β-decay anisotropies of the mirror nucleiO15andF

Abstract

Measurements of the p anisotropy of the mirror nuclei ls O and 17 F, using on-line isotope separation and low-temperature nuclear orientation, are reported for the first time. Results of this novel approach are discussed in connection with the study of weak interactions in nuclei.PACS numbers: 27.20.+n, 29.30.Fq Precise measurements of the asymmetry coefficient A \ in the angular distribution of p particles emitted from oriented nuclei are difficult to perform. Even the famous A i p asymmetry of 60 Co (the first proof of parity violation) has been reported with high precision only recently. l Most values tabulated in the literature are of limited accuracy and often conflicting. 2 Still, reliable pasymmetry measurements, if performed with sufficient accuracy, allow one to obtain new information in many cases. For the mirror nuclei, they could give valuable information on, e.g., the presence of a V+A component in the weak interaction. Up to now, only two A\ symmetries of mirror nuclei (besides the neutron) have been reported, viz., those of 19 Ne (Ref. 3) and 35 Ar. 4 ' 5 The difficulties of these measurements are compounded by the short lifetimes and/or absence of a suitable technique.During the last decade, a large number of short-lived nuclei have become accessible in on-line conditions. At the start of this same period it was proposed to engage in the study of on-line separated nuclei by nuclear orientation. 6 After an initial period described in other papers, 7 we started particle detection in millidegree environments, 8 which makes it possible to study A\ asymmetries of p transitions. Previously, we have measured p asymmetries of long-lived nuclei off-line, using semiconductor detectors operating at liquid-N2 temperature. 9 Now the detectors are at liquid-helium temperature 8 without any thermal radiation shield and therefore without any absorbing material between them and the on-line separated source. This eliminates the energy loss and scattering of the /?'s that would occur in the radiation shields if the detectors were to operate at higher temperatures.The first on-line /3-anisotropy experiments we report here were performed on ls O it 1/2 = 122 sec) and 17 F (f 1/2-65 sec) with the KOOL facility, 7 which is directly coupled to the LISOL mass separator on-line with the cyclotron CYCLONE at Louvain-la-Neuve. 17 F was produced by irradiating a mixture of MgO and AI2O3 powders, placed inside a forced electron-beam-induced arc-discharge ion source, 10 with 10-MeV deuterons. For 15 0, the graphite catcher of the ion source was bombarded with a 32-MeV a beam. Singly charged ion beams of 10 3 -10 4 atm/sec could be extracted from the source.

Precision half-life measurement ofF17

“…The only existing measurement of a correlation parameter is the measurement of A β = 0.960(82) from Ref. [29]. While this value agrees with our prediction in Table IV, it is found that ρ varies very weakly with A β , leading to a large uncertainty in ρ.…”

“…Upon close inspection, the article does not mention the presence of 15 O contamination. Furthermore, the 1969 measurement [14], which reported a lower half-life and used the same 29 Si. Hence, for these reasons, we did not include the 1960 measurement in our averaging.…”

A GEANT4 Monte-Carlo simulation code for precision spectroscopy

“…in the amplitudes (14)) require the angular distribution coefficientà (see below) to be determined with a precision better than about 1 %. In the past we have performed already several measurements of the angular distribution of β radiation with the LTNO method (17), (18), (19). However, in order to reach the 1 % precision level the method had to be further optimized.…”

14 Particle Decay Study of Oriented Nuclei

“…II-D were used for detection. The hyperfine field of oxygen in iron and the /^-coefficient of the 5/2 + ->5/2 + transition of 17 F were determined [35][36], The latter agrees with recent large-scale shell-model calculations. In ISOLDE/CERN the first particle anisotropy, more specifically on the ß-asymmetry of 24 Na, has been measured with the NICOLE refrigerator, yielding the hitherto unknown hyperfine field of Na in Fe [37].…”