Вариабельность постоянной распада для электронного захвата наблюдалась в эксперименте, но она лежит в пределах процента во всём доступном в лаборатории диапазоне давлений и температур. Период полураспада в этом случае изменяется в связи с некоторой (довольно слабой) зависимостью плотности волновой функции орбитальных электронов в окрестности ядра от давления и температуры. Существенные изменения постоянной распада наблюдались также для сильно ионизованных атомов (так, в предельном случае полностью ионизованного ядра электронный захват может происходить только при взаимодействии ядра со свободными электронами плазмы; кроме того, распад, разрешённый для нейтральных атомов, в некоторых случаях для сильно ионизованных атомов может быть запрещён кинематически). Все эти варианты изменения постоянных распада, очевидно, не могут быть привлечены для «опровержения» радиохронологических датировок, поскольку погрешность самого радиохронометрического метода для большинства изотопов-хронометров составляет более процента, а высокоионизованные атомы в природных объектах на Земле не могут существовать сколько-нибудь длительное время.
// Период полураспада — Википедия
Поиск возможных вариаций периодов полураспада радиоактивных изотопов, как в настоящее время, так и в течение миллиардов лет, интересен в связи с гипотезой о вариациях значений фундаментальных констант в физике (постоянной тонкой структуры, константы Ферми и т. д.). Однако тщательные измерения пока не принесли результата — в пределах погрешности эксперимента изменения периодов полураспада не были найдены. Так, было показано, что за 4,6 млрд лет константа α-распада самария-147 изменилась не более чем на 0,75 %, а для β-распада рения-187 изменение за это же время не превышает 0,5 %[2]; в обоих случаях результаты совместимы с отсутствием таких изменений вообще.
The changes of decay rates of radionuclide 111In (electron capture) and 32P (β decay) induced by external mechanic motion are studied. The results indicate that, in the external circular rotation in clockwise and anticlockwise centrifuge on Northern Hemisphere (radius 8 cm, 2000 r/min), the half life of 111In compared with the referred (2.83 d) is decreased at 2.83% and increased at 1.77%, respectively; the half life of 32P compared with the referred (14.29 d) is decreased at 3.78% and increased at 1.75%, respectively. When the clockwise and anticlockwise rotations increase to 4000 r/min, the half life of 111In is decreased at 11.31% and increased at 6.36%, respectively; the half life of 32P is decreased at 10.08% and increased at 4.34%, respectively. When the circular rotation is removed, the decay rates of 111In and 32P, return back to the referred, respectively. It is found that the external circular rotations in clockwise and anticlockwise centrifuge selectively increased and decreased the decay rates of 111In and 32P, respectively, and the effects are strongly dependent on the strength of circular rotation. It is suggested that these effects may be caused by the chiral interaction
Early workers tried to change the decay constants of various members of the natural
radioactive series by varying the temperature between 24K and 1280K, by
applying pressure of up to 2000 atm, by taking sources down into mines and up
to the Jungfraujoch, by applying magnetic fields of up to 83,000 Gauss, by
whirling sources in centrifuges, and by many other ingenious techniques. Occasional positive results were usually understood, in time, as the result of changes
in the counting geometry, or of the loss of volatile members of the natural decay
chains.
PERTURBATION OF NUCLEAR DECAY RATES
G. T. EMERyl
Physics Department
Indiana University, Bloomington, Indiana
CONTENTS
1. INTRODUCTION . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... 165
2 . DECAY MODES DlRECfLY INVOLVING BOUND ELECfRONS.... 167
2.1 ELECfRON CAPTURE........................ . . ...... . . . . . . . . . . . . . ... 167
2.2 INTERNAL CONVERSION. . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . .. 169
2 .3 MATRIX-ELEMENT EFFECfS AND KINEMATIC EFFECfS. ................. " 175
3. CASES STUDIED. .................................................. 175
3.1 7Be (EC, to>., ESCA)... . . . . . . . . ... ................................ 176
3.2 i7Fe [IC, to>', to(N/M), ME].. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... 178
3.3 8DZr (EC, to>.); 86Sr (EC, to>.)... . . . . ......... . . . . . . . . . . . . . . . . . . ...... 180
3.4 DONb (IC, to>.).... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... 181
3.5 DOTe (IC, to>.)....... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... 182
3.6 119Sn [IC, to(O/N), ME]...... . . . . . . . . . . . . . . . . . . . . . . . . .... .... . . . ... 183
3.7 126Te [IC, to>', to(O/N), ME]. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... 184
3.8 160Tm [IC, to(P/O), ME]........................................... 185
3.9 ID3Pt (IC, to>.)... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... 186
3 .10 la6U (Ie, to>.)..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... 186
3 .11 OTHER CASES... . ............. ...................... ............. 187
4. MACROSCOPIC WAYS OF CHANGING THE RATES OF ELECTRON
CAPTURE AND INTERNAL CONVERSION. ........................ 189
4.1 CHEMICAL STATE... . ......, ........... . . . . .......... . . . . . . . . ...... 189
4.2 PRESSURE.... .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... 191
4.3 SUPERCONDUCTiVITY. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... 191
4.4 INTERNAL ELECTRIC AND MAGNETIC FIELDS.. . .. . . . . . . . . . . . . . . . . . . . ... 192
4 .5 TEMPERATURE... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... 193
4.6 PLASMAS. ........ . .............. ......... . . . ........, . . . . . . . . . . . .. 193
5. SPECULATIONS AND POSSlBILmES...... . . . . . . . . . . . . . . . . . . . . . . . . . 194
5.1 ALPHA DECAY, BETA DECAY, AND FISSION......... ....... . . . . . . . . . . ... 194
5.2 GAMMA-RAY EMISSION AND HIGHER-ORDER PROCESSES........ .... . . . ... 195
5.3 OTHER POssIBILmES...... . . . . . . .......... . . . . . . .............. ...... 196
SUPERCONDUCTIVITY
Gentle as the superconducting transition is, its effects on total internal conversion rates havc apparently been observed in the cases of 99Tc (149) and 90Nb
(145). In both cases the samples were held at 4.2°K, and comparisons were made
with and without an applied magnetic field large enough to destroy superconductivity. The effects werc relatively large, 0.5 X lO-s and 2.0XI0-s, respectively,
and of opposite sign. The application of the magnetic field decreased the rate in
Tc and increased it in Nb. In both cases s and d valence electrons are available
for conversion. The difference in sign of the effect is probably related to the dominance of p-electron conversion in the E3 Tc transition, in contrast to the more
or less equal contribution of S1/2 and P3/2 bound states in theM2 Nb transition.
It is possible, but less likely, that the change in occupancy of the 4d orbital, from
less than half-filled in Nb metal to more than half-filled in Tc metal, is relevant.
No convincing explanation of the magnitudes of the effects has been presented.
An experiment by Snyder (227) on the temperature dependence of the l19Sn
Mossbauer-effect isomer shift did not show a discontinuity across the superconducting phase transition, but the upper limit is not inconsistent with the
rate-change effects (24).
An influence of superconductivity on the half-life of 90mNb has been studied
by Olin & Bainbridge (145). Sources prepared in a manner similar to those in
(143) were held at liquid helium temperature. A 4-kG magnetic field was used
to quench the superconductivity. When the field was removed, the recovery of
the 122-keV gamma-ray intensity to equilibrium followed. It was found that
90mNb decays more slowly in the superconducting state than it does when the
superconductivity is quenched by a magnetic field: >..(normal)->..(superconducting)= (0.195 ± 0.055) 10-2 >..(normal). An earlier attempt by Cooper (146), in
which the superconducting transition was induced by temperature change and
observed by flux explusion, led to an upper limit of ",0.2X10-2 for I Ll>"I I>".
The influence of temperature and the superconducting transition on this
lifetime were studied by Byers & Stump (149). No difference was found between
the decay constant at 77°K and 293°K. At 4.2°K measurements were made without
magnetic field, and thus in the superconducting state, and with a field of 5.3
kG (normal state) with the results X(4.2°K, superconducting)-X(293°K)
= (0.64± 0.04) lO-a X(293°K), X(4.2°K, normal)-X(293°K)= (0.13 ± 0.04) lO-a
X(293°K). The sign of the superconducting effect is opposite to that found for
gQmNb (145) (see Sec. 4).
PRESSURE
With increasing pressure, valence-electron densities in the region of the
nucleus generally increase, thus increasing most capture and conversion decay
rates. Experiments of this type have been done with 99Tc (150, 151) and 90Nb
(146), and estimates of the expected size of the effect have been made by Porter &
McMillan (152). In addition to the direct squeezing of the wavefunctions, a pressure increase can result in transfer of electrons from one band to another (for
example, from an s band to a d band) ; such effects are important in the analysis
of pressure-induced isomer shifts (224-226).
The effect of high pressure on the decay rate of 90mNb was investigated by
Cooper (146) who found that >"(0.1 megabar)->"(0) = (6.3 ± 7) to-3 }"(O).
Effects of compression on the decay rate of 99mTc in Tc metal were studied
by Bainbridge (150), who found that X(O.1 megabar)-X(O)= (0.23 ± 0.05) 10-a X(O),
and by Mazaki, Nagatomo & Shimizu (151), who found X(O.1 megabar)-X(O)
= (0.46 ±0.23) lO-a X(O). Effects of just this order of magnitude were calculated
for this case by Porter & McMillan (152).