Activation measurements of thermal neutron capture cross-sections and resonance integrals

Journal

of Nuclear

Energy.

Vol. 24. pp. 35 to 42.

Perganton

Press 1970.

Printed

in Northern

Ireland

ACTIVATION MEASUREMENTS OF THERMAL NEUTRON CAPTURE CROSS-SECTIONS AND RESONANCE INTEGRALS T. B. RYVES National Physical Laboratory,

Teddington,

Middlesex, England

(First received 23 May 1969 and in final form 29 October 1969) Abstract-The standard thermal neutron flux at the NPL has been used to measure a number of 2200 m/s activation cross-sections and resonance integrals relative to those of gold. Thecalculatedaccuracy of the cross-sections in most cases was 2 per cent or better, the main uncertainty being due to the determination of the p-counting efficiency of the activated samples. 1. INTRODUCTION

A KNOWLEDGE of thermal neutron capture cross sections and resonance integrals is needed for reactor calculations, activation analysis and other theoretical and experimental situations concerning the interaction of thermal neutrons with matter. Many of these cross sections have not been measured accurately and recent measurements do not agree very well with each other. In order to add to the amount of such information available the recently commissioned standard thermal neutron flux at the NPL, described in detail by RYVES and PAUL (1968), has been used to measure a number of 2200 m/s activation cross sections and resonance integrals relative to those of gold. This paper describes the methods used, and the cross-section results quoted mostly have calculated accuracies of the order of 2 per cent or better. 2. EXPERIMENTAL

METHODS

The activation measurements were made in the known flux and spectrum of the NPL standard thermal neutron flux. Most of the measurements were made at a flux level of I.26 x 10’ n/cm2/s whose value is known, via the gold cross section, to 0.7 per cent or better. The samples were either in the form of pure metallic foils or of salts evaporated onto thin backings of aluminium or plastic. An exception was silicon which consisted of thin flakes of quartz (SO,). The reported values are the means derived from several measurements carried out in slightly different flux densities and slightly different spectra. This was a check on the consistency of the measurements. The samples were counted in 47$ argon-methane gas flow proportional counters and in all favourable cases the absolute activity was determined by the p-6-ycoincidence technique, as described in the next section. The saturated absolute activities per atom of the foils after irradiation bare, A, and after irradiation encased in 1 mm cadmium, Aod, were used to determine the 2200 m/s cross section b,,, and the reduced resonance integral I’ from the two equations: A = ntbc#,,g + GfW, OW, W’hl 4, = nw,[g/R -fW4 T)W+ FG,fC(B, VO, G,X’/q,l These equations were derived previously by RYVES and PAUL (1968) for the standthermal flux, in which the measured slowing down spectrum varies as l/,??+fi where E was the neutron energy. C(/I, 7’) was a renormalisation correction to the epithermal flux fraction fat temperature T, h(@, G,) was a correction to the resonance integral defined for a l/E slowing down flux, no0 was the conventional flux and W was the

ard

35

36

T. B. RYVES

small correction term used by WALKERet al. (1960). Gth and G, were the thermal and resonance self-shielding factors and F was the transmission factor for resonance neutrons through 1 mm cadmium, whose values are given in Table 1. R was the cadmium ratio for “l/v” detector and g was the Wescott function, which has been taken as unity except where otherwise stated in Table 1, column 10. The absolute activities A and Acd depend upon the /I efficiency of the &r/3 counter. The measurement of this efficiency presents considerable experimental difficulty. 3. DETERMINATION OF THE B-COUNTING (a) Isotopes with a simple decay scheme

EFFICIENCY

The b efficiencies of most of the samples were determined by 47$-y coincidence counting. In the cases of Na, Al, V and 82Br the y-rays are in coincidence with a single b-ray and they were counted in the conventional manner, CAMPION(1959), using a 3” x 3” NaI(T1) crystal. (b) Isotopes with a complex decay scheme For isotopes with a complex decay scheme a prominent y-ray photopeak was selected, and the efficiency of the ps in coincidence measured using the 47+--y coincidence counting technique. It was necessary to correct this measured efficiency to obtain the true overall p efficiency. These corrections depended on the decay scheme, and the first step in their calculation was to find the escape probability P(E, t) of monoenergetic electrons of energy E from an infinite homogeneous slab of thickness t expressed in mglcm 2. CAMPIONet al. (1960) give the expressions (inserting the appropriate numerical values). P(E, t) = O-3570 E/t

for

E
P(E, t) = 1 - t[0*7632-3.519E,(1.9t/E)]/E

for

E>t

(1)

which are based on the electron range-energy relations of LANEand ZAFFARANO (1954). The electron range-energy relations of KATZ and PENFOLD(1952) were also used. These assume a definite range R(E) and it is found simply that: P(E, t) = R(E)/2t

for

R(E) < t

P(E, t) = 1 - t/2&q

for

R(E) > t 1.

(2)

For a given /?-ray, the average escape probability is averaged over the b-ray spectrum , E

P(E,,,,

t) =

msr s0

P(E, t)N(E) dE

(3)

where N(E) is the number of electrons in the b-ray spectrum between energies E, E + dE, (normalised to unity when integrated from zero energy to the maximum /?-ray energy Em,=) as predicted by Fermi beta decay theory, and has been taken from SIEGBAHN(1955) using the calculations of ROSEet al. (1953). In the &y coincidence measurements the decay scheme data compilation of LEDERERet al. (1967) were used. The photopeaks of one or two prominent gamma-rays, yi, in the decay schemes were channel-selected, and the apparent @ efficiency, .Q’, was found from E@’ = NC/N, (4)

Cu foil

2.56 h

12.8 h

2Po h

25.0 m

Z-mylar

CSI

PbBrz 4.38 h on 35.3 h 1thin plastic In foil 54.0 m

Cu foil

4.5

6-l 1

15-17

4-9

21

13 14-17 15-36

of E#

-

l+Xl 1xKl 1.00 1x)0

1.00 1.00 103 1.00

0.93 0.99

1.00 0.95

1.00 1.00

1.00

0.4 0.93

0.95 0.95

0.99 0.95

0.95

1.00

1.00 I.00 -

0.98

0.99 0.9 0.98 1.00

G

1.00

0.95

I.00 0.9 lXY.3 0.97

0.95 1.00

1.00 1.00

1.00 I.00

1.00

1.00

1.00

1.00

1.00 1.00 103 l+O 1.00

Gt,

1.00 1.00 l+Xl 1.00 1.00

F

B-y Co with 0.60 Mev 1’ 1.00 p-r Co with 044 Mev y 0.96

p-y Co with 1.29 Mev y fi-r Co with 0.56 Mev y

0.7-0.8 0.92

0.78 0.92

,f-r Co

-

Calculated B-Y Co Calculated Calc. from meas. es of arNa Calc. from mass eP of 24Na formed by 27A1 (n, a) in backing /J-r Co with 0.32 Mev y B-r Co /I counted under Ni foil, then in calibrated NaI well crystal p-y Co with 1.12 Mev and 1.48 Mev ys 6-y Co with 0.51 Mev y and talc. from meas. E of Wu @-{Co with 1.04 Mev y Calc. from meas. EB of **Br

P-6-y co

Measurement

0.7-0.8

-

0.95 0.89

0.43

0.84

0.97 0.98 0.330.4 effective

0.99

o-95 0.98 0.98 O-98 0.97

Total eB

DETAILS OF THE MEASUREMENTS

0.4

7

a = 1.08% EC 43.5 % &EC= 1.7 i 0.5%

eEc = 0.06 + 0.06 %

EC 6.3%

EC6% exe = o-3 + 0.1%

8

3 8

-

1.6 0.6

0.5 0.2

1.7

0.2 1.0

0.3

6

0.33 %

a =

2 5

0.6 o-2 0.6

a = 5.34%

4 6 8

0.4 0.7 0.3 0.4 0.6

-nannnm -=*=*----%

0.4

9 5 6 2 10

measurements

-

1.3 1.4

I.3 1.4

1.7

1.3 2.5

4.0

1.5

1.3 1.1 1.0

1.5

l-4 2.0 1.2 1.0

I.0

Svstematic -I’ I.,“I %)

Errors in n“0

5

LI = 6.88%

a = 3.09%

a = 11.3%

Notes

No. of

* The value of the z*AI half-life measured at NPL, which is significantly lower than the value given by LEDERER et a/. (1967).

134CS

somBr ““Br

BUBr

5.10 m 17.6 m

21

Ni foil

5.79 m 3.77 m 451 d

e8cu

16

Ti foil V foil Fe foil

12.4 h

42K

NaCl on Al Mg foil Al foil SiO, flake NaCl or KC1 8

KC1 on Al

15.0 h 9.5 m *2.25 m 2.62 h 37.3 m

mg/cm2

24Na 2’Mg “BAl ?Yii WI

Composition

Half-life

Target

Product nucleus from (a, Y) reaction

TABLE L-EXPERIMENTAL

T. B. RYVES

38

where NCand NYwere the measured coincident and y channel count rates, after correction for dead-time, background and accidental coincidences. To find the true B efficiency equation (4) was first expanded as 3

= 7 a&E@

NV

(5)

3

aiE;4

where the ai were the relative abundances of the yi, with detection efficiencies syi and associated p-efficiencies E@~. The experimentally observed y-spectrum in the gamma channel gave the relative aici directly, since these are proportional to the areas under the respective gamma-rays, designated Ai. The ssi were interpreted from a detailed knowledge of the decay scheme. To do this, an approximate sample thickness t was first assumed and the individual B-efficiencies spj for the particular /?-ray branches ,5”were calculated by integrating the average escape probability given by equation (3) over the allowed shape predicted by Fermi #? decay theory for each branch. Each b-ray had a relative abundance j& where the double s&ix referred to the j-th p-ray feeding the i-th y-ray. Thus z %jfij ‘Bi =

%@-

=

EpIki

(6)

where ~~~ was the &efficiency of the principal b-ray I, and ki was a calculated coupling constant between &Biand ~~~ Equations (4) and (5) were rewritten as C A&i i

The process was iterated if necessary by choosing a better value of r to recalculate the EBj,and hence k, until the correct sBr was obtained. The overall &efficiency of the sample was finally calculated from this value of t for all the fls in the decay scheme. The two formula equations (1) and (2) usually agreed to better than I per cent especially for thin samples with Ed > 0.9. As a check on the method, two separate coincidence measurements were made on 65Ni, with the I.48 Mev y which is coincident with a O-65 Mev B, and with the I.42 and l-48 Mev ys which are coincident with 1.02 and 0.65 Mev /I’s and others; the measured b-efficiencies were O-696 & 0.003 and O-730 f 0.003 respectively. For &Ni 58 per cent of the @s of 2.14 Mev go direct to the ground state. The calculated overall p-efficiencies were 0.846 and 0.831, and the mean value of O-839 was used to evaluate the cross-section. In order to further test the method, the calculated p-efficiencies for a range of gold and indium foils up to 50 mg/cm2 ( a0 - 0.5) were compared with efficiencies measured by the 477/?-y technique. It was found that the final t values were close to the physical thicknesses of the foils using the electron escape probability formula based on the range energy relations of KATZ and PENFOLD(1952) which is given in equation (2), but about half the physical foil thicknesses when using the LANEand ZAFFARANO (1954) range energy relation together with equation (1). This latter difference between t and

Activation measurements of thermal neutron capture cross-sections

39

the foil thickness was probably due to a breakdown in the range energy relation at low energies. However in the present experiments the foil efficiencies have all been derived from effective p-efficiencies measured directly by the coincidence technique, using the above calculations only to find the relative efficiencies of the various p-rays in the assumed decay schemes and hence calculate the overall efficiencies. For many of the isotopes the highest energy p-ray goes direct to the ground state without emitting a y-ray, and hence is not measured by the 4r@-y coincidence method. Very often this &ray is also the most abundant, and contributes more to the total p-efficiency of the foil than the other p-rays, which are of lower energy and consequently of lower efficiencies. In these cases the final error in the overall p-efficiency is likely to be smaller than might be expected because as the efficiency approaches 100 per cent the formulae become progressively more accurate, the error in t becoming less important. Conversely the formulae diverge more and more for lower energy /l’s as t assumes progressively greater importance. (c) The /?-counting eficiency for 27Mg, 31Siand 5gFe For 27Mg and 31Si the /3--y coincidence method could not be used due to the low specific activity obtainable or the unsuitable decay scheme. The p-efficiency was calculated from the empirical formula quoted by AXTON (1963) and gave about 98 per cent in both cases. The calculated efficiency using this formula agreed with the measured efficiency, by the 47$-y coincidence method, for aluminium (also 98 per cent) to better than 1 per cent, and accordingly it was assumed that these calculated efficiencies were accurate to about 1 per cent. In the case of iron, the 45d 6gFe and 2.6~ =Fe activities were both observed, the latter decaying by electron capture. Foils of several thicknesses were assayed in a 47$ counter, covered with thin nickel foil to absorb the Auger electrons from 55Fe. Because of the low isotopic abundance of 58Fe and long half life of 5gFe the observed @-count rate was very low (< 1 count/set). The j3-efficiency for 5gFe was determined by subsequently irradiating the foils in a high flux reactor, counting in the 4~- counter, and then dissolving the foils in dilute nitric acid and counting as liquid samples in a NaI well crystal against calibrated 5gFe solutions, setting the gate to include the 1.095 and l-292 Mev gamma peaks. The iron samples were also used to measure the approximate p-counting efficiency for electron capture (EC) as described in the next section. (d) The /?-counting eficiency for electron capture and conversion electrons The p-efficiency for electron capture, .zEC, was obtained by calculation, using Gold’s formula for the absorption of X-rays by a plane parallel film as quoted by CAMPIONand MERRITT(1957) and integrating over the volume of the foil. If the X-ray absorption coefficient is ,u and the foil thickness t, the probability P,(p) of X-rays escaping from a foil is Pz(P) = c+Jl + (tdt - l)@ - (@)2Er&t)l/2@ where ok is the fluorescence yield of K X-rays. The observed @-counts resulting from the passage of the X-rays through the counting gas (90 %ArlO %C!H,) can be calculated from the X-ray absorption coefficient and counter geometry. The number of p-counts from the Auger electrons and the few L-X-rays present are small in the present experiments with relatively thick foils and have been ignored. Values of ok were taken from the paper by FINK et al. (1966).

40

T. B. RYVES

The calculated cEC for 64Cu, 80Br, lz2Sb and 12g1are given in Table 1, where the decay scheme branching ratio has been included, and except for 64Cu are much less than 1 per cent. A further check on these values was obtained from the iron foils previously referred to. The 2.6~ half life 5jFe decays by 100 per cent EC, and the relative amounts of 55Fe and 5gVe present after the irradiation of an iron sample can be calculated assuming the neutron capture cross sections. The bare iron foils were counted by 47rfl-y coincidence, gating on the 1.095 and 1.292 MeV y-rays of 5gFe, to give the b-efficiency of 5gFe alone. Also the NaI well crystal measurements previously mentioned gave the overall effective /3-efficiency. Hence aEC could be derived, and for a 15 mg/cm2 foil was 5.9 per cent, in reasonable agreement with the calculated value of 6.5 per cent. No allowance was made for conversion electrons, since for the thin samples of low and intermediate Z used it was shown that their effect in the 47rp counter was not significant within the errors. 4. ERRORS

The random and systematic errors in the cross-sections are given in columns 11 and 12 of Table 1, and the total error in Table 2. This total error is the linear sum of the random and systematic errors. The random errors are the standard errors on the spread of the measurements, and the number of measurements has also been given in Table 1, so that the standard deviations can be found if required. The systematic errors include estimates of the uncertainty in neutron flux (0.7 per cent), weighing, half-life and p-efficiency, added in quadrature. The errors in isotopic abundance have been excluded, values used being given in Table 1, column 10. For 5gFe the error in half-life was also excluded. In this case the measured activity, and hence cross-section was almost proportional, to the half-life (because the irradiation time was much less than the half life) so that any adjustment can readily be made. The /?-efficiencies for the bare and cadmium covered foil activations were assumed to be equal. This assumption is not strictly true because of the resonance absorption which occurs preferentially near the outer surfaces of the foils. However, in the present set of measurements, the foils were sufficiently thin to make the effect negligible within the errors. 5. RESULTS The cross-sections co and reduced integrals (less l/v part) I’ are given in Table 2, and are relative to Au 0, = 98.8 & O-3b and I’ = 1514 f 60b. They supersede the previous provisional results of RYVES (1969). The errors quoted are the sums of the standard errors describing the experimental spread in the measurements and of the estimated systematic errors; the standard deviations can be found from the number of measurements taken. Some published values of cross-sections and resonance integrals are also listed and show good overall agreement within the errors for most of the cross-sections. It is certainly difficult to know which particular cross-section value from the literature to select in most cases, because the spread on individual measurements is so great, and therefore usually when possible the preferred value from BNL 325 (1965) has been given for purposes of comparison. However it should be remembered that these preferred values are not actual measurements in themselves. In some cases, e.g. aluminium, the majority of previous measurements have been

Activation measurements of thermal neutron capture cross-sections TABLE 2.--MEASURED

41

ANDLITERATURE VALUES OF NEUTRON CAPTURE CROSSSECTIONSAND RESONANCEINTEGRALS

Measured values

Product nucleus from (n, Y)

I’

00

80 & 12 mb 8.0 rt 1.2 mb 66i-Pmb

00

2PNa 2’M g 28A1 31Si Wl ‘2K 51Ti 52V 69Fe

529 38.2 233 108 423 1.46 179 4.93 1.14

GjNi

1.49 & 0.03 b 4.4 f 0.2 b

044 i_ 0.14 b 2.5 A 0.2 b

2.17 f 0.03 b 8.70 f 0.30 b (2.7 rt 0.2 b) assumed 2.69 f 0.09 b 161 z!z 3 b 6.21 i 0.10 b 4.14 f 0.12 b 6.12 i 0.12 b (2.6 i 0.2 b) assumed

1.17 rt 0.12 b 92 f lob 34.5 zt 4 b

534 * 5 mb 27f5mb 235 & 5 mb 103 * 3 mb 440kl3mb 1.50 f 0.045 b 140 & 30 mb 4.9 h 0.3 b 1.18 * 0.03 b 1.09 2 0,03 b 1.52 i 0.14 b 4.51 f 0.23 b 4.03 & 0.17 b 2.2 f 0.2 b 8.2 i 0.4 b 2.7 f 0.2 b

50&5b 2710 k 200 b 206 zt 15 b 120 A 12 b 145 f 9 b 30&6b

3.23 rt 0.2 b 162&3b 6.10 i 0,25 b 4.03 & 0.16 b 6.2 & 0.2 b 2.6 & 0.2 b

Wu 6Yzu 80Br aomBr 8ZBr “GrnIn YSb =*Sb WI 13”m~s

* =t h + & i rt rt 5

7 mb 0.8 mb 5 mb 2 mb 7 mb 0.03 b 3 mb 0.06 b 0.02 b

Literature values

120 0.77 38 0.48

& i i i

60 15 11 09

mb b mb b

Ref.

I’

Ref

75 i_ 10mb

(c)

60f3mb

(k)

:l$ i;; (f) (b) (a)

0.36 + 0.10 b

(j)

(g) (h) (a) (a) (1) (b)

428 f 3.17 i 11;; ;

(r) (c) $)

0.04 b 0.18 b 0.23 b

:; (0)

(P) (q) Yi; (:)

41.3 f 1 b 2500 + 85 b 159b 136 b 146 & 7b

1 n :;

References:-(a) BNL 325 (1965); (b) BNL 325 (1958); (b) BAUMANN(1963); (d) CARRE and VIDAL (1966); (e) KOHLERand KNOPF (1966); (f) KAPPE (1965); (g) FABRYand DEWORM(1967); (h) CARPER(1966); (j) GEIGERand VAN DER ZWAN (1967); (k) BREITENHUBER and PINTER (1968); (i) F~NONCELLI and FABRY (1967); (m) DAHLBERCer al. (1961); (n) ANL 5800 (1963); (0) EMERY (1965); (p) BECKURTSei al. (1963); (q) ORVINI and MAXIA (1967); (r) ANDERSON(1964).

of the absorption cross-section using the pile oscillator.

Such measurements have the

advantage of being independent of the decay half-life of the product nucleide, which is particularly important for short lived isotopes. The 28A1half-life has been remeasured as 2.25 m (to be published) which is about 2 per cent less than the value given by LEDERER et al. (1967). change in the activation

cross-section

The resulting fractional

in the present experiment

is approximately

double the fractional change in the half-life, since the 2EA1was counted about 6 m after irradiation.

A systematic error of 2 per cent has been allowed, which covers the half-

life uncertainty.

The two most recent measurements of the aluminium activation

cross-section listed in BNL 325 are both unpublished, so that the half-life values used are unknown.

The 52V half-life was also remeasured as 3.77 m, in excellent agreement

with the value of LEDERER er al. Several other isotopes had large (i.e. >2 per cent) systematic errors associated with the measurements.

In the case of Wu

the uncertainty in the decay scheme branching

ratio was included, and for the bromine isotopes there was some uncertainty in the measured and calculated @ efficiencies and half-lives (for example the effect of the 6 m 82mBrdecay, which was not directly observed). Three results in Table 2 differ significantly from the literature values. cross-section (including direct and indirect production

The *lBr

via the 6m 82mBr to s2Br) of

42

T. B. RYVES

2-69 & 0.09b is lower than the value of 3.23 A 0.2b of EMERY (1965), but the spread of earlier measurements in BNL 325 (1965) ranges from about 2 to 4b. The capture cross-sections of 2eMg and 50Ti were much higher than the literature values quoted in the table, but both of these were from early measurements with large errors, and the comparison is not really very meaningful. Since the other activation cross-sections measured in the present experiments form a coherent and consistent set of results, in fair agreement with the literature values, and since the estimated errors for the 2eMg and 50Ti measurements are of the same order of magnitude (-2 per cent) as the rest, it seems likely that they are more reliable than the older values. The resonance integrals have been compared with values from the literature when these could be found and are in general agreement. For half the isotopes whose resonance integrals have been measured, no previous measurement has been discovered. The rather large errors (typically &-lo per cent) were due to the uncertainty in the corrections which were needed for the departure of the epithermal flux from a l/E dependence per unit energy interval, and also in several cases to the very low counting rates of the samples irradiated under cadmium in the well moderated NPL thermal neutron flux assembly. Acknowledgments-Many of these measurements were made jointly with the late Dr. E. B. PAUL. Much of the foil preparation and counting was done by Mr. D. R. PERKINSand MISS JOAN DAURIS. The calibrated 68Fe sample was prepared by Mr. I. W. GOODIER. I am indebted to Dr. P. J. CAMPION for many helpful discussions on coincidence counting, and to Mr. J. R. JOHNSONfor prolonged operation of the Van de Graaff accelerator. REFERENCES 1. 2. 3. 4. 5.

6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

16. 17. 18.

19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.

ANDERSONL. L. (1964) Health Phys. 10,315. ANL, 5800 (1963) Reactor Physics Constants, 2nd Edn. AXTON E. J. (1963) Reactor Sci. and Tech (J. Nucl. En Parts A/B) 17, 125. BAUMANNN. P. (1963) Rep. No. DP-718. BECKURTSK. H., BROSEM., KNOCHE M., KRUGER G., PONITZ W. and SCHMIDTH. (1963) Nucl. Sci Engng 17, 329. BNL 325 (1958) Neutron Cross Sections, 2nd Edn. BNL 325 (1965) Neutron Cross Sections, 2nd Edn. Supplement No. 2. BREITENHUBER L. and PINTERM. (1968) Rep. No. EANDC(OR) 68 “L”. CAMPIONP. J. and MERRITT W. F. (1957) Rep. No. CRP-745. CAMPIONP. J. (1959) Znt. J. Appl. Rad. Isotopes 4, 232. CAMPIONP. J., TAYLOR J. G. V. and MERRITT J. S. (1960) Znt. J. Appl. Rad. Isotopes 8, 8. CARRE 3. C. and VIDAL R. (1966) IAEA Conference on Nuclear Data, Paris. CARTER P. (1966) Conference on Radiation Measurements in Nuclear Power. Berkley Glos. Paper No. 54. DAHLBERGR., JIRJ..~WK. and JOHANS~~NE. (1961), J. Nucl. Energy. EMERY J. F. (1965) J. Znorg. Nucl. Chem. 27,903. FABRY A. and DEWORM J. P. (1967) Rep. No. EANDC(E) 76 “U”. FINK R. W., JOPSONR. C., HANS MARK and Swm C. D. (1966) Rev. Mod. Phys. 38,513. GEIGER K. W. and VAN DER ZWAN L. (1967) Nukleonik 10,277. KAPPE D. S. (1965) Pennsvlvania State Universitv dissertation abstract 27B 919. KATZ L. and PENFOLDA. ‘s. (1952) Rev. Mod. Phys. 24,28. KOHLERW. and KNOPF K. (1966) Z. Naturf 21a, 829. LANE R. 0. and ZAFPARANOD. J. (1954) Phys. Rev. 94,960. LEDERERC. M., HOLLANDERJ. M. and PERLMANI. (1967) Table of Isotopes. 6th Edn. Wiley, New York. ORVINI E. and MAXIA V. (1967) Energia nucleare 14, 541. PINONCELLIB. and FABRY A. (1967) Rep. No. EANDC(E) 76 “U”. ROSE M. E., PERRYC. L. and DISMUKEN. M. (1953) Rep. No. ORNL-1459. RYVEST. B. and PAUL E. B. (1968) J. Nucl. Energy 22, 759. RAKEST. B. (1969) Rep. No. EANDC(UK) 11OAL. SIEGBAHNK. (1955) Beta and Gamma Ray Spectrosocopy. North-Holland, Amsterdam. WALKER W.M., WE~TCOIT C.H. and ALEXANDERT.K. (1960) Canad J. Phys 38, 57.