The half-life of the second excited state in Cs133

Abstract

To obtain spins and parities of the levels of Cs 13a many investigations were performed on the decay of both Ba iza and Xe laa [1]. It has been demonstrated that for three out of the first four exeited states of Cs laa reliable spin and parity assignments [2] might be aehieved. This is true especially for the first and the third exeited levels where the values 5/2 + and 3/2 + are reliably established.It is doubtless that the most problematic level is at 161 keV, i.e. the second exeited level. Aecording to certain measurements [3, 4] spin 3/2, while in others [2, 5, 6] 5/2 are supposed. The problem is even more interesting since Cs laa is near the double elosed shells and the investigation of sueh kind of nuelei is very promising theoretically also.It would be very useful in the given situation ir there were any available experimental data referring to the lifetime of the 161 keV level. Unfortunately, up to now only the approximate estimation of BODENSTEDT et al. [5] has been known in literature aecording to which Tll 2 < 5 9 10 -10 sec.That was the reason why we decided to measure the lifetime of the seeond excited state of this isotope.The measuring instrument eonsisted of a time to amplitude convcrter with slow-fast system. The signals of two NaJ(TI) crystals in connection with two 6810/A photomultipliers were connected with a fast coincidence circuit with tunnel diodes [7]. The slow part of the eonverter was the usual one, attached to a multiehannel analyzer.The resolving time of this instrument was 2 z = 3,0-10 -9 see for Co 8o isotope.Measuring the lifetime of the seeond exeited level, the slow parts were set to the 161 keV and 276 keV energies, respeetively. It is clear from the level seheme that no other eascades interfered in the measurement of this level. The transition being weak, the chance eoincidences were reduced by placing a lead and tin shield between the source and the erystals. The lead shield made a cut off about 200 keV and the tin redueed the intensive 81 keV peak. We used Co 6o isotope asa prompt gamma souree.

Transition Probabilities of Excited Nuclear States in133Cs

“…After correcting the measured centroid-shift (Fig. 3) for the shift due to different single counting rates for the two sources ( -2 1.2 ps), our final result for the 160.5 keV level was T + = 1 9 0 i 1 5 PS This result is not in agreement with the previous values obtained by gamma-gamma coincidences [13] (97 k26 ps), and electron-gamma coincidences [12] (85 i 1 6 ps), where the electrons were detected by a plastic scintillator (NE 102). In the Physica Scripta 2 present work five independent measurements were made, each of them consisting of about twenty centroid-shifts, all giving consistent results.…”