A method for the study of short-lived nuclear isomers

3950

Notes I

I

I

+45

I

I /

A

+10

_o

+5

K

N -5

I

I

200

250

,I

,

300 Wavelength

1

I

3,50

400

, m F.

Fig. 1. Optical rotatory dispersion of [Rh(phen)2Cl2] +. It seems likely that the attempt to resolve [Rh(phen)2Cl2] + by diastereo isomers fails neither because of the thermal instability of the ion nor because of catalytic racemization by rhodium(I). Instead the solubility difference of the diastereo isomers must not be sufficient to allow separation.

Acknowledgement-This research was carried out under Grant G M 11989 from the National Institutes of Health. D O N A L D E. SCHWAB J O H N V. R U N D

Department of Chemistry University of A rizona Tucson, Arizona 85721 U.S JI .

J.inorg. nucl. Chem., 1970, Vol. 32, pp. 3950 to 3953.

Pergamon Press.

Printed in Great Britain

A method for the study of short-rived nuclear isomers (Received 24 April 1970) THE MEASUREMENT of nuclear isomerism is important for several reasons. It allows the transfer of angular momentum in nuclear reactions and the spin dependence of the nuclear level density to be studied, while, once determined, the variation of the isomeric cross-section ratio with incident particle energy may be used to measure that incident energy. It is thus important that the isomeric cross-section ratio should be capable of accurate measurement. For nuclear isomers having half-lives of 1 rain or less the isomeric cross-section ratio is often known very inaccurately, if at all. We have developed a technique for measurement of the isomeric cross-section ratios of these short-lived isomers and have studied the isomer aSK produced by the agK(n, 2n)3SK react/on using 14.7 MeV neutrons. Here the half-life of the isomeric state of 3aK is 0.95 sec and that of the ground state 7.7 min. We have been unable to find any other measurement of this isomeric ratio.

Notes

3951

EXPERIMENTAL 14"7 MeV neutrons were produced by bombarding a tritiated titanium target with deuterons accelerated in the University's 500 KeV Van de Graaff accelerator. The experimental arrangement is shown in Fig, 1. The sample was placed in close proximity to the tritium target, with a shielded 2 in. × 2 in. NaI(T1) scintillation detector shielded by 1.5 cm of

Deuteron beom

detec~t~A Tritium forget !

---~

.....

I

I

Pb

\

/

2 cm --

"~ .........

Somple

/

To

0

onalyser Scinfillotion defector 20 cm

.

Fig. 1. Schematic diagram of experimental arrangement. lead placed so that it received 3,-radiation only from the irradiated sample. The scintillation counter was connected via a single-channel analyser to a 120-channel pulse-height analyser used in the multiscale mode. The neutron output was monitored using the associated a-particle technique[l]. A solid-state surface-barrier detector was placed at the end of the evacuated arm A to detect a-particles corresponding to the neutrons produced on bombarding the tritium target. The output of the c~-detector was thus used to measure the number of neutrons produced/sec at the tritium target. PROCEDURE A simplified decay scheme for *8K[2] is shown in Fig. 2. Both isomeric and ground states decay with fl+-emission and the resulting anihilation radiation was counted in each case with the same geometry. In this way the counter etiiciencies cancel when the isomeric cross-section ratio is calculated. The single-channel analyser was calibrated with sources of 13rCs and 54Mn and was then set to allow the multichannel analyser to multiscale 0.511 MeV anihilation radiation. In both cases the multichannel analyser was switched on near the end of the irradiation. The accelerating voltage of the Van de Graaff was then shorted out, and the multichannel analyser automatically plotted a decay curve for the irradiated sample. Preliminary tests were performed to ascertain both the cut-off time of the Van de Graaff (5 ms), which sets the ultimate limit on the shortest half-life measurable by the technique, and the multiscale spectrum after a dummy run with no sample present during the irradiation. The results of the dummy run are shown in Fig. 3(a).

1. D. Crumpton, Ph.D. Thesis (1968). 2. C. M. Lederer, J. M. Hollander and J. Parlman, Table of Isotopes 6th Edn. Wiley, New York (1967).

3952

Notes

3e~

O. 9 5 sec

O. 13 MeV

7.7 rain

0 38

19K

0 38

IB:A+ Fig. 2. Decay scheme of 38K.

(a) 800

600

400

0

...... 200 Dummy run

I00 0

I I

I 2

Time , sec

Fig. 3(a). Decay oPSmK.

.- -.-

Notes

3953

For the isomeric state the sample of spectroscopically-pure KBr was irradiated for 3 sec (i.e. to saturation). The resulting decay curve from the multichannel analyser, for 0' 1 sec per channel, is shown in Fig. 3(a). To study the ground state decay the same procedure was adopted but the sample was irradiated for 25 rain and the analyser was used with 10 see per channel (Fig. 3(b)). This decay curve contains not only the 7.7 rain half-life expected from the ground-state decay of aSK but also a half-life of 25 rain due to activation of iodine in the NaI crystal. This was subtracted from the decay curve before calculating the isomeric ratio.

6oo I 400

--~.

~ 7-7 rain

'

0200

I000

"

I

I

I

I

I

500

I

I Time

I

"

I

I

~

I

tO00

I

I

I

I

I

1500

I

, sec

Fig. 3(b). Decay of the ground state of aaK. The isomeric cross-section ratio was calculated from the results using the activation equation A = n(r~b ( 1 - e -at) where A is the activity produced by a flux of 4, neutrons/sec/cm z on a sample of n atoms and a crosssection ~r. X is the decay constant and T the irradiation time. Hence the cross-section ratio (r,~/(r, is given by tr.__~= A.m~' ~b, (1 - e-~,ro) o', A u (am (l--e-hmrm) where the suffix m refers to the isomeric state and g to the ground state. The isomeric cross-section ratio for the 39K(n, 2n)3SK reaction was found to be 0.46±0.03 for 14-7 MeV incident neutrons. Physics Department The University o f Aston in Birmingham Gosta Green, Birmingham 4 England

A. J. C O X M. J. K E A R I N