PHYSICAL REVIEW E VOLUME 49, NUMBER 1 JANUARY 1994 Fluctuations in radioactive decays. II. Experimental results Roberto Boscaino and Giorgio Concas Dipartimento di Scienze Fisiche, Universita di Cagliari, via Ospedale 79, I 091-2$ Caglian, Italy Marcello Lissia Istituto Nazionale di Fisica Nucleare, via Negri 18, I-09127 Cagliari, Italy Sergio Serci Dipartirnento di Scienze Fisiche, Universita di Cagliari, via Ospedale /g, I 09Igg-Cagliari, Italy and Istituto Nazionale di Fisica Nucleare, via Negri 18, I-09127 Cagliari, Italy (Received 2 August 1993) We experimentally investigate the decay statistics of a p source of 50Sn. We Snd that, for counting periods of the order of 10 h, the variance is higher than the Poissonian value by more than one order of magnitude. A careful study of the relative Allan variance and the measurement of the generalized Allan variance demonstrates that the observed anomaly is uot caused by 1/f noise. Rather, even if counting periods are much shorter than the nucleus lifetime, the variance excess is a consequence of the nonequilibrium nature of the decay process. This conclusion is supported by the quantitative agreement between our experimental data and the theoretical formulas derived in the preceding paper [Boscaino et al. , Phys. Rev. E 49, 333 (1994)). PACS uumber(s): 05.40. +j, 23.20. — g, 02.50. — r, 29.90.+r I. INTRODUCTION The decay of nuclei through the emission of particles or radiation is often cited as an archetype of the sim- ple Poisson process, since the unit-time probability of the decay events is time independent and the events are uncorrelated Rom each other. Nevertheless, in the past decade, it has been conjectured that the statistics of the particle counting in a nuclear decay experiment could deviate from the pure Poisson behavior because of the existence of additional sources of fiuctuations superim- posed onto the intrinsic shot noise: essential 1/f noise of the emission rate, Lorentzian noise, originating from cooperative eEects, and other detector-related nontrivial noise sources, e. g. , solid angle Buctuations and spatial 1/f noise in track detectors [1 — 7]. In the past ten years, many authors have studied ex- perimentally the counting statistics of emitted particles or photons and compared their results to the standard Poisson theory. Results have been confhcting even for the same type of source. Experiments on n sources ( 49sAm [8 — 10] and zs~40Po [11]) and on a P source of ssCs [12] have confirmed the Poissonian nature of these decay processes. Conversely, a counting variance in ex- cess of the Poisson value had been measured, for long counting periods, in experiments on the o. decay of 2ssAm, s4Pu, and 49sCm [7, 13 — 15] and on the P decay of sfT1 and suY [16,17]. In the preceding paper [18], hereafter referred to as I, we have shown theoretically that the statistics of the decay, as measured by counting the particles emitted by a system of N decaying centers in a given period T, re- veal an additional contribution to the variance due to the exponential decrease of N in time. The relevant feature of this contribution is that it manifests itself even for counting periods T much shorter than the nu- clear lifetime. As shown in I, this aspect has been over- looked [8,9, 11 — 14, 16,17] or underestimated [10,15] in pre- vious investigations. The present paper is intended as the experimental counterpart to I. We report experimental results on the statistics of the 5pSn nuclear decay. We 6nd that the relative Allan variance of the emitted p-photon counts, for periods longer than a few hours, is higher than the Poisson value, and the excess ratio amounts to more than one order of magnitude for periods of the order of 10 h. The agreement between our experimental results and the theory derived in I is quantitative, thus strongly support- ing our explanation of the phenomenon [19]: the nonequi- librium nature of the decay process is responsible for the excess of variance. In Sec. II we describe the source and the experimental apparatus. The experimental results and their statistical analysis are reported in Sec. III, where the comparison with the theoretical predictions is also carried out. Fi- nally, Sec. IV is reserved for our conclusions. II. EXPERIMENTAL SETUP A. The source ln our experiments we use a crystal of CaSn03 contain- ing 5pSn nuclei. These nuclei are in a metastable state with an energy of 89.5 keV above the ground state and a lifetime r = 422. 7 days [20,21]. The decay scheme of the nucleus &om the excited to the ground state is shown in the inset of Fig. 1 [20]. The first step is an isomeric 1063-651X/94/49(1)/341(6)/$06. 00 49 341 Q~ 1994 The American Physical Society