Lifetime measurements of certain excited states of lead isotopes
Singh, Ramjee Prasad
As described in Chapter IV, the method of lifetime measurement presented in this thesis is applicable to the El transitions of energy % 1 Mev in medium and high Z K-capture nuclei. In such cases, nuclear and atomic level lifetimes are of comparable order and hence the nuclear transitions of this type are expected to be associated with K X -ray satellites. The fact that the nuclear level lifetime can be expressed in terms of atomic level life- times permits an estimation of the former as already dis- cussed. That the satellite emission does occur has been experimentally demonstrated by the observed differences in the intensity reductions R and R' on the one hand, and R2 and R2' on the other, for two different absorbers. The intensity reductions suggest that the K X -ray satellites are certainly very close to the K -edge of the absorbers used in the experiment. The mean energy of the satellite group according to the experimental values of intensity reductions for the iridium absorber is 76.03 keV whereas for osmium it lies in the range 75.8 - 80.05 keV. These values agree well with the Wentzel- Druyvestyn type of estimate for the lead KK satellite energy (N76 keV) calculated by Slater's method(il° The experimental values of Ks = 5.97 gm,/cm.2 and 8.53 gm. /cm. 2 for iridium and osmium absorbers respectively are also within the expected limits 2.2 - 9.4 and 2.3 - 9.8 for the two absorbers. The relatively small intensity reduction observed in the case of osmium is mainly because the Ka, component of the lead K X -rays lies on the higher energy side of the osmium K edge. While calculating the satellite intensity (S) from equation (4.28), it was found that the value of S depended very sensitively on the difference between Rl and Rlt or R2 and R2t. The relatively large error in estimation of S from osmium measurements as compared with that from iridium results is in accordance with this observation.Since the osmium absorbers were made from a powder of ammonium chlorosmanate, any uncertainty in its thickness or uniformity might also have contributed to the results. It was, however, not possible to take into account the effects of these contributions.. For these reasons, more reliance may be reposed on the results of iridium than those of osmium measurements. The osmium results can then be regarded as a corroborative piece of evidence in favour of the iridium results. The general contribution of the X -ray Compton background and of the Compton back- ground in the neighbourhood of the conversion line to the coincidences observed, from which the satellite intensity was estimated has already been discussed. As explained in Section 5,11, the procedure adopted for recording coincidences was meant to eliminate the contributions from such effects. Another possibility of some contribution to the observed satellite intensity arises from the theory of Primakoff and Porter( 110) already referred to in Section 3.5.4. According to these authors, when a nucleus decays by orbital electron capture, there is a definite probability for the simultaneous excitation of a non -captured orbital electron as a consequence of sudden charge alteration. The probability per K capture for the production of a double hole in the K shell by this process is, however, very small (P(KK)~3/16Z(2)). The only nucleus in which such a process has been observed is Ge71 l for which the theory predicts a value of 10 -4 for P. For lead, P(KK) will be ~ 10(-5), and hence even if such an effect exists, its contribution to the satellite in- tensity will be obviously negligible.The mean life of the 1720 keV El transition as obtained in the present experiment from the iridium measurements is 15 x 10 -16 seconds. The single particle estimate for this transition gives Z = .54 x l0 -16 seconds. The observed value, therefore, corresponds to a retardation of about 27. As mentioned in Section 1.5, two values for the lifetime of this transition are already available. Brunner et al. have reported a retardation of about 50, while Wu et al. obtained a retardation of about 600. None of these authors have quoted errors on the results reported. But since both these results were obtained by observations on monoenergetic positrons, they are expected to involve huge uncertainties. It has already been remarked in Section 2.7 that the estimation of lifetime by this method relies on the theoretical value of monoenergetic K- positron emission coefficient (ae+) for which the results of Lombard and Rys(59) differ from those of Sliv by a factor of 2. Besides, the estimation of K- positron intensity in the presence of a large background also introduces a considerable error. The X -ray satellite method, on the other hand, used in the present experiment does not involve such uncertainties. The quantities involved in the calculations are either experimentally known or can be computed with a reasonable degree of accuracy. The value of lifetime obtained by this method is, therefore, expected to be better than that given by the method of monoenergetic positron emission.The results of the present experiment seem to favour the value reported by Brunner et al. rather than the value obtained by Wu et al. In order to see whether the observed retardation of the 1720 keV El transition in Pb206 is reasonable, reference may be made to section 1.5 where the retardation of El transitions has been briefly discussed. The relatively small retardation obtained in the present experiment rules out any possibility of K- forbiddenness as the cause. This statement is supported by Rusinov's empirical rule (eq. (1.30) , Chapter I) , regarding the K -forbiddenness of El transitions in even - even nuclei. Moreover, the nucleus 82(Pb(206)) with only two neutrons short of doubly closed shell is less likely to be described in terms of the Nilsson model. The unusually high first excited state of Pb(208) and the slow electric quadrupole transition rates in other lead isotopes provide strong evidence for the rigidity of the core and a weak surface coupling in these nuclei. The works of Alburger and Pryce( 118) , True and Ford(119) and Kearsley( 120) have shown that a very successful description of the energy levels in Pb206 up to about 3 Mev can be obtained from simple shell model considerations where the interaction between the two neutron holes is taken as a perturbation and the effects of nuclear deformations are ignored. Thus, most of the energy levels shown in the Bi206 decay scheme (Fig. 33) are accounted for in terms of simple two hole neutron configurations. The upper two levels (3403 and 3280 keV), however, have been ascribed by True and Ford to the core excitation formed, possibly by proton configurations (S1 ) 1 h/ and (d3/2) -h 9/2 respectively. Such an assignment involves a change in the orbitals of three nucleons (2 neutrons and 1 proton) which is very unlikely because many particle transitions are generally expected to be slower compared with single - particle transitions by several orders of magnitude. The result of the present experiment seems to strengthen the confidence in the shell model description of the Pb206 nucleus. An application of the method described in this work to the lifetime measurement of the 1863 keV El transition in Bi(205) may further strengthen this contention.