Response functions of spherically moderated neutron detectors

NUCLEAR

INSTRUMENTS

AND

METHODS

169 ( 1 9 8 0 )

115-120;

(~) N O R T H - H O L L A N D

PUBLISHING

CO.

RESPONSE FUNCTIONS OF SPHERICALLY MODERATED NEUTRON DETECTORS M . P . D H A I R Y A W A N , P. S. N A G A R A J A N and G. V E N K A T A R A M A N

Division of Radiological Protection, Bhabha Atomic Research Centre, Bombay-400 085, India Received 11 June 1979 and in revised form 2 October 1979 This paper presents the results of Monte Carlo calculations on the response functions of spherical moderator - detector s y s t e m s as a function of neutron energy in the energy range from 1 0 - 7 - 1 4 M e V . The diameter of the polyethylene moderator varied in the range of 2 inch to 12 inches and the central thermal neutron detectors considered were LiI scintillators and BF 3 counters of varying diameters. T h e best microscopic cross section data available in the literature was used for all the constituents o f the moderator and of the detectors. Anisotropy of neutron scattering was taken into account. Variations in the neutron response due to change of thermal neutron detector size or the detector material itself as well as due to the presence of an annular air gap surrounding the thermal neutron detector were studied. T h e air gap reduces the sensitivity of the system to neutrons but the nature of the energy response curve is not affected. C o m p u t e d response functions of the rem counter devised by J. W. Leake of Harwell are also presented.

1. Introduction Neutron energy response characteristics of detectors making use of a spherical polythene moderator ;and a central thermal neutron detector had been the :subject of study for the last twenty years~-6). The moderating spheres had diameters ranging from 2 " - 1 2 " . The central thermal neutron detector was ,either a cylindrical 6LiI scintillator, plastic or glass :scintillator containing boron or lithium or I°BF3 or 3He gas fillled proportional counters. These detectors ranged from being perfectly black to being reasonably transparent to thermal neutrons. In addition to these simple systems we have a class of instruments with a perforated cadmium shell in the flesh of the moderator used for neutron monitoring purposes, which are supposed to have a satisfactory energy dependence over a range of neutron energies. The response functions reported so far have fallen broadly between the two main contenders for acceptability viz., the tabulated values of Burrus 7) based on the experimental values of Bramblett et al. 8) and the theoretical results obtained for a 10" sphere by Hansen and Sandmeir 9) and results given by others for spheres of radii other than 10" by using the method developed by Hansen and Sandmeir.

2. Work done In the present work the response of neutron detectors using spherical moderators for a broad parallel beam of monoenergetic neutrons was calculated using a Monte Carlo code developed for this purpose. The most recent cross section data avail-

able was used for the components of polythene and the central thermal neutron detector. Anisotropy of neutron scattering was taken into account using the data available in the compilation of Garber et al.l°). The response per unit fluence at any energy is the number of In, ~) or In, p) events for a broad parallel beam of unit fluence incident on the spherical moderator. Neutrons were tracked through the moderator material and the point energy cross section data was used. Neutrons of energy below 0.1 eV were classified as thermal neutrons and the 0.025 eV cross section was utilized for their subsequent tracking. 5000 histories were followed at each energy. The overall inaccuracies in the calculation including the error in the neutron data and statistics is about _+20%. Details of the calculation are to be published as a report of this Research Centre.

3. Results and discussion Figures 1 and 2 show the results of the present calculations for a 0.245 cm radius 6LiI scintillator kept in the centre of polythene spheres of various sizes. The density of the scintillator is taken as 4.061 g cm 3 and the 6Li enrichment as 100%. This scintillator size is equivalent to a cylindrical scintillator of size 4 mm diameter×4 mm height. Also shown in the figure for comparison are the results of various authors7'lt-13). In the intermediate energy region our results agree with the values of Wetzel et al.ta). The results of Wetzel et al., agree with a calibration done by Harrison et ai.~4). On the whole the present results are nearer to those obtained using Hansen and Sandmeir calculations. It must be remembered that the results

M.P. DHAIRYAWANet al.

116 0.3

0

=

I--~... . ^ ^ A

~ .-/

2

4

I I '

I I ,

o 0.3

_,,~"

J

I I i

T

,

O') z

0

o

I

II

I

II

WORK

I

I

J I- "~[-'J-q':'~<~-

[

I

I

I

J

I

t

I

I

"

-

]

I

A

z

0.3

PRESENT ~- .,..,.'~..,.~_._.~

i 1 t i

/..t-----f-~.t.j

Wl=

~"'~,,

.....

i I ~

~

IZ

""

-- ,...~.

lit

------K.JBRIEN et al ( 1 9 6 5 ) V BOOT ( 1 9 7 8 ) ...... BURRUS (1962) A W E T Z E L ct al ( 1 9 7 0 )

" ~ . ~ A

="~ ....

1 I

~ A

A

0.1

,111

[

w

I

I

I

I

J

I

I

I

I

J

I

I

J

, 1 I

I

I

J

,

I

, "-"

1

I

J

I

I

l

......

'tlDj A =

I

11

,

r

J

I I

]

11

.2

I

1

l

=

=

1J

Z---

I

------

-

-

=

1

~

-

-% 5" D I A 0

I -7 10

Fig. l. Response

,

I -6 10

i

"~ ,

I -5 10

,

,

I -4 10

J l

I -3 10

~

I

J

J

-2 10

I

-1 10

NEUTRON ENERGY ( McV ) of a 0.49 cm diameter Lil scintillator inside polythene

quoted in figs. 1 and 2 are for the case of the scintillator being in perfect contact with the moderator i.e., no annular air gap is present between the scintillator and the moderator. It is known that the earlier calculations of Hansen and Sandmeir 9) allowed for an annular gap of v a c u u m of 7 m m thickness around the scintillator. The thickness of the air gap in the experiments or calculations of other authors is not known. Figure 3 shows the results of our calculations for a 2" diameter BF3 counter in moderating spheres of diameters 3", 5", 8", 10" and 12". Also shown for comparison are the results of Maerkar et al.2). Mostly our results are less than those quoted by

I

|

I

I

1

I

,

I 10

spheres of different diameters.

Maerker et al. The differences are attributed mostly to the energy limit below which neutrons are taken to be thermal. Whereas we considered a neutron thermal if its energy fell below 0.1 eV, Maerker et al., had used a value of 10 eV for this purpose. But in the case of the three inch diameter polythene sphere we find that our results are higher than those of the other authors. The effect of scintillator size on response was also studied. From figs. 4 and 5 it is seen that the response curves for a single moderating sphere with different sizes of LiI crystals (viz., 0.49 cm, 0.98 cm and 2 . 0 0 c m diameters) are parallel to each other• The responses are nearly proportional to the surface

NEUTRON DETECTORS 0.3

. . . . .

1

----

K, Ot BRIEN

....

BURRUS

0.2

'

'

I

'

'

I

~

'

'1

~

'

0.1

'

'

'

'

i

//

//

.... o_-

I

,

i

[

,

'

'

I

'

i

0.2

o.

10

.......

"A 'A

/"

',~,

7

I

,

,

I

=

i

I

,

,I

I

'

'

I

'

'

t

,

'

INCH

I 1

0

I

'

, '

. . . . . . . . . . 18 ?

16 6

'

]

'

'

I

i

I

'

p4"-'l-"-r='P'-~ 1l~ 5

i

of a0.49

cm

diameter

,

I

".-,

11~/' NEUTRON

2. R e s p o n s e

I

\

/

\

i

,

,

I

i

,

I

i

'

i

'

'

I

'

'

I'

' /~.

P--T=--'T-"I'~'"'~'T''m

I()2

ENERGY

LiI s c i n t i l l a t o r

~\

,

16 3

inside

= I

161

J '

////'/~

0.2-

Fig.

DIA

I

0

,

:.-_'\

/ tU V) Z 0 Q. U)

'

/

.////

I

'

//,"

PRESENT WORK

. . . . '

'

the" ~ ' ~

(19S2)

. . . . . . . .

0

'

i t at (1965)

" WETZEL .t ,,t (197o v BOOT (197e) --

I

1 17

,

I

,

i '

,

I

10

(MeV) polythene

area of the scintillator. This is understandable since about 2 m m thickness of 6LiI is perfectly black to thermal neutrons and thermal neutron absorption will only be a surface effect. The deviation from the strict proportionality with respect to area may be attributed to thermal neutron flux perturbation in ~Lhe vicinity of a large thermal neutron sink, the small variation in moderator thickness as a consequence of the detector size changes and epithermal and fast n e u t r o n components of the response. Our :findings are in general agreement with those of Rohloff and Heinzelmann 15) and Sanna3). The above studies were done with no annular air gap present between the scintillator and the moderator whereas an air gap is necessarily present in an actual system. Calculations were done to study the influence of the air gap on the response of the scintillator. For an airgap of 0 . 7 c m thickness surrounding the scintillator the response is found to

spheres

of different

diameters.

go down by a factor of five. The factor by which the response goes down is higher at lower energies. But the nature of the energy response curve with or without air gap remains nearly the same over most of the energies as is seen in figs. 4 and 5. Figures 6 and 7 shows the response of 2" and 1" diameter BF 3 counters kept in the centre of polyethylene spheres. Since BF3 counters (90% B-10 e n r i c h m e n t + 6 0 cm Hg filling pressure) are translucent to thermal neutrons, the sensitivities depend on the ratio of the volume for the 10" diameter moderating sphere. For the 3" diameter moderating sphere no such law can be ascribed for ratio of responses because the thickness of moderator itself varies for the two different sized BF 3 counters. During the course of the calculations it was found that in a 3" diameter moderating sphere epithermal neutrons contribute about 25% of the response whereas for larger spheres (10" diameter

118

M.P.

DHAIRYAWAN

et al.

1.6 9

X--

X'--X

-

ORNL-TM -3452

O

~

- PRESENT

( 1971 ) M ~ , E R K A R

et,al

RESULTS

.8

5"

.7

?-~" X

/

.6 LI,I ,03 Z 0 O. if} UJ rr

J

/

x

'~x / X\ I

\

.5

\

x x x,×

I \ 3"

k.... \ \

/

x

5o

/

X

/

k/X

X,.

.~,\ k J 12~.) X/

/ \

.4

X\ /X

/

J

/.17,

/

.2 "X~I

2 0,--'-

_ _x

x.

.

.

.

. . . . . .

.

.

.

,. . . . .

10 5

.

- - -x-

.

10"

j . . . . .

16 4

;

10 3

~"~xJ ~

16 2 NEUTRON

~ "Xx~ ~

-=

~'~---L

181 ENERGY

"~ ~_-3L-'~

1

14

(MeV)

Fig. 3. Response of a 2" diameter BY3 counter inside polythene spheres of different diameters.

small sized spheres and hence when one replaces one type of thermal neutron detector by another, the epithermal neutron behaviour should be taken into account. Block ~7) has recognized the importance of the epithermal neutron response behaviour

and above) the epithermal response is hardly 5% of the total response. Our findings are in general agreement with the results of Mijnheer~6). The results point out that the epithermal neutrons contribute a sizeable percentage of the counts in

101

f

I

I

I

f

I

r

I

I

I

~

I

i

I

I

I

~

~

I

i

~

I

3° DIAMETER POLYTHENE SPHERE •" .--..,_~ ..

lOc

~.

o I~I

-..

\

"-. 0%,9CM D I A M E T E R SURROUNDING IT

1(~2

.....

0.49CM

DIAMETER

LiI

SCtNTILLATOR

.....

0.98CH

DIAMETER

LiI

SCINTILLATOR

.... 10 3

t

167

Li! S C I N T I L L A T O R WITH 0 . T C M THICK

2.00CH DIAMETER I

I

,G e

I

i

I

16 s

I

Li!

SCINTJLLATOR

I

[

~

I

I

WIrH

NO

WITH NO W i T H NO

I

i

r

16 3

NEUTRON ENERGY

AIR I

lo-2

AIR

AIR AIR

"\ ~

GAP

~

,,

GAP GAP

i

I

I

~6'

I

I

I

I

I

~o °

(MeV)

Fig. 4. Effect of Lil scintillator size and air gap on the response of a moderated neutron detector.

lo ,~

NEUTRON 101

i

i

i

i

I

i

i

i

i

i

i

i

I

DETECTORS

i

i

i

I

1 19

i

i

i

I

I

i

i

//

10 DIAMETER POLYTHENE SPHERE

,o~

o

. ~

o

U3 I,IJ =z:

1(~2

~ ~ ! _

D.AMETERL,, SC,N~UATOR WiT.0., CMTHICK IT

0.,gCM AIR GAP SURROUNDING

0.49CM DIAMETER Li[ SCINTILLATOR WITH NO AIR GAP 0.98CM DIAMETER L i l SCINTILLATOR WITH NO AIR GAP 2.00CM DIAMETER

16z.

I

J

l

IG?

I

I

I

1G6

I

I

I

1G5

I

I

1G/,

Lil

I

SCINTILLATOR I

I

I

163

NEUTRON

WIIH

I

I

NO AIR

I

10-2

I

GAP

I

I

iff 1

I

I

I

100

10 l&

ENERGY (MeVI

Fig. 5. Effect of Lil scintillator size and air gap on the response of a moderated neutron detector. 5X10I -

~

POLYTHENE SPHERE

10I

Ro~io of 2"~O I"

5 10° DIAMETER POLYTHENESPHERE "~

32

--

~ ~ X ~ d i a

i'

I

I

I

I

t

l

16 4

10 2

1(~2

lbl

100

101

1~3

~:

102

1(5"

•

.

I

.

1(~6

,

BF3 c

I

.

,

1()5

3.5

PRESENT WORK

z

==

,F I

I

1() 7

1(~ 6

I 1(~ 5

I

I

I

I

I

I~ 4

1(~ 3

I(~ 2

101

1

NEUTRON ENERGY ( MeV )

Fig. 8. Calculated response of the Leake counter.

,

I

n

,

,

t

I

e

,

.

r

I

~

.

,

I

1

.

,

l;~

14

Fig. 7. Effect of detector size on the response of a moderated neutron detector.

3

1(~ 8

.

u

10E' 10-3 10-2 10-I NEUTRON ENERGY (MeV)

NEUTRON ENERGY{ M¢V)

Fig. 6. Effect of detector size on the response o f a moderated neutron detector.

I

o

10

120

M.P.

DHAIRYAWAN

of the central detector in his moderator system (gold foil) and had it covered with a thick tantalum foil to off-set any resonance activation. Figure 8 shows the response of the commercially available Leake counter~8). This consists of a spherical 3He counter of 3 cm diameter and a filling pressure of 2.5 atm, at the centre of a 8" diameter polyethylene sphere with a 22 % perforated concentric cadmium shell in the flesh of the moderator. For the sake o f comparison experimental results 14'19'20) wherever available are also shown in the figure. Our results indicate an over-response by a factor of about 3.5 at 20 keV. Alberts :~) has recently done a calibration of this instrument at 24 keV. The results o f this author agrees with present results after allowing for experimental errors. The results of the present calculation can be taken to be reliable since the code has made use of the latest available data and the calculated results agree with experimental data wherever available. The most important finding of the present calculations is that an annular gap around the central thermal neutron detector brings down the overall sensitivity of the system even though the air gap does not affect the nature of the energy response curve. References i l D. Nachtigall and G. Burger; in Topics in radiation dosime-

try, suppl. 1 to Radiation Dosimetry, eds. Attix and Roesch, (Academic Press, New York, 1972). 2) R.E. Maerker, L.R. Williams, F.R. Mynatt and N . M . Greene, report ORNL-TM-3451 (1971).

et al.

-~) R. Sanna, Report ttASL - 267 (1973). 4~ L.S. Andreeva, E.B. Keirim-Markus, A . K . Saveinski and

L.N. Uspenski, Prec. Synlp. on Neutron nlonitoring for radiation protection purposes, Vol. 1 IIAEA, Vienna, 1973} p. 97. ~ V.E. Aleinikov, V.P. Gerdt and M . M . K o m u s h k o v Proc, Symp. on Neutron monitorirLg for radiation protection purposes, Vol. I (IAEA, Vienna, 1973} p. 31. M. Bricka, Mylene Deltas, J. Lamberiux and J. Caizergues Prec. Synip. on Neutron monitoring [br radiation protection purposes, Vol. I IIAEA. Vienna, 1973) p. 297. 7~ W. R. Burrus, Report ORNL-3360 (1962) 269. ~) R.L. Bramblett, R.I. Ewing and T W. Bonner. Nucl. Instr. and Moth. 9 (1960} I. ~) G. E. thnlsen and [t. A. Sandmeir, Nucl. Sci. t'~ilg. 22 {1965) 515. >) D. I. Gardner, k. O. Stromberg, M. D. Goldberg. D. E. Cullen and V. M. May, BNL-400 (1970~. iI) K. O'Brien. R. Sanna and J.C. Mch.iughlin, Prec. t:irst Syrup. on Accelerator radiation dosinletry and experience, Report US AEC-CONF-651109 (1965)p. 346. ~-'1 S.J. Boot. tlarwell, personal conlmtinication 11978). ~31 L. Wetzel, G. Kneske, G. Thant, V.R. Aleijniko,~, V . A . Archipov. V. M. Komozkov, B. M. Nazarov and P. Klapper, Prec. Synip. on Radiation protection problems in accelerators o f charged particles (Dubna, 1970) p. 201. ~4~ K. G. ttarrison, J. R, llarve) and S. J. Boot, Nucl, Instr. and Moth. 148 (1978~ 511. ~) F. Rohlofl" and M. tteinzehllann, Proc. Syrup. on Neutron monitoring for radiation protection purposes, Vol. I I I A E A , Vienna, 1973) p. 269. B.J. Mijnheer. Thesis IUnivorsity of Anlsterdanl, 1971 ). i7} S. Block, ttealth Phys. 16 (1964) 93. iS} J. W. Leake, Nucl. Instr. and Moth. 63 (19681 329. I. M . G . T h o m s o n , A. Lavender. R.G. Shipton and Jane Goodwin, Prec. S~mp. on Advances in physical and biological radiation detectors (IAEA. Vienna, 1971)p. 505. 20) It.V. Larsoll, R.L. Kathren and I. M G . T h o m s o n . Prec. Syrup, on Advances ill physical and biological radiation detectors (IAEA, Vienna. 19711 p. 533. ~,1 W . G . Alberts, Nucl. Instr. and Moth. 155 (1978~ 307