves for the (n,p) wtd (n ® ~) reactions . The spectral change could efeated fey a change in the elective thickness of 'bis-

i

q5(E)ei(E)N,(1 --

i--(n,z ) .(n .P),(n .Y) . . . .

e -,") C A " dE

neutron flux incident on the crystal cross section for the iP' reaction number of atoms available for the i" reaction decay constant for decay of isotope which results from the i`s reaction t =-. activation time 0 = delay time before counting. This expression, after subtracting the activities due to 1 1 " and A1 27 , reduces to eq. (2) _ ONNa ( ON.(n, p) (I - e -~ Na(n,P)t) e - "Nat n,P)G + e-'Na(n'a)') e -dN a (n,a)0 1 (2) + 07NA 11 , «) ( 1 ® where it has been assumed that q5(E) and a(E) are constant for the fast reactions . This expression at saturation value for 0 = 0 reduces to Ana, . = 0NNa { QNa(n, 13) -+.'

t7Na(n, OC) 1 -

Therefore, the ratio of the saturation activities of the (n,p) and (n,a) rea(tions, is seen to be given by the ratio of the cross sections for the production of these two activities . Bormann et al .') measured the cross sections for the individual reactions as a(Na23(n, p)Ne 23) = 9 ± 4 rnb a(Na"(n, a)F2°) ---- 29 ± 9 mb which yields a value of

aNa23(n, a) ! aNa 23 (n, p) = 3.3 ± 0.8. This value is in agreement with the values which are obtained in the present set of experiments . Unfortunately the wide spread in our results (2.1--4.4), prevents us from making more definite statements about the "goodness" of the data of Bormann et al. Kaul 3) and Bizetti et al.2) measured the cross section for the (n,a) reaction in Na 23 as 1,.35 mb ± 15 and 147 ± 18 mb respectively. The large discrepancy between these values and that of Bormann et A'4) has not been resolved. A short series of activation experiments were also performed on dt Csi crystal . A typical decay curve analysis, along with a similar curve fear Nal, is shown in fig. 8 . These curves have not been normalized, but

G, D. HICKNIAh AND P. HEGEDÜS Te4Jztiki ~'' Activities oP - served n J " x 1 Comn ents

..-.~.

Catoff enav ( e l

"vation time ~ (sec)

Paraffin

..14

105

nom

0.14

35

none

0.14

105

4

0.66

105

5

1 .17

105

6

2.5

105

8

Crystal . ', ~!1 fiuu~kL g

10

1

none none

s

none

1

none none

2.5

305

none

0.14

a

105 105 1415

12

0.14

105

3

0.14

105

14

2:.5

15

0.14

36

0.14

17

105 1Yi 105

236 -t 4* 4860 ± S0t 92t4 56.60 :t 240 41t} 10 8M 200 194 4 4000 ± RO 75±3 155 t 60

10cm Pb + Cd 5.5 cm Bi .i- Cd

35

0.14

Act 25 min

~~ Cd 5.5 cm Bi

2.5

014

Activ Yies of various companents at t

Crystal located 10` rcn from neutron tube

nonc

( _

3.85 cm in Cd 7.7 c m i 1.11 I d 11 .3 cm iq Cd ; 110ne

5.5 cm Bi -F- Cd

5.5 cm Bi Cd 5.5 cm Bi

1

± ± t t ± f ± ± ± i t

80 80 60 70 60 60 80 80 50 50 60 60 20.

696

100

2.5

0.5

816

120

2.1

0.4

340 ± 70

5.2 ± 1 .0

724

- 140

2,7 0.5

504

80

2.2 ± 0.6

590

100

1 .9 i 0.5

600

100

3.1

705

80

2.6 f 0.4

0.5

480 r 50

3.3

560 ± 50

2.8 ± 0.4

136 ± 20

6.4

1.8

1l ± 24

5.5

1.5

± 0.5

84 ± 5

9,1 i 0.8

854 ± 22

178 ± 10

4.8 ± 0.4

214 t 20

5.5 t 0.7

--f- Cd

1180 ± 20

214 :t 20

5.3 ± 0.7

cated 1-8.4 cm from neutron tube

.15.5 cm Bi + Cd :5 .5 cm Bi + Cd 5.5 cm Bi

126 ± 2*

940 ± 20

180 ± 30

5.2 ± 1.0

2600 ± 40t

940 ± 20

210 ± 34

4.4 -r 0.9

119 ± 3

680 ± 26

168

16

4.0

0.8

2460 ± 60

680 j 26

196

36

3.4

0.7

544

20

108

20

5.0

1.3

544

20

126

24

4.3

1 .1

87±3

420 ± 10

100

16

4_2

0.9

1784 ± 60

420 ± 10

118

20

3.6

0.8

920

146

30

6.3 :

1 .8

90t3 1872 ± 60

+ Cd 5.5 cm Bi + Cd 5.5 cm Bi

168 ± 4

5.5 cm Bi

7.7 cm

Act 38 sec

1180 f 20

172 ± 34

354 ± 10

41+8

8.6

2.0

354 ± 10

48

10

7.3

1 .7

700 ± 30

132 ± 20

5.3

1.2

4260 ± 1+00

700 ± 30

154 ± 24

4.5 ± 1.0

185 ± 5

630 ± 24

98 ± 18

6.4 ± 1,7

630 ± 24

114+22

180 i 6

18±3

10.2 ± 2,5

180 ± 6

21 ±4

8.7 ± 2.0

474 ± 20

80

14

5.9

1.5

94

16

5.0

1 .3

207 ± 5

5.5 cm Bi

3820 ± 100

15

105

7.7 cm

1

5.5 cm B

0.14

105

11 .5 cnn f

5.5 can .li

30

920 ± 30

5.5 cm Bi

3.85 cm

11 sec

5.5 cm Bi

3480 ± 80

none

._._. . Act

_ 0 '~

Act Act 11 sec38 sec ~. __ .__ .

1760 1760 1760 1980 1120 1120 1840 1840 16 .10 1610 900 900 760

+- Cd

Crystal l 9

_

Nal crysta

_

---

180 ± 5

3750 ± 100

474 ± 20

5.4 ± 1 .5

5.5 ± 1 .4

Crystal located 36.5 cm from neutron tube 19

É

20 21

4

0.14 0 .1-4 0.14

105 105 105

676

6~

none nore à 1 .5

cin

in Cd

? i

5 .5 cm Bi + Cd

5 .5 cm Bi + Cd 5.5

cm Bi

+ Cd

"fhe PA number u :ac~i squarc is the measured activity. e d il, aber i. each s t.Rare iç the -aturation activity.

i

'

48 ± 2* 986 ± 32t

320 ± 20 320 ± 20

51 ± 2 1052- 36 33

f

2

-f- 40

300 ± ; t 300 16 1

98±20

1

114+24 84 ± 16

3.

98±20

150 ± 10

31

± 10

150 ± 10

36±12

33±1 .0 2.8±0 .9

3.1 ±

±0.7

.9+ 2.5 4.2±2.0

THE TIME-ENERGY RESPONSE OF Nal AND CSI CRYSTAL

83

TABLE 4 Activities observed in 3" x 3" Nal crystal located 18 1 cm from neutron tube Comments

Cutoff energy (MeV) 22

f ~ ~

0.14

ctivation Palrrffrtx 1 time 1 moderator , (sec) _~ ._ . 105

nonc

23

105

Donc

24

105

none

25

105

none

26

305

noce

Crystal .11; 114 .ng

5 .5 cm + Cd 5 .5 cm + Cd 5 .5 cm + Cd 5 .5 cm + Cd 5 .5 cm + Cd

Bi Bi Bi Bi Bi

Activities of various components at t = 0

Act 11 sec

Act 25 min

Act 38 sec

Aret 38 :aec

1360+ 160 1594 -t . 200 1220 ± 240 1430 280 1140±200 1336 240, 480 ± 60 562+ 70 443 _t 80 441+80

2 .6 ± 0 .5 2 .2 ± 0 .4 2 .6 t 0 .8 2 .2 ; 0 .7 2 .7±0 .7 2 .3 ± 0 .6 5-0 v 0 .9 4 .3 ± 0 .8 3 .9 ± 1 .1 3 .9 ± 1 .1

640 ,3200 410 8460 144 2960

± 20* S00t ± 20 ± 400 ± 10 ± 2W

Act 11 sec

1

3580 ± 150 3580 ± 160 3160+200 3160 - 200 3060 ± 2W 30b0 00 2420 ~- 60 2420 ± 6&' 1720 ± 900 1720 ± 100

4

* The first number in each square is the measured activity . t The second number in eirch square is the saturation activity.

were plotted directly after subtraction of background, aluminum and iodine activities. The activity which is observed in the CsI crystal is substantially less than that which is observed in the Nai crystal. However, a short lived activity in the order of 12 sec is found in the Csi crystal activation. Thi: activity is thought to be due to the Mg24(n,2n) Mg 23 11 A123 reaction. This reaction is possible as thin layer of MgO was used around the crystal as a reflector . Another reaction of 2.3 min was observed in both the Nai and CsI crystal

Fig.

7. Decay of Nat

foi

various low

cutoff

and is thought to arise from either one or bc~h of the following reactions Alj'(n, y)A1 2a p)A128 Si2s(n,

__.+ si2s

2 .3 min

The aluminum is present in the canning ofthe crystal, while the silicon is in the glass of the sealed "neutron tube" itself. Experiments to determine the major contributor of the observed activity were not conclusive .

energies. Crystal activation : 14 MeV neutrons,

MIeV) where Ev = 0 .14, 0 .66, 1 .17, 2 .5 MeV .

for

100 sec, Energy

interval : (Fe < E < 5

G ` ? . HICI~M~1,I~ AND F. HE~rEDtUS

Fig. 8 . Comparison of NaI ar~d CsI crystal decays (normalized to h~$ X25 min) atctivities) .

®

or pvxsED ~Etr3r~oN ExrT~iu~t~rra~s he pa~~~ble~n of interest is that of converting the results o:f the activation experiments to pv~lsed tnea,sureg nts . ~Chis can t~; accomplished by analyzing the ~~ctivity ~a~rhich would be produced by a series of pulses as a function of pulse width, frequency, delay and ct~a~xnting; intervals. 'l`hhis i:~ ~depict~d in fg. 9. °tee f~cnillowing ¬~eantities are used in the pulsed n~:utron ~~ra~alysis. = t~~~dutron flux (netarons/cn1Z ~ sec) during a burst (c~su~ne constant) l:urst frequency try = cr4V u~ron burst width (sec) ~~ _~ ~ticne delay between end of burst and beginning ~e~f counting interval (sec) t~ _ :c.~~unting interval (sec) t = aE°'lapsed clockïixne (Ç~~c) r~,; _ ,~~~ ctivation cross section (cmz) for the i`n reac~t ion (assume energy dependent) _ :r~ttaanber of atoms available far the i` h reaction `~. _ ~clecay constant ~f the i` n ruction, (sec`1). ~ .~oA~°~oN

~c ~.

~P

=

The activity which is produced, due to the i`n reactior±, from a neutron burst can be expressed as ~~

p

~' Q~ ` ~~( ~

~

e

~~+r

")~

The number of counts (from the i`" reaction) which are measured after the rrst burst during toj are c~) while, the numt~er ofcounts that are me~~au.red after the n`n pulse during t4 ïs easily seen to be where

_ d ~'~ = B { 1 + e x + . . . . . . ~ e ~` t°~ ° ~ ~x }

(~)

n .~ tv x _ ~c/v B ~

~ 1 -- e_z``° ~ ~,~

e ._~~tA

'

The accamu .lated counts after time ~ are obtained by sunttr ing the counts from all counting intervals, i.e. ~T r = ~ {rt~-(n--1)e`°~`-~t- . . .+ [~~~-(n .~-1).~eT~»~'>x~ (7)

u

and ha~ally

~t b

._.__.._.. _..~ .___ . . ___ . ._.~.~_~ ,~`~. __ ~ B Cg) o~ L,i~Q~-~~ ~~~~ ~~t~~tir~ titre

¬;t~~r~scs~i~.ti¬~~

icii~rval~ .

THE TIME-ENERGY RESPONSE OF Nal AND CSI CRYSTALS

Eq. (9) can be used to calculate the build up activities of Ne23, FZe, Ah' and I"' in a Nal crystal during a pulsed measurement . The quantity B in equation (9) can be obtained from the activation experiments in the following manner. From an activation experiment the activity of each isotope at 0 = 0 can be obtained . (10) A®. o = 0 - aiNi(l - e-À'`). This yields e-d `) O'orl-Ni -. Ae=o/(1 The quantities 0,ai and Ni are the same which appear in equation (4) for the activity produced during a neutron burst and enables one to calculate Ap. Once Ap is known the quantity B can also be determined . 4. Conclusions The results of these activation and pulsed experiments should be most helpful to those who plan to use a crystal such as Nal or CsI in either pulsed or continuous fast or thermal neutron beams. The exact numbers which are given in this report are of no great concern as the exact magnitudes of the various radioactive components must be determined in the same geometry and under the same conditions in which the crystal is to be used. The results of these experiments can be summarized as follows 1 . The fast neutron induced reactions in Na (11 s and 38 s) and the 20 msec metastable state in Na" are avoided by using CsT . 2. No short lived activities in Csi have been identified, which can be attributed to Csl. 3. A short lived activity (- 12 sec) has been observed in Csx but it is thought to originate in Mg, as the crystal is covered with a MgO reflector. This point should be investigated in greater detail. 4. By performing activation experiments one is able to obtain intensity ratio factors which can be used to describe analytically the activity which is produced in the Nal crystals . 5. If one performs pulsed and activation experiments in the same geometry and under the same conditions, the result of the activation experiments can be used to

85

calcuhite the buildup activities which arise from the Na and I activities . 6. It is found that it is possible with the proper choice of gamma and neutron shields, to use Dither Nal or CsI crystals in various neutron experiments . Lead and bismuth shields have been investigated with the result that both find importance as shields, but the proper choice of shield is very dependent on the time region of interest . 7. There is good evidence for believing that the fast reaction in bismuth producing the 2.7 msec state, could be used to make a "bismuth fast counter" . The authors would like to thank Mr. G. Moser of tin Philips Corporation for making possible the loan of the Philips Neutron Generator . Appreciation is also given to Drs. R. Vernon and T. Auerbach for recognising the solution to equation (7). We wish to thank Dr. J . Brunner for helpful discussions, and Mr. A. Biichli for solving the electronic problems of the multichannel analyser: Finally one of the authors (G.D.H) would like to express his sincere appreciation to Dr. R . Meier and the Swiss Government for making possible his year's appointment at EI R. References ') M. A. Grace, H . R. Lemmer and H. Halban, Proc. Phys. Soc ., 65 (1952) A.

2) P. C. Bi=ti, A. M . Bizzeti-Song and M. Boccioiini, Nucl. 3)

a) s) 6) 7) 8) 9) 10) 11) a?) 13)

Phys., 36 (1962) 38. O. N. Kaul, Nucl . Phys., 33 (1962) 177. M . Bormann, H . Jeremie, G . Andersson-Lindstrom, H. Neuert and H. Pollehn, Z. für Naturf., 15 (la) (1960) 200 . W. R . Dixon, Nucl. Phys., 42 (1963) 27. G. D. Hickman and F. Hegedizs, EIR-Bericht Nr. 59 (1963). O. Reifenschweiler, Philips Res . Report, 16 (1961) 401 . L. J. De Vries and F. Udo, Nucl . Instr . and Meth., 13 (1961) 153 . Yu. S. Zamyatnin, N. I. Ivanova and I. N. Safina, J . Nucl. Energy part A : Reactor Science, 12 (1960) 83. Yu. A. Vasilev, Yu. S. Zamyatnin, P. V. Toropov and E. F. Fomushikin, J. Nucl . Energy II, 9 (1959) 43 . Yu. S. Zamyatnin, E. K. Gutnikova, N. I. Ivanova and I. N. Safina, J. Nucl. Energy II, 9 (1959) 41 . L. Ruby and J. B. Rechen, Nucl . Instr. and Meth., 15 ~ 1962) 74. L. C. Thompson, Nucl. Instr. and Meth., 25 (1964) 333 .