Concerning the time dependence of the decay rate of 137Cs.

TL;DR: It is found that the PTB measurements of the decay rate of (137)Cs show no evidence of an annual oscillation, in agreement with the recent report by Bellotti et al., and is consistent with the finding that different nuclides have different sensitivities to whatever external influences are responsible for the observed periodic variations.

About: This article is published in Applied Radiation and Isotopes.The article was published on 2013-04-01 and is currently open access. It has received 26 citations till now. The article focuses on the topics: Nuclide.

Summary

2. Analysis of PTB measurements

• For present purposes, the authors have found it convenient to adopt sections of 500 measurements.
• This spectrogram exhibits a strong annual oscillation from 2003 to 2005.
• The plot derived from 133Ba data is shown in Fig.

3. Discussion

• The authors next review in more detail previous reports concerning the decay rate of 137Cs.
• Thermal expansion of the lucite, which has an expansion coefficient of 7:1 10 5 cm=cm=1C (10.02, 2012), would have been the same for the two standards, and hence expansion due to temperature would not be a likely source of any seasonal variations.
• It is well known that the output of the detector is relatively insensitive to changes in the detector bias voltage, since the voltage is high enough to prevent recombination of the electron/ion pairs generated, but too low to cause electron multiplication.
• Notably, the decay rate of 226Ra shows strong evidence of an annual oscillation in the earlier dataset but weaker evidence for such an oscillation in the later dataset (Fig. 4).

4. Conclusions

• The authors purpose here has been to present new data, for 133Ba and 137Cs, which were measured on the same detector system for the same time period, where one isotope (133Ba) exhibited a clear annual periodicity and the other (137Cs) did not.
• This result, in addition to the results of decay experiments listed in Table 1, indicates that the failure to observe the annual (or other) periodicity in one isotope does not exclude that possibility in others.
• More specifically, the results presented in Table 1 in support of time-varying nuclear decay rates cumulatively represent over 60 years of data collection, in comparison to the 0.5 years of data from Bellotti et al. (2012).
• Clearly, a new series of experiments with a variety of nuclides and a variety of detectors would help to determine definitively whether the amplitudes and phases of annual oscillations are steady or variable and, if variable, the characteristics of the variability.

Figures

Fig. 1. Time–frequency display (spectrogram) of measurements of the decay-rate of 137Cs made at PTB over the time interval June 1999 to November 2008. There is only a slight suggestion of an annual oscillation from 2002 to 2003. The power, S, is represented by the color bar. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 2. Spectrogram of measurements of the decay-rate of 133Ba made at PTB over the time interval June 1999 to November 2008. There is evidence of an annual oscillation from 2003 to 2005. There is also evidence of the first harmonic of this oscillation. The power, S, is represented by the color bar. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 1 Experiments where time-dependent decay rates have been observed.

Fig. 5. Time-phase display (phasegram) of measurements of the decay-rate of 226Ra made at PTB over the time interval June 1999 to November 2008. The phase of the annual oscillation is approximately 0.36, corresponding to a peak in the modulation on or about 12 May. The power, S, is represented by the color bar. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Frequently asked questions

Q1. What are the contributions in "Concerning the time dependence of the decay rate of 137 cs" ?

In this paper, the authors present new data for 133Ba and 137Cs, which were measured on the same detector system for the same time period, for which no annual oscillatory behavior was observed. 

Q2. What future works have the authors mentioned in the paper "Concerning the time dependence of the decay rate of 137 cs" ?

This result, in addition to the results of decay experiments listed in Table 1, indicates that the failure to observe the annual ( or other ) periodicity in one isotope does not exclude that possibility in others. In light of Table 1, the authors can state in general that their studies to date suggest the following: ( a ) not all nuclides exhibit variability in decay constants ; ( b ) among nuclides that do exhibit this variability, the patterns of variability ( e. g., amplitude and phase of any oscillation ) are not all the same ; and ( c ) for nuclides that do exhibit variability, the patterns themselves may vary over time. 

Q3. How do the authors analyze the PTB data?

The authors have also analyzed the PTB measurements in terms of ‘‘phasegrams,’’ which are analogous to spectrograms, displaying the power as a function of time and phase for an assumed annual oscillation. 

Q4. What is the significance of the ionization chambers?

One of the significant factors leading to the selection of ionization chambers in these types of measurements is their inherent stability with respect to systematic and environmental effects. 

Q5. How long did the rod remain in the irradiator?

The rod was placed in the neutron irradiator for 5 min, activated, and then was placed in the counting bed and counted for fifteen minutes. 

Q6. How many sources were used to determine the decay rate of 137Cs?

The 137Cs standard set referenced above, which was used to determine system stability and calibration, consisted of nineindividual sources of 0:5 mCi each. 

Q7. What was the standard deviation of the 137Cs counts over a six month period?

Over a six month period of initial set-up and calibration, as reported in Cohn et al. (1969), the standard deviation of the 137Cs counts was 0.54%, implying good stability, and the background over the same period was 2:32 10470:56% counts/min over a 2:5 MeV spectrum window. 

Q8. What is the simplest explanation for the difference in decay rates?

Based on the care and thorough effort in the design, construction and calibration of the counting system in the Ellis experiment, aimed at reducing possible background and systematic variations, it seems probable that the observed differences in the fluctuations in the measured decay rates are intrinsic to the 56Mn and 137Cs decays themselves. 

Q9. What was the use of the counting system?

This counting system was utilized in conjunction with a broad beam neutron irradiator (Cohn et al., 1972), for the in vivo NAA experiments described in Ellis (1990), as mentioned above.