кто же "за"

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\section{\label{sec:introd} INTRODUCTION}

Since the early days of nuclear science, nearly a century ago, it has been widely accepted that the decay constants of radioactive isotopes decaying by $\alpha$, $\beta^-$ or $\beta^+$ emission are independent of all physical or chemical conditions such as pressure, temperature and material surroundings. This belief was based on numerous measurements in the early 1900Тs, some of which claimed remarkable precision (see \cite{Em72} for an interesting review): for example, Curie and Kamerlingh Onnes \cite{Cu13} in 1913 determined that the decay constant of a radium preparation did not change by more than 0.1\% when cooled to 20K. In contrast, decays proceeding by internal conversion or electron capture (ec), to which atomic electrons contribute directly, were placed in a different category, being potentially susceptible to their chemical --- though not physical --- condition. There is a long history of $^7$Be decay measurements that demonstrate small but detectable effects on that isotope's decay constant caused by its chemical environment.

Quite recently, however, measurements have been reported claiming relatively large changes in half lives for $\alpha$, $\beta^-$, $\beta^+$ and ec decays depending on whether the radioactive parent was placed in an insulating or conducting host material, and whether the latter was at room temperature or cooled to 12K. Specifically, $^{210}$Po, an $\alpha$ emitter, when implanted in copper was reported to exhibit a half life shorter by 6.3(14)\% at 12K than at room temperature \cite{Ra07}; the $\beta^-$ emitter $^{198}$Au in a gold host reportedly had a half-life longer by 3.6(10)\% at 12K \cite{Sp07}; $^{22}$Na, which decays predominantly (90\%) by $\beta^+$ emission, was measured as having a 1.2(2)\% shorter half life at 12K \cite{Li06}; and $^7$Be, which decays by pure electron capture, apparently had a half-life longer by 0.9(2)\% at 12K in palladium and by 0.7(2)\% in indium \cite{Wa06}. The authors of these reports also proposed a theoretical explanation of their observations based on quasi-free electrons --- a ``Debye plasma" --- causing an enhanced screening effect in metallic hosts. This would lead to host-dependent half-lives and a smooth dependence of half-life on temperature in a metal.

Needless to say, these claims led to considerable popular interest, not least because they could potentially have contributed to the improved disposal of radioactive waste \cite{Mu06}. Not remarked on at the time, though, was the impact that such a result would also have on all half-lives that have ever been quoted with sub-percent precision. Of greatest concern to us were the half-lives of superallowed $0^+$$\rightarrow$$0^+$ $\beta^+$ transitions, essential to fundamental tests of the Standard Model \cite{Ha09}. Their precision has typically been quoted to less than 0.05\%, well below the temperature and host-material dependence claimed by the new measurements \cite{Ra07,Sp07,Li06,Wa06}.

Based on this concern, we first repeated the measurement on the decay of $^{198}$Au ($t_{1/2}$ = 2.7\,d) in gold \cite{Go07}. While the original measurement by Spillane et al. \cite{Sp07} followed the decay for only a little over one half-life, we recorded the decay with much better statistics for over 10 half-lives at both room temperature and at 19K. Our results showed the half-lives at the two temperatures to be the same within 0.04\%, a limit two orders of magnitude less than the difference claimed by Spillane et al. This null result was subsequently confirmed by two other measurements of $^{198}$Au, which set limits of 0.13\% in a Al-Au alloy host \cite{Ku08} and 0.03\% in gold \cite{Ru08}. The latter reference also reported a new $^{22}$Na decay measurement, which set an upper limit on the temperature dependence of that $\beta^+$ decay at 0.04\%, again nearly two orders of magnitude below the earlier claim, in this case by Limata et al. \cite{Li06}. For $\alpha$ decay, the $^{210}$Po measurement has not yet been repeated but low-temperature measurements on a variety of other $\alpha$ emitters \cite{St07,Se07} have set upper limits of 1\% on any possible temperature dependence in those cases. Though significantly lower than the temperature dependence claimed to have been observed in reference \cite{Ra07}, this 1\% limit is considerably less stringent than the limits obtained for $\beta^-$ and $\beta^+$ decays.

The status of electron-capture decay is also less definitive. One new measurement of $^7$Be decay in copper \cite{Ku08} found no temperature dependence greater than 0.3\% but another \cite{Ni07} actually found a small change in half-life --- 0.22(8)\% --- depending on whether the host material was a conductor (Cu or Al) or an insulator (Al$_2$O$_3$ or PVC), both at room temperature. In neither case is the result as precise as has been achieved for $\beta^-$ and $\beta^+$ decays. Furthermore, since $^7$Be is known to show effects from its chemical environment, it is difficult to be certain about the cause of any observed effect and even more difficult to generalize its behavior to the electron-capture decay of other nuclei for which the $K$-shell electrons are much better shielded from the external environment.

We thus set out to determine the temperature dependence for the ec-decay half-life of a nucleus with a $Z$ that is considerably larger than that of $^7$Be. Our goal was to achieve a precision comparable to that obtained for $\beta^-$ and $\beta^+$ decays, {\it i.e.}\,$\leq$0.1\%. For our measurement we sought a nucleus that decays entirely by electron capture with a few-day half-life and a delayed $\gamma$ ray that can be cleanly detected. It also had to be producible by thermal-neutron activation so that we could obtain statistically useful quantities without serious contaminants. Although there are not a lot of candidates to choose among, we found $^{97}$Ru satisfied all our conditions. Its decay scheme appears in Fig.~\ref{fig1}. We report here measurements of the half-life of $^{97}$Ru at room temperature and at 19K as measured via its 216-keV $\beta$-delayed $\gamma$ ray. We have found no temperature dependence in the results. Our upper limit is 0.1\%, an order of magnitude below the effect claimed for $^7$Be \cite{Wa06}.

\section{\label{sec:Appar} Apparatus and set-up}

Рисунок во введении и текст до библиографии здесь не приведен (см Other formats)

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