A study of some natural radio-elements of low specific activity

A description is given of a low background proportional counter system suitable for the examination of long-lived radioactive isotopes occurring in nature, or of other artificially produced isotopes with a low specific disintegration rate. An investigation of neodymium shows no evidence for the reported beta-activity and it is concluded that Nd[150] is stable against single beta decay. A slight alpha-activity, which may be characteristic of Nd[144] is observed. No experimental proof of instability in either of the 'neighbouring' isobars Re[187] and Os[187] is found. Since recent geophysical work suggests that Re[187] is beta-active, it is concluded from the results of the study that the energy of the electrons released in the transition is very small, of the order of 1 Kev or less. Assuming a first-forbidden transition this suggests that the half-life of Re[187] is 10[11] years or greater. The emission of a low intensity L x-radiation from heavy elements, under bombardment by cosmic or other background radiation, is noticed. The effects on the ultimate sensitivity of the proportional counter and some methods of mitigating the interference are considered. The radio-active transitions undergone by Lu[176] are studied in detail. The beta decay is found to have a half-life of (4.13 +/- 0.20) x 10[10] years and seems most likely to correspond to a third-forbidden transition with a spin change of 4 units. The maximum energy of the beta spectrum is 425 +/- 15 Kev. The energies of the three excited states of Hf[176], occupied in succession by the decaying nucleus, confirm predictions from the Bohr-Mottelson rotational model of the nucleus. Strong electron peaks from the internal conversion of the 89 Kev gamma ray of Hf[176] and internal conversion electrons from 190 +/- 10 Kev and 310 +/- 10 Kev gamma rays permit classification of the transitions as E.Q. The intensity of the L x-ray emission from Lutetium is consistent with the presence of an electron capture mode of decay, amounting to (9 +/- 1) % of the beta transitions. Some evidence for the emission of gamma rays following electron capture in Lu[176] is demonstrated. The ratio of the rates of emission of gamma rays and beta rays from K[40] is re-measured. Two separate investigations give values of 0.124 +/- 0.002 and 0.121 +/- 0.004 respectively, confirming the discrepancy between recent physical and geological work on the K[40] radio active transition. The half-life of the isotope is estimated to be (1.28 +/- 0.02) X 10[9] years. The energy of the gamma ray is confirmed to be 1.46 +/- 0.01 Kev. New coincidence experiments to check the accepted decay scheme of K[40] are described. Experiments to determine the amount of backscattering of electrons and positrons from thick sheets of steel and aluminium into 2pi solid angle are also treated