Search for Superscreening Effects in a Superconductor

Abstract

The decay of19 O(β − ) and 19 Ne(β + ) implanted in niobium in its superconducting and metallic phase was measured using purified radioactive beams produced by the SPIRAL/GANIL facility. Half-lives and branching ratios measured in the two phases are consistent within one-sigma error bar. This measurement casts strong doubts on the predicted strong electron screening in superconductor, the so-called superscreening. The measured difference in screening potential energy is 110 (90) The cloud of electrons surrounding the atomic nucleus is known to act as a screening potential. The effect of this screening on a charged projectile is to increase or reduce the penetrability of the projectile through the Coulomb barrier of the positively charged nucleus. Such an electron screening is known to play an important role in astrophysics since it modifies the nuclear cross sections at low energies [1]. When measuring the cross sections at energies well below the Coulomb barrier, it is important to take into account the composition of the target since insulator, metallic or alloys materials have different electron screening effects [2,3]. In superconductors, electrons are organized in Cooper pairs obeying the Bose-Einstein statistics. Due to their different nature, Stoppini [4] predicted that the local density of electrons around the nuclei could be higher than in the normal phase, a strong electron screening effect, called "superscreening", could happen in these materials. It is of high interest to study this predicted superscreening effect because of the possible theoretical implications and because of its potential use in the management of nuclear waste disposal [5].Electron screening also affects decays of radioactive nuclei [5,6]. From the theoretical point of view, the effects of the environment on the radioactive decay can be reduced to the effect of one parameter called the screening potential energy U e . In most cases, it is difficult to calculate the exact value of U e since it depends on several effects, including the electron screening and atomic bonding. Since these environmental effects do not modify the nuclear matrix element, the product of the β-decay Fermi function f (Z, Q) and the half-life t remains constant. Hence, the change of the β-decay rate can be calculated should be used for each individual branch i in order to calculate the new partial half-lives t ′(i) . It results in a new half-life 1/t ′ = i 1/t ′(i) and new branching ratios BR ′(i) = t ′ /t ′(i) . Hence, the environment can modify the half-life of the nucleus and also the β-decay branching ratios (BR). This last effect had not been previously discussed.

High precision measurement of the Ne19β -decay half-life using real-time digital acquisition

“…Our final estimation of the half-life of 19 Ne, T 1/2 = 17.2569 ± 0.0019 (stat) ± 0.0009 (syst) s, is found in perfect agreement with the former measurements of Triamback et al [11] and Ujic et al [4], as illustrated in figure 14. However, it differs by 3.2 standard deviations from the value obtained by Broussard et al [12] and tends to discard the latter.…”

“…Thanks to a real-time digital multiparametric acquisition system, the half-life of 19 Ne has been measured with the unprecedented relative precision of 1.2 × 10 −4 . The result, in agreement with the measurements of Triambak et al and Ujic et al [4,11], is not consistent with the last measurement by Broussard et al [12]. The detailed analysis of the data provided by the multiparametric acquisition system has shown that at this level of precision, pile-up, baseline variations and primarily gain variations have to be considered carefully.…”

“…Nuclear β decay offers an attractive electroweak process for the study of fundamental interactions [1,2] and plays an important role in nucleosynthesis, particularly in r -process nucleosynthesis [3] where the half-life of exotic β emitters is often a key parameter. Precise halflife measurements are also important in the investigation of the influence of electron screening on radioactive β decays [4]. During the last decade the main motivation for accurate half-life measurements of β emitters has been related to the determination of the up-down element V ud of the Cabibbo-Kobayashi-Maskawa (CKM) [5,6] quark mixing matrix.…”

“…The uncertainty on V ud is still dominated by the precision of the Fermi fractions measurements [10], but the question is raised about the reliability of the previous half-life estimation of 19 Ne. This is due to significant discrepancies of more than two standard deviations between the three most recent and most precise measurements: 17.262 ± 0.007 s [11], 17.254 ± 0.005 s [4] and 17.2832 ± 0.0051 (stat) ± 0.0066 (syst) s [12].…”

“…We show here that, with appropriate precautions, relative uncertainties of 10 −4 and below can be easily achieved for half-life measurements of nuclei with production rates on the order of 10 5 pps. This opens up new perspectives in a context where large efforts are dedicated to improved precision in half-life measurements [4,8,11,12,[15][16][17] and Fermi fraction measurements [9,18] to extract V ud from mirror transitions. Our analysis also shows evidence for sources of systematic errors at the level of 10 −4 in relative precision that must be addressed using a multiparametric acquisition system.…”