The probability for transitions between the first 2+ and the first 0+ excited states of O16

Nuclear Physics 66 (1965) 638--640; ( ~ North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprtnt or m i c r o f i l m without written permission from the publisher

THE PROBABILITY FOR TRANSITIONS B E T W E E N T H E F I R S T 2 + AND T H E F I R S T 0 + E X C I T E D STATES OF 016 H. FUCHS t, K. HAGEMANN and C. GAARDE Institute for Theoretical Physics, University of Copenhagen, Denmark Received 1 December 1964 The probability for y-transitions from the 6.92 MeV 2+ state in Oae to the 6.06 MeV 0+ state has been measured relative to the probability for ground-state transitions. The branching ratio obtained is R = (2.7-t-0.7) × 10 -~, in agreement with an earlier result.

Abstract:

I E

NUCLEAR REACTIONS Fee(p,u7), E = 0.874 MeV; 7(e+e-)-coin. Oxe deduced B(E2), deformation parameter.

measured

Recently there has been an increasing interest in attempts x) to describe some of the excited states of 016 in terms of rotational bands. The lowest of these is a band built on the 0 + state at 6.06 MeV, with the 2 + state at 6.92 MeV corresponding to the first rotational excitation. Decisive for this interpretation is the magnitude of the probability for radiative transitions between the two states. The corresponding 7-radiation is of very low intensity compared to the 6.92 MeV radiation to the ground state and therefore difficult to observe. The ratio R between the two transition rates has been measured by Gorodetzky et al. 2), who later 3) corrected their result to R - (2.9-1-1.1)x 10 -4, which, combined with the lifetime o f the 6.92 MeV state 4), yields a B(E2) value for the rotational transition of 10 single,particle units. Because of the general interest in the interpretation of the states in question, we made an attempt to observe the 860 keV radiation with an improved signal-tobackground ratio. The experimental procedure was essentially the same as that applied by Gorodetzky et al. 2). The 6.92 MeV level was populated through the F19(p, ~)O 16 reaction by means of the resonance at a proton energy OfEp = 874 keV. A 7.6 c m x 7.6 cm N a I crystal was employed in order to detect the 860 keV quanta in coincidence with the electron-positon pair emitted in the subsequent monopole transition. The electron pair was recorded by means o f two N E 102 scintillators in coincidence with each other. The resolving time was 2z = 18 ns, whereas the resolving time in the coincidence with the N a I detector was 2z = 30 ns. The pulses from the two N E 102 detectors were added and then pulse-height analysed in order to discriminate against pulses which did not correspond to the proper energy sum, t On leave from Hahn-Meitner-Institut ffir Kernforschung, Berlin, Germany. 638

Ole TRANSITION

639

PROBABILITIES

i.e. 6:06 M e V - 2 m 0 c2 = 5.04 MeV. If this condition was fulfilled to within __+10~o simultaneously with the two coincidence conditions, a linear gate was opened and the pulses from the NaI detector thereby transmitted to a multi-channel analyser. The thickness of the applied D y F 3 target was chosen to be about 6 keV at Ep = 874 keV, which is approximately equal to the resonance width. With this thickness one obtains a favourable ratio of the resonant y-ray production to the contribution from neighbouring resonances which populate the pair emitting oxygen state directly and thus contribute to the background. In order to lower the random coincidence rate, the beam current was reduced to approximately 0.05 pA. ~010

,.-,,

<-z-

z 30-

~rn[~

0

l

20-

rn

[0.~

Z --1 0

u 10"

0

fl n,..L,g,g r 0.5

1.0

E~ (MeV)

Fig. 1. Spectra of 7-rays coincident with electron pairs, obtained in two independent m e a s u r e m e n t s . The total accumulation time was about 30 h.

The results of two independent measurements performed in this manner are shown in fig. 1. The 860 keV transition is clearly exhibited in both coincidence spectra. The efficiency o f the pair detecting arrangement was determined in two different ways, which within their uncertainties yielded the same result, viz. 2.0 x 10 -2. One method corresponded to the procedure described in ref. 2), whereas the other exploited the ratio of coincident-to-singles counting rates in the plastic detectors. This ratio has to be corrected for the angular correlation between the electron and the positon, for which we used the measured 5) dependence W(O) ~- l + a c o s 0, with a -~ 1. By means of this relation, the efficiency can be estimated from the geometry as well, and this also yields a value consistent with the one given above. Combining this value with the v-detection efficiencies tabulated by Wolicki et aL 6), one can from the experimental yields calculate the branching ratio R = (2.7-1-0.7) x 10-*. The uncertainty indicated includes a combined standard deviation of 15~o as well as an estimate of possible systematic errors. The above value of R agrees well with the

64"0

H. FUCHS el al.

corrected value of Gorodetzky et aL a) and thus confirms the collective nature of the 860 keV transition. If the transition is interpreted as rotational, we can deduce the magnitude of the corresponding deformation of O 1~ in the 0 + band. The result is fl = 0.31_+0.07, in agreement with theoretical estimates 1). One of us (H.F.) wants to thank the Institute for Theoretical Physics for the hospitality extended to him. He is indebted to Die Deutsche Forschungsgemeinschaft for a grant. References 1) D. M. Brink and G. F. Nash, Nuclear Physics 40 (1963) 608; J. Borysowich and R. K. Sheline, Phys. Lett. 12 (1964) 219; G. E. Brown, in Proc. Int. Conf. on Nuclear Physics, Paris (1964) 2) S. Gorodetzky et al., Phys. Lett. 1 (1962) 14 3) S. Gorodetzky et al., J. de Phys. 24. (1963) 887 4) F. Ajzenberg-Selove and T. Lauritsen, Nuclear Physics 11 (1959) 1 5) S. Devons and G. R. Lindsey, Nature 164 (1949) 6) E. A. Wolicki, R. Jastrow and D. F. Brooks, Naval Research Laboratory, Washington D.C., Report 4833 (1956)