Activation cross-sections of fast neutron detectors

Heertje, I. Aten Junior, -4. H. W. 1962

Physica 28 661-666

ACTIVATION FAST

CKOSS-SECTIONS

NEUTRON

by I. HEERTJE Instituut

voor

OF

DETECTORS

and A. H. W. ATEN

Kernphysisch

Onderzoek,

Amsterdam,

JR.

Nederland.

synopsis The cross-sections of several fast-neutron detectors have been measured in the neutron flux of a radium-beryllium source. The results suggest that the energy spectrum of the neutrons is similar to that suggested by Hess and that the cross-sections published for the reactions 33S(n, p) 33P, 3rP(n, p) 3rSi, 37Al(n, p) 37Mg, ssFe (n, p) s3Mn and 37Al(n, a) 34Na are sufficiently accurate to be used as a basis for neutron flux measurements.

Introduction. For the determination of fast neutron fluxes and of neutron energy spectra the use of threshold detectors proves to be the most convenient method in a large number of cases. Evidently for this purpose one needs to know the absolute values of the cross-sections as a function of neutron energy. For the most important threshold reactions cross-sections have been measured and interpolation curves are given in the Brookhaven compilationr). Before data are used as a basis for measurements, it is important to check their reliability. The best way to do this, would be to repeat the determinations in a series of independent measurements, but this evidently would be an extremely tedious procedure. However, a certain amount of information can also be obtained by measurement of the activation of these detectors in a known flux of fast neutrons having a known energy distribution. This will not, of course, provide a complete control of the crosssection curve, but if the total activity observed agrees with the calculated one, this will certainly indicate a high probability that the published values are correct. In several laboratories such tests have been carried out by simultaneous activation in a flux of fission neutronss)s)4)5)6) and critical compilations of these results have also been publisheds) 6). Most of the results obtained suggest that the cross-section curves selected in Brookhavenl) are fairly reliable, but in several cases the agreement has been far from satisfactory. In some experiments the disagreement may have been due to the fact that the energy distribution of the neutrons did not correspond to a pure fission spectrum, but this explanation can not account for all discrepancies by -

661 -

662

I.

HEERTJE

AND

A. H. W.

ATEN

JUNIOR

any means. The most evident case is that of the reactions W(n, 9) 32P and 3lP(n, p) 3lSi, where the cross-section vs. energy curves have roughly the same shape, with the CJvalue for 3% about twice as large as that for 3lP. Integration of the Brookhaven curves over the fission spectrum gives a calculated ratio of 2.34)s) 6). It is evident that deformation of this spectrum can affect the observed ratio of the cross-sections only to a minor degree, and the variations which have been observed for this ratio - between 2.04) and 1.17) - therefore will have to be explained in some other way. The difficulties mentioned in the preceding paragraph suggest that it would be of interest to activate threshold detectors with neutrons of known energy but of a different origin. For this reason we have activated several threshold detectors by means of a radium-beryllium source. This method has two advantages over the use of a fission flux. In the first place the neutron flux, as it leaves the source, is likely to be much less deformed than the fission flux available in most experiments. Secondly the total fast flux from the radium-beryllium source is known by absolute calibration, which means that not only relative but also absolute cross-sections can be obtained, but, of course, integrated over the energy distribution of the neutron source. A very serious complication is presented by the uncertainty of the shape of the energy spectrum of the Ra-Be neutrons. Theoretical arguments together with some experimental data suggest that a fairly large fraction of the neutrons have energies below about 1.5 MeV8)9), but in most experimental investigations this group has not been observedlo). If a certain shape of the neutron energy spectrum is assumed, it is possible to calculate from this spectrum and from the assumed cross-section US. energy curve an average cross-section from the formula: I&“,,

/ a(E)N(E)

Cl=

0

K,,>,X /N(E) 0

dE

dE

The cross-section curves we have used, are given in figure 1. These have been taken from the Brookhaven compilationl) and from Mani e.a. 11) and Schmitt e.a.11). The curve of Terre11 and Holml) for the 56Fe(n, p) 56Mn reaction has been normalized to a value of 130 m barn at 14.0 MeV. For the neutron spectrum we have used on the one hand the spectrum given by Houtermans and Teucher12) and as an alternative the spectrum given by Hesss). The calculated values of the average cross-sections are given in table 1. In the third column we have given the values of u calculated from the spectrum as it was drawn by Hess, but omitting the contribution of the neutrons with energies below 1.5 MeV.

ACTIVATION

Experiments.

CROSS-SECTIONS

The experimental

OF FAST

NEUTRON

determination

was done with a radium-beryllium source, the of which had been determined in a large MnS04 be 1.47 x 106 nlsec f 3%. We also had a value given by the Union Mini&e du Haut Katanga,

DETECTORS

663

of the average cross-sections rate of neutron production solution bath and found to for the neutron production which amounted to 1.54 x

x 106 n/set &Is%. Another measurement of the neutron production had been performed at the Physical Laboratory of the Vrije Universiteit of Amsterdam by comparing this neutron source to another calibrated one by means of a long counter *) . (This gave a value of 1.5 1 x 106 n/set f 2.6%). As a basis for our calculations we used the average value of 1.50 x 106 n/set. m

C

35c

3oc I-

)-

20(

15(5-

IO< 3-

51o-

i OO

Fig.

1. Activation

I

2

cross-sections

the neutron

energy

3

4

5

of the

6

7

various

8

9

10

threshold

II

En h+ev

detectors

as used for the calculations

as functions

of

in this paper.

As the source has a cylindrical shape, we have determined the anisotropy of the neutron emission by means of the reaction arP(n, p) arSi. Three tablets of red phosphorus, all having the same weight and size, were activated *) We wish to thank measurement.

Professor J. Blok and Dr. J. J. Vasmel

for kindly performing this

664 in three

I. HEEKTJE

positions,

ANIl

at 5 cm distance

A. H. W.

ATEN

JUNIOR

from the centre

of the source

(axial,

equatorial and under an angle of 45” with the axis). As the activities in the three samples were in the proportion of 106 to 100 to 98, and as our activation experiments were performed in the equatorial position, we did not apply any corrections for anisotropy. The materials irradiated consisted of metal discs of aluminum and iron and, in the cases of sulfur and phosphorus, of tablets formed under high pressure. In the case of the phosphorus it was necessary to add 50% of cellulose powder before the tablets were compressed. Part of the irradiations were performed at a distance of 5 cm, which served to give us a quantitative value of the cross-section. A number of relative measurements were also carried out on the surface of the source. For the latter irradiations the reactions ssS(n, p) 32P, 3lP(n, p) 31Si and 27Al(n, p) 27Mg were used to measure the neutron flux. As the activities observed were in general quite low, measurements were carried out by means of an anti-coincidence G.M. counter. This counter had to be calibrated and furthermore in all activity measurements corrections for selfabsorption had to be applied. For the pure p-emitters (31% and 32P) this was done with the aid of experimental selfabsorption curves, - obtained on 24Na and 27Mg in Al discs - , which were extrapolated to zero thickness. As the counter efficiency for zero thickness can easily be determined, it was not difficult to calculate the counter efficiency for a certain sample weight. 24Na and 27Mg formed in the Al discs were calibrated with the same selfabsorption curves and also by means of 4np - y coincidence measurements. In additionsrSi and 24Na can be standardized fairly well with the 1.33 Me\’ P-radiation of 40K in a KC1 tablet of the same weight as the samples. 56Mn formed in the iron discs was calibrated with a NaJ(T1) crystal of known efficiency

on the 850 KeV photopeak

of the 56Mn.

The experimental values obtained are included in table 1. Discussion. It will be noted that these figures are in good agreement with 6 calculated from the spectrum according to Hess, but are quite different from the values obtained from that of Houtermans and Teucher’s or from that of Hess’, if the low energy peak is omitted. The results of our measurements thus indicate that Hess’ spectrum is more likely to be correct than the spectra which do not include a low energy neutron peak and that it may very well give a fairly accurate representation of the actual neutron distribution of a radium-beryllium source. We may also conclude that, though we have not been able to give arguments for the correctness of any individual cross-section curve, the fact that, with one single neutron energy distribution, average cross-sections can be calculated for all five nuclear reactions which

ACTIVATION

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CROSS-SECTIONS

OF FAST

NEUTRON

665

DETECTORS

are in good agreement with our observed values, suggests that probably the interpolated curves in the Brookhaven compilation give a fairly good representation of the actual values for each one of these reactions. Evidently, the course of our argument depends very much on the reliability various experimental data are considered to have. In a recent critical compilationra) arguments are given that for the reaction 33S(n, p)33P two independent series of measurementsi4) 15) agree within about 15% as a rough average in the energy range 2.5 - 6 MeV and that a third seriesr6) agrees with the other two between 2.5 and 4 MeV. If we are willing to accept this as proof that the cross-section curve for this reaction is essentially correct, we have a much stronger support for the neutron energy spectrum proposed by Hess (at least in as far as it contains a contribution of about 3076 of neutrons with energies below about 1.5 MeV) and, through this spectrum, also for the correctness of the cross-section curves of the other nuclear reactions included in table 1. TABLE Average

cross-section

B calculated Reactions

for

the spectrum Hess in mb

-szs(lt,p)3sp

164

of

I

for radium d calculated the spectrum Houtermans Teucher

(a) beryllium for of and

in mb

neutrons

3 calculated for the spectrum of

3 observed in mb

Hess with intensity = 0 below 1.5 MeV in mb

247

235

105

98

69.1

I

154.3

slP(n, p) slSi

68.5

s’Al(n,

19.3

31.8

27.6

20.6

9.7 7.6

15.7 11.5

13.9 10.9

9.0 6.8

p) s?Mg

“sFe(n, p) 5’3Mn s’Al(n, o) s4Na

For the reaction 27Al(n, oz)24Na the Euratom compilation shows good agreement in the low energy part of the energy range - between 6.7 and 8.3 MeV - between two independent series of measurementsr7) is). These values are represented with satisfactory accuracy by the Brookhaven interpolation. The Euratom collection, however, also gives a third measurementrs) which suggests that the cross-section should be much lower in this region. The difference between the values of G calculated from the two different cross-section curves is, however, not sufficiently large to make possible a decision between the two alternatives on the basis of our experimental value of c?. Acknowledgement. We wish to thank Mr. H. Turksma and Mr. P. J. de Valois for their valuable assistance. This investigation was performed as part of the programme of the “Stichting Fundamenteel Onderzoek der Materie (F.O.M.)” (F oundation for Fundamental Research on Matter) with

666

ACTIVATION

CROSS-SECTIONS

OF

FAST

NEUTRON

DETECTORS

the financial support of the “Nederlandse Organisatie voor Zuiver-Wetenschappelijk Onderzoek (Z.W.O.)” (Netherlands Organisation for the Advancement of Pure Research). Received

17-3-62

1)

Hughes,

D. J.

a~ld Schwartz,

R.

I<., “Neutron

2)

Report B.N.L.-325, 2nd ed. (1958). Hughes, D. J., Pile Neutron Research

3) 4)

Rochlin, Ricabarra,

5)

177. Passel,

6)

Mellish,

7)

Sealand,

8)

Hess,

K. S., Nucleonics I)., Turjanski, T. 0. and Heath, C. I<., Nucleonics E., Lykaas,

W. N., Annals

cross-sections”

(AddisomWeslcy,

17 (1959) 54. Ii. and Atcn

Junior,

RI. and Samsahl, (i (1959)

Reading,

Mass.

A. H. W., J. Nuclear

R. L., liuclear Sci. Engng. 1’3 (1961) 114.

of Physics

Brookhaven

I<., hsted

10 (1961)

11) 12) 13)

Teucher, Liskien,

14)

hold reactions, Euratom, November 1961. Klema, E. D. and Hanson, A. O., Phys.

15) 16)

Allen, L., Biggers, W. A., Prestwood, R. J. and Smith, R. K., Phys. Hdrlirnann, T. and Huber, P., Helv. phys. Acta 28 (1955) 33.

Phys.

M., Z. Physik 126 (1949) 410. H. and Paulsen, A., Compilation

17)

Grundl,

18) 19)

Schmitt, Tewes,

J. A., Hcnkel,

H. W. and Halperin, A reference to a special

ography

of the Euratom

R. L. nud Perkins,

Rev.

D.C.),

121

Measurements

(1961)

A 12 (1960)

of cross-sections Rev.

73 (1948)

B. L., Phys.

of neutron

flus

anti

827. for wloe

mutroll

induced

thre<-

106.

Rev.

Rev. 107 (1957)

1363.

10’3 (19581 425.

J., Pbys. Rev. 121 (1961) 827. report, which was not available

compilation.

Energy

ill (5) alld (6).

spectra for physical and biological applications, fig. 11 page 63. Mani, G. S., MC Collum, G. J. c.a., Nucl. Physics l!l (1960) 535; J.,

1953).

115.

De Pangher, J., quoted in ref. 10, page 67. NBS Handbook 72, July 15, 1960 (Washington

H. W. and Halperin,

Lab.

308.

9) 10)

Schmitt,

National

to us, is gi\cn

in the bibli-