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Physics Letters B
www.elsevier.com/locate/physletb
Evidence against solar influence on nuclear decay constants
^jjjjj^ CrossMark
S. Pommé3 *1, H. Stroha, J. Paepena, R. Van Ammela, M. Marouli3, T. Altzitzogloua, M. Hulta, K. Kossertb, O. Nähleb, H. Schraderb, F. Jugetc, C. Bailatc, Y. Nedjadic, F. Bochudc, T. Buchillierc, C. Michotted, S. Courte d, M.W. van Rooye, M.J. van Stadene, J. Lubbee, B.R.S. Simpsone, A. Faziof, P. De Felicef, T.W. Jackson g, W.M. Van Wyngaardtg, M.I. Reinhardg, J. Golyag, S. Bourkeg, T. Royh, R. Galea h, J.D. Keightleyi, K.M. Ferreirai, S.M. Collinsi, A. Ceccatellij, M. Unterwegerk, R. Fitzgeraldk, D.E. Bergeronk, L. Pibidak, L. Verheyen', M. Bruggeman', B. Vodenikm, M. Korunm, V. Chisté n, M.-N. Amiotn
a European Commission, Joint Research Centre (JRC), Retieseweg 111, B-2440 Geel, Belgium b Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100,38116 Braunschweig, Germany c Institut de Radiophysique, Lausanne (IRA), Switzerland
d Bureau International des Poids et Mesures (BIPM), Pavillon de Breteuil, 92310 Sèvres, France e Radioactivity Standards Laboratory (NMISA), 15 Lower Hope Road, Rosebank 7700, Cape Town, South Africa
f National Institute of Ionizing Radiation Metrology (ENEA), Casaccia Research Centre, Via Anguillarese, 301—S.M. Galeria I-00060 Roma, C.P. 2400, I-00100 Roma A.D., Italy
g Australian Nuclear Science and Technology Organisation (ANSTO), Locked Bag 2001, Kirrawee, NSW 2232, Australia h National Research Council of Canada (NRC), 1200 Montreal Road, Ottawa, ON, K1A0R6, Canada i National Physical Laboratory (NPL), Hampton Road, Teddington, Middlesex TW11 OLW, UK
j Terrestrial Environment Laboratory, IAEA Environment Laboratories, Department of Nuclear Sciences and Applications, International Atomic Energy Agency (IAEA), Vienna International Centre, PO Box 100, 1400 Vienna, Austria
k Physical Measurement Laboratory, National Institute of Standards and Technology (NIST), 100 Bureau Dr., Gaithersburg, MD 20899-8462, USA * Belgian Nuclear Research Centre (SCK-CEN), Boeretang 200, B-2400 Mol, Belgium m Joief Stefan Institute (/SI), Jamova 39,1000 Ljubljana, Slovenia
n CEA, LIST, Laboratoire National Henri Becquerel (LNHB), Bât. 602 PC 111, CEA-Saclay 91191 Gif-sur-Yvette cedex, France
A R T I C L E I N F 0
A B S T R A C T
Article history:
Received 1 July 2016
Received in revised form 18 August 2016
Accepted 19 August 2016
Available online 24 August 2016
Editor: V. Metag
Keywords:
Half-life
Decay constant
Uncertainty
Radioactivity
Neutrino
The hypothesis that proximity to the Sun causes variation of decay constants at permille level has been tested and disproved. Repeated activity measurements of mono-radionuclide sources were performed over periods from 200 days up to four decades at 14 laboratories across the globe. Residuals from the exponential nuclear decay curves were inspected for annual oscillations. Systematic deviations from a purely exponential decay curve differ from one data set to another and are attributable to instabilities in the instrumentation and measurement conditions. The most stable activity measurements of alpha, beta-minus, electron capture, and beta-plus decaying sources set an upper limit of 0.0006% to 0.008% to the amplitude of annual oscillations in the decay rate. Oscillations in phase with Earth's orbital distance to the Sun could not be observed within a 10-6 to 10-5 range of precision. There are also no apparent modulations over periods of weeks or months. Consequently, there is no indication of a natural impediment against sub-permille accuracy in half-life determinations, renormalisation of activity to a distant reference date, application of nuclear dating for archaeology, geo- and cosmochronology, nor in establishing the SI unit becquerel and seeking international equivalence of activity standards.
© 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3.
1. Introduction
The exponential-decay law is one of the most famous laws * Corresponding author °f physics, already carved in stone since the pioneering work of
E-mail address: stefaan.pomme@ec.europa.eu (S. Pommé). Ernest Rutherford [1], Maria Sktodowska-Curie [2] and others. It
1 Fax: +32 (0)14 571 864. has withstood numerous tests [3-5] demonstrating that the de-
http://dx.doi.org/10.1016/j.physletb.2016.08.038
0370-2693/© 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3.
cay of a radionuclide can be characterised solely by a single decay constant - or equivalently by the half-life - which is invariable in space and time. However, observations of periodic oscillations in measured decay rates of radioactive sources [6-13] have been heavily debated in the last decade [6-25]. Controversy arose at two levels: (i) at the observational level, with experimental data sets showing significant differences in stability of decay rates with time, and (ii) at the interpretational level, either ascribing the observed modulations to instabilities in the detection system, or advocating new physics to explain variability in the decay constants.
As much as the instability claims attract interest as inspiration for new physical theories and applications [14,15], if true they would have major implications on traceability and equivalence in the common measurement system of radioactive substances. Variability of decay constants at permille level would limit the precision by which a half-life value could be assigned to a radionu-clide, as well as the accuracy by which the Sl-unit becquerel could be established through primary standardisation [26] and international equivalence demonstrated through key comparisons and the Système International de Référence (SIR) [27]. The implications at metrological level would eventually affect science built on the decay laws, from renormalisation of activity to a reference date for nuclear dosimetry to precise nuclear dating for geo- and cos-mochronology.
At the heart of this controversy are the metrological difficulties inherent to the measurement of half-lives [28-30]. From a metrological point of view, it is obvious that instruments, electronics, geometry and background may vary due to external influences such as temperature, pressure, humidity and natural or man-made sources of radioactivity. Claims of variability of half-lives on the basis of deviations from an exponential decay curve can only be considered when the instrumental effects have been fully compensated and/or accounted for in the uncertainty budget. Jenkins et al. [9] claim to have done so before proposing their hypothesis that permille sized seasonal variations of decay rates of 226 Ra and 36Cl are caused by solar influences on their decay constants [6-8]. Evidence has been collected to demonstrate instabilities in the decay of other radionuclides [10,11] and by means of time-frequency analysis periodicity at shorter and longer term than 1 year have been claimed [11-13]. However, this interpretation is being challenged by the publication of data sets confirming a close adherence to exponential decay with residuals in the 10-5 range [16,18,20,21, 23].
Authors of both convictions expressed the need for collecting evidence for different radionuclides measured with different detection techniques [7,11,13,18,23]. At national metrology institutes (NMls) taking responsibility for establishing the unit becquerel, mono-radionuclide sources are kept and regularly measured for standardisation purposes as well as for determining half-lives. In addition, gamma-ray spectrometry laboratories keep records of quality control measurements on their spectrometers which provide useful information on long-term trends in activity measurements of a reference source. ln this work, the hypothesis that decay constants vary through solar influence in phase with Earth-Sun orbital distance has been tested through the analysis of a unique collection of activity measurements repeated over periods of 200 days up to four decades at 14 laboratories distributed across the globe.
2. Measurements & analysis
Precise activity measurement series were performed for alpha decay (209Po, 226Ra series, 228Th, 230U, 241Am), beta minus decay (3H, 14C, 60Co, 85Kr, 90Sr, 124Sb, 134Cs, 137Cs), electron capture (54Mn, 55Fe, 57Co, 82,85Sr, 109Cd, 133Ba), a mixture of electron cap-
ture and positron decay (22Na, 65Zn, 207Bi), and a mixture of electron capture and beta minus/plus decay (152Eu). More than 60 data sets were collected, some of which were performed over several decades. Some data sets excel in precision, others reveal vulnerability of different measurement techniques to external conditions. Characteristics of the data sets are summarised in Table 1.
The measurement techniques employed are as follows: ionisation current measurements in a re-entrant ionisation chamber (IC) or a hospital calibrator (HIC) [31,32], net area analysis of full-energy y-ray peaks (and integral spectrum counting) by y -ray spectrometry with a HPGe detector (HPGe) [33], particle counting in a planar silicon detector in quasi-2^ configuration (PIPS) [34], X-ray counting at a small defined solid angle with a gas-filled proportional counter (PC) [35,36], live-timed /)-y anti-coincidence counting (LTAC) [37], triple-to-double coincidence counting with a liquid scintillation vial and three photodetectors (TDCR) [38], liquid scintillation counting (LSC) [38], particle and photon counting in a sandwich Csl(Tl) spectrometer (Csl) [39], internal gas counting (IGC) [40], and a-particle counting at a small defined solid angle with a large planar silicon detector (aDSA) [35,36]. An overview of standardisation techniques and their sources of error can be found in the special issues 44(4) and 52(3) of Metrologia [41,42] and references in [25,28].
Exponential decay curves were fitted to the data and the residuals were inspected for annual modulations. The data sets were first compensated for (1) the presence of occasional outlier values, (2) abrupt systematic changes in the detector response, e.g. due to replacement of the electronics or recalibrations of the instrument, and (3) systematic drift extending over periods of more than 1 year, e.g. due to gas loss from an ionisation chamber, uncompen-sated count loss through pulse pileup in a spectrometer, activity build-up from decay products in a source, etc. The residuals were binned into 8-day periods of the year and averaged to obtain a reduced set of (maximum) 46 residuals evenly distributed over the calendar year. To the averaged residuals, a sinusoidal shape A sin(2^(t + a)/365) has been fitted in which A is the amplitude, t is the elapsed number of days since New Year, and a is the phase shift expressed in days. The fitted amplitude values can be considered insignificant if they are of comparable magnitude as their estimated standard uncertainty (see Table 1).
3. Discussion
The controversy started with the interpretation [7,8] of A ^ 0.15% modulations in the decay rate measurements of a sealed 226 Ra reference source in an IC at the PTB between 1983 and 1998. The averaged residuals, shown in Fig. 1A, have a sinusoidal shape with amplitude A = 0.083 (2)% and phase a = 59 days. An explanation through solar influence on the alpha or beta decay constants of nuclides in the 226 Ra decay series seems unlikely, since the residuals are out of phase with the annual variation of the inverse square of the Sun-Earth distance, 1/R2 (renormalised to 0.15% amplitude in the Figs. 1-2 of this work). The real cause is of instrumental nature, since the modulations were significantly reduced after changing the electrometer of the lC [22,25]. There is a remarkable correlation with average seasonal changes of radon concentration in air ( A = 16 (2)%, a = 57 days) measured inside the laboratory from 2010 to 2016, but causality has not been proven.
At other institutes, annual modulations of smaller amplitude and different phase have been observed, which demonstrates the local character of the non-exponential behaviour. The data sets for 226 Ra show a different level of instrumental instability, but the most stable 226 Ra measurements prove invariability of its decay constant against annual modulations within 0.0025% to 0.005%. An
Table 1
Characteristics of the decay rate measurement sets analysed. The method acronyms are explained in the text. The period indicates the first and last year in which data were collected. The standard deviation is an indication of the uncertainty on the annual averaged data (maximum 46 data, covering 8-day periods), derived from the spread of the input data and the inverse square root of the number of values in each data group. The amplitude and phase are the result of the fit of a sinusoidal function to the averaged data. In bold are the amplitudes at 10-6-10-5 level. The estimated standard uncertainty on the amplitude is indicated between parentheses, its order of magnitude corresponding to that of the last digit of the value of A.
Decay Nuclide Laboratory Method Period #Data Rel. std Amplitude A Phase shift a
mode(s) (year) dev in % in % in days
a 209 Po JRC PIPS 2013-2016 1539 0.024 0.006 (5) 6
a + p- 226 Ra PTB IC 1983-1998 1973 0.011 0.083 (2) 59
a + p- 226 Ra PTB IC 1999-2016 2184 0.005 0.016 (1) 194
a + p- 226 Ra ENEA IC 1992-2015 161 0.025 0.043 (5) 324
a + p- 226 Ra NIST IC #1 2008-2016 99 0.016 0.015 (3) 255
a + p- 226 Ra NIST IC #2 2012-2016 272 0.036 0.002 (8) 8
a + p- 226Ra BIPM IC 2001-2015 136 0.015 0.004 (3) 4
a + p- 226Ra JRC IC 2005-2015 1737 0.005 0.003 (2) 363
a + p- 226Ra NPL IC #1 1993-2016 4055 0.014 0.0025 (18) 60
a + p- 226Ra NPL IC #2 1993-2016 3996 0.005 0.005 (1) 73
a + p- 226Ra NMISA IC 1992-2015 276 0.343 0.106 (60) 67
a + p- 226Ra ANSTO IC 2012-2015 700 0.015 0.005 (3) 256
a + p- 226Ra ANSTO HIC 2008-2014 1749 0.077 0.009 (18) 82
a + p- 226Ra LNHB IC #1 1998-2016 455 0.026 0.026 (6) 328
a + p- 226Ra LNHB IC #2 1998-2016 498 0.028 0.042 (7) 294
a 228Th NIST IC 1968-1978 70 0.107 0.031 (22) 327
a 230U JRC aDSA, PIPS, Csl, LSC, HPGe 2010-2011 5451 0.083 0.007 (7) 173
a 241 Am JRC PC 2004-2008 245 0.022 0.101 (16) 104
a 241 Am SCK HPGe #8 2008-2016 430 0.13 0.024 (28) 55
a 241 Am SCK HPGe #26 2013-2016 166 0.12 0.002 (23) 304
a 241 Am SCK HPGe #11 2008-2016 402 0.12 0.055 (22) 242
a 241 Am SCK HPGe #16 2008-2016 382 0.13 0.079 (26) 290
a 241 Am SCK HPGe #25 2011-2016 245 0.12 0.079 (22) 236
a 241 Am SCK HPGe #10 2008-2016 466 0.14 0.115 (27) 259
a 241 Am SCK HPGe #27 2011-2015 238 0.45 0.095 (91) 280
a 241 Am SCK HPGe #13 2008-2016 434 0.12 0.167 (26) 235
a 241 Am PTB LSC 2014-2016 574 0.004 0.0006 (7) 260
p- 3H JRC LSC 2002-2014 706 0.112 0.048 (24) 197
p- 3H NIST lGC 1961-1999 21 0.75 0.18 (20) 149
p- 14C JRC LSC 2002-2014 706 0.075 0.013 (16) 92
p- 14C NMISA TDCR 1994-2014 32 0.250 0.067 (80) 59
p- 60Co NIST lC 1968-2007 250 0.050 0.007 (7) 0
p- 60Co NIST LTAC + IC 2006-2014 26+7 0.036 0.007 (9) 18
p- 60Co JSI HPGe #1-6 1998-2016 15254 0.079 0.041 (14) 161
p- 85 Kr NIST lC 1980-2007 98 0.035 0.036 (15) 153
p- 90Sr PTB TDCR 2013-2014 4493 0.009 0.004 (2) 362
p- 90Sr PTB lC 1989-2016 2207 0.009 0.018 (2) 26
p- 124Sb JRC lC 2007 59 0.005 0.003 (2) 241
p- 134Cs JRC lC 2010-2015 1065 0.002 0.0051 (5) 48
p- 137 Cs IRA lC 1984-2012 276 0.043 0.018 (9) 342
p- 137 Cs NRC lC #1-3 1995-2009 62 0.074 0.006 (22) 147
p- 137 Cs PTB lC 1997-2016 2149 0.005 0.014 (1) 29
p- 137 Cs NIST lC 1968-2011 254 0.034 0.004 (6) 33
p+, EC 22Na JRC lC 2010-2016 443 0.003 0.0047 (6) 53
EC 54Mn JRC lC 2006-2009 156 0.007 0.005 (1) 28
EC 54Mn PTB lC 2010-2016 716 0.011 0.014 (2) 78
EC 55 Fe JRC lC 2004-2005 595 0.007 0.004 (3) 187
EC 57Co NIST lC 1962-1966 97 0.089 0.055 (22) 324
EC, p+ 65 Zn JRC lC 2002-2003 140 0.026 0.008 (4) 163
EC(,p+) 82Sr/82Rb + 85Sr NIST lC 2007-2008 158 0.011 0.0006 (27) 240
EC(,p+) 82Sr/82Rb NIST HPGe 2007-2008 23 0.46 0.073 (75) 255
EC 109Cd JRC lC 2006-2010 125 0.017 0.015 (4) 18
EC 109Cd JSI HPGe #3, 4 1998-2016 5414 0.139 0.035 (24) 346
EC 109Cd NIST lC 1976-1981 167 0.058 0.013 (15) 220
EC 133 Ba NIST lC 1979-2012 131 0.042 0.028 (8) 74
EC, p-, p+ 152 Eu IAEA HPGe #1, 2 2010-2016 143 0.113 0.020 (24) 162
EC, p-, p+ 152 Eu SCK HPGe #8 2008-2016 1228 0.10 0.006 (19) 242
EC, p-, p+ 152 Eu SCK HPGe #26 2013-2016 499 0.10 0.027 (23) 178
EC, p-, p+ 152 Eu SCK HPGe #11 2008-2016 1168 0.10 0.048 (21) 280
EC, p-, p+ 152 Eu SCK HPGe #16 2008-2016 1260 0.08 0.062 (18) 285
EC, p-, p+ 152 Eu SCK HPGe #25 2011-2016 723 0.10 0.080 (23) 213
EC, p-, p+ 152 Eu SCK HPGe #10 2008-2016 1374 0.08 0.094 (16) 206
EC, p-, p+ 152 Eu SCK HPGe #27 2011-2015 698 0.16 0.155 (34) 228
EC, p-, p+ 152 Eu SCK HPGe #13 2008-2016 1249 0.11 0.161 (24) 214
EC, p-, p+ 152 Eu NIST lC 1976-2011 96 0.040 0.021 (9) 214
EC, p-, p+ 152 Eu PTB lC 1989-2016 2199 0.007 0.018 (1) 11
EC(,p+) 207 Bi NIST lC 1971-2011 152 0.05 0.004 (11) 23
Fig. 1A. Annual average residuals from exponential decay for 226 Ra activity measurements with an IC at PTB from 1983 to 1998. The line represents relative changes in the inverse square 1/R2 of the Earth-Sun distance, normalised to an amplitude of 0.15%.
Fig. 1B. Same for 226Ra activity measurements with the Vinten IC of NPL from 1993 to 2016, after renormalisation per calendar year.
example is shown in Fig. 1B, comprising 40 0 0 226 Ra ionisation current measurements over a period of 22 years at the NPL.
Stability is best achieved where the detector efficiency is least influenced by geometrical and environmental variations and where the signal of the radiation is easily separated from interfering signals and electronic noise. For example, measuring 241Am decay through alpha-particle detection with close to 100% detection efficiency would typically be more stable than through fractional detection of its low-energy photon emissions in a gas-filled proportional counter. For the alpha emitters, 209Po, 226Ra, 230U, and 241Am, the invariability of the decay constants was confirmed within the 10-5 level.
Comparably lower stability could be anticipated for beta-minus decay. Parkhomov [10] found 7 data sets of beta-decaying radionu-clides exhibiting periodic variations of 0.1% to 0.3% amplitude with a period of 1 year. Fischbach et al. [8,14,15] suggested new theories in which the variable flux of anti-neutrinos from the Sun would significantly modulate the probability for emission. From metrological point of view, instability in the detection efficiency for a pure beta emitter can be expected due to the continuous energy distribution of the beta particle which makes the count rate subject to threshold variations at the low-energy side and possibly
Fig. 2A. Annual average residuals from exponential decay for 134Cs activity measurements with the 1G12 IC at the JRC from 2010 to 2016.
Fig. 2B. Same for22 Na.
incomplete detection probability at the high-energy side. However, measurements based on y-ray emission subsequent to the emission - possibly through the decay of a short-lived daughter nuclide - can be made more robust.
High-quality measurement data were collected for emitters in Table 1, mostly obtained by IC but also with primary activity measurement techniques such as the triple-to-double coincidence ratio (TDCR) method and live-timed 4nfi -y anticoincidence counting (LTAC). It was demonstrated for 36Cl [20], 60Co (Table 1) and 90Sr/90Y [23] that primary standardisation techniques like TDCR and LTAC are more stable than routine counting techniques, because each measurement provides information about the detection efficiency and automatically corrects for its fluctuations. Some IC measurements show remarkable stability, too, and refute the conclusions made about variability of the decay constants as well as the hypothesis of a significant solar influence on the decay rate. 1n Fig. 2A, averaged residuals for 134Cs in an 1C demonstrate stability within the 10-5 range. Evidence of stability down to the 10-5 level was found for the beta minus emitters 60Co, 90Sr, 124Sb, 134Cs and 137Cs, and down to the 10-4 level for 3H, 14 C, and 85 Kr. These results are in direct contradiction with the permille level oscillations for 3H, 60Co, 90Sr, and 137Cs reported by Parkhomov [10] and Jenkins et al. [11].
E 0.10
241Am-152Eu (SCK)
Annual oscillations
0.05 0.10
241 Am amplitude A (%)
Fig. 3. Amplitude of average annual oscillations in the decay rates of 241 Am and 152Eu measured by y-ray spectrometry with 8 HPGe detectors at SCK between 2008 and 2016. The index refers to the detector number. A mixed 241Am-152Eu point source was measured 166-466 times in a fixed geometry at about 11 cm from the endcap using the 59 keV line of 241 Am and the 122 keV, 779 keV and 1408 keV lines of 152Eu.
Radionuclides disintegrating by electron capture (EC) and p+ decay - 22Na, 54Mn, 55Fe, 57 Co, 65Zn, 82Sr/82Rb+85Sr, 109Cd, 133Ba, 152Eu, and 207Bi - were investigated by the same techniques as a and p- emission and, also here, stability within the 10-5 to 10-4 range was observed in most cases. An example is shown in Fig. 2B for 22Na measured in the same period with the same 1C as 134Cs in Fig. 2A. The tiny modulations in the residuals for both nu-clides are highly correlated, which is most likely a seasonal effect of instrumental origin. Clear evidence of annual modulations being of instrumental origin has been found in thousands of y -ray spectrometry measurements with 8 HPGe detectors at the SCK, as shown in Fig. 3: the modulations in measured decay rates for the alpha decay of 241 Am and mixed EC, p-, and p + decay of 152Eu are highly correlated but the amplitude differs from one detector to another. In other words, the modulations are linked to the instrument, not to the type of decay.
4. Conclusions
The experimental data in this work are typically 50 times more stable than the measurements on which recent claims for solar influence on the decay constants were based. The observed seasonal modulations can be ascribed to instrumental instability, since they vary from one instrument to another and show no communality in amplitude or phase among - or even within - the laboratories. The exponential decay law is immune to changes in Earth-Sun distance within 0.008% for most of the investigated a, p-, p+ and EC decaying nuclides alike.
Owing to the invariability of decay constants, there is no impediment to the establishment of the becquerel through primary standardisation at 0.1% range accuracy nor to the demonstration of equivalence of activity at international level over a time span of decades. 1t is normal for repeated activity measurements to show varying degrees of instability of instrumental and environmental origin and such auto-correlated variability should be taken into account next to statistical variations when setting alarm levels in quality control charts. Taking into account such instabilities and adhering to proper uncertainty propagation, no fundamental objections need to be made against half-life measurement with sub-permille uncertainties, nor against applying exponential decay
formulas to calculate activity at a future or past reference time or to perform accurate nuclear dating.
Acknowledgements
The authors thank all past and present colleagues who contributed directly or indirectly to the vast data collection over different periods spanning six decades.
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