NOTES

THE CROSS SECTION FOR THE NP8(n, a)Fes GROUND-STATE TRANSITION

Bayinan et al. (1961~) have ineasured the cross sections for (9, a ) reactions leading to the ground state for several nuclides in the f7/2 shell with 18-iVIev protons and have interpreted their results in terins of a triton pickup. Their data show that the cross section for even mass-number targets is a factor of 20 to 50 times larger than for odd mass-number targets and their calculation, assuming lowest seniority for nucleons outside closed shells, is able to explain most of this ratio. However, an even-odd ratio of a factor of 3 to 8 still reinains and they suggest that a detailed DWB calculation will be needed to check further the piclcup hypothesis.

The ineasureinent of similar (n, a ) cross sections is complicated by the large background of alpha particles produced in the detector itself by the neutrons and has not yet been extensively undertaken. By trying a case which is favor- able froin the experiinental point of view we have, however, shown that such ineasureinents inay be possible. We used a 1-mg/cm2 natural niclcel target and a solid-state junction-counter as detector. Two proportional counters were placed between the nickel target and the alpha detector and appropriate pulses froin thein were used to gate the channel analyzer which recorded the alpha spec- trum. Our neutrons had a mean energy of 14.7 Mev.

The higher-energy end of the spectrum is shown in Fig. 1. The background without the nickel target is also shown. The ground-state transition in Ni" is resolved from the excited-state transitions and we have estinlated froin the number of counts and from the nuinber of neutrons that the cross section for this transition is lf 0.3 mb. From our geoinetry we can say that this cross section is integrated over limits which are a t inost O0 to 90'. We can set a better limit by observing that the alpha counts appear within a 400-liev interval. Froin the kineinatics of the T(d, n)He4 and the Ni59(n, a)FeSS reactions we find that this observed width corresponds to limits of 0" to 45'. Thus we measure the cross section integrated between 0" and 45' to be l f 0.3 rnb for the NiS8(n, a)Fe5j ground-state transition a t 14.7 Mev.

T o compare this with the (p, a ) cross section we integrate the Nij8(p, a)Co6j ground-state data of Bayinan et al. (1961b) over the liinits O0 to 45' and find 1.5f 0.5 inb for the (p, a ) cross section over these limits; the error represents our uncertainty in the integration and not the error in the original data. Thus the (n, a ) and ( p , a ) cross sections for NiS8 are similar. T o estiinate the ratio of these cross sections on the basis of piclcup reactions we use the expressions

*Present address: C.R.S.R., Cornell University, Ithaca, N.Y., U.S.A.

Canadian Journal of Physics. Volume 41 (1963)

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FIG. 1. The measured spectrum of the alpha particles from (n, CX) reactions in a target of natural nickel a t a n incident neutron energy of 14.7 Mev.

100.L

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80-

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quoted by Bayman et al. and assume that the integrated product of the wave functions for the picked-up neutron and two protons in the (n, a ) case is identical with that for the two neutrons and one proton in the ( p , a ) case. The ratio then becomes equal to the ratio of the spectroscopic factors n~ultiplied by the velocity factors and by the penetrabilities of the bombarding particles. This gives a predicted ratio of the (n, a ) cross section to the ( p , a ) cross section of 0.8 with an error which is primarily due to this assumption and which may be as large as a factor of three. Thus in the case of Ni58 a pickup mechanism predicts that the (n, a ) and ( p , a) reactions should have about the same cross sections, as is in fact observed. I t should be noted also, however, that a process involving a knockout of a preformed alpha cluster would predict a ratio which is mainly dependent on the penetrabilities; these are in the ratio of 1.5 for the 14.7-Mev neutrons to the 18-Mev protons. Again a considerable uncertainty is introduced into the expected ratio of the cross sections because of the different nature of the ground states produced by the two reactions. The measurements therefore agree with either a pickup or a knock-on mechanism.

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BAYMAN, B. F., BRADY, F. P., and SHERR, R. 1961a. Proceedings of the Rutherford Jubilee International Conference (Heywood, London), p. 553.

1961b. Private communication.

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