We have set limits on contributions of scalar interactions to nuclear beta decay. A magneto-optical trap provides a localized source of atoms suspended in space, so the low-energy recoiling nuclei can freely escape and be detected in coincidence with the beta. This allows reconstruction of the neutrino momentum, and the measurement of the beta-nu correlation, in a more direct fashion than previously possible. The beta-nu correlation parameter of the 0(+)-->0(+) pure Fermi decay of (38)K(m) is a =0.9981+/-0.0030+0.0032 / -0.0037, consistent with the standard model prediction a =1.
A new technique, full neutrino momentum reconstruction, is used to set limits on the admixture of heavy neutrinos into the electron neutrino. We measure coincidences between nuclear recoils and positrons from the beta decay of trapped radioactive atoms and deduce the neutrino momentum. A search for peaks in the reconstructed recoil time-of-flight spectrum as a function of positron energy is performed. The admixture upper limits range from 4 x 10(-3) to 2 x 10(-2) and are the best direct limits for neutrinos (as opposed to antineutrinos) for the mass region of 0.7 to 3.5 MeV.
A magneto-optic trap (MOT) can provide a well-polarized, backing-free, localized source of radioactive atoms for b-decay experiments. We have trapped approximately 6000 atoms of 38 K m (t 1͞2 0.925 s) and 2000 atoms of 37 K (1.226 s) produced at the TRIUMF on-line separator TISOL in a vapor-cell MOT. We have measured optical isotope shifts and deduced the nuclear charge radii, which show an unusual lack of change at the neutron number N 20 shell closure. Plans include a search for scalar contributions to the b 1 -n correlation in the 0 1 ! 0 1 decay of 38 K m . [S0031-9007(97)03637-5] PACS numbers: 23.40. Bw, 29.25.Rm, 32.80.Pj, 32.80.Ys The novel use of magneto-optically trapped radioactive atoms promises improvements in performing symmetry tests of the standard model. Optically trapped atoms are confined in a small volume in space (a few mm 3 ), have negligible source thickness, and can be optically pumped to achieve close to 100% atomic and nuclear polarization. These conditions are favorable for carrying out experiments to study weak interaction symmetries in b decay [1-3], and to measure isotopic dependence of parity nonconservation (PNC) in heavy atoms [2,[4][5][6]. Here we measure nuclear properties of ground and isomeric states of the trapped atoms.Among alkali atoms, which have simple electronic structure convenient for laser trapping, potassium isotopes offer rich opportunities for b-decay experiments. 37 K and 38 K m each decay predominantly by a single superallowed transition. The b-n correlation in a I p 0 1 ! 0 1 Fermi decay is sensitive to the exchange of hypothetical scalar bosons. Limits on the scalar interaction are poor, both from b decay and from high-energy experiments, and a 1% measurement of the b-n correlation coefficient a would be competitive [7]. Among alkali atoms, isomeric 38 K m has the only such pure Fermi decay. The mixed Fermi-Gamow-Teller (I p 3͞2 1 ! 3͞2 1 ) decay of 37 K is suitable for b-asymmetry experiments and positron longitudinal polarization measurements [8], which are sensitive to the presence of right-handed currents in the weak interaction.The half-lives of 38 K m (0.925 s) and 37 K (1.226 s) are an order of magnitude shorter than isotopes trapped in related work elsewhere [1,4,9]. This creates an experimental challenge in the production and trapping of these isotopes; however, the shorter lifetimes make it easier to achieve trap lifetimes long enough that nearly all trapped atoms undergo radioactive decay while in the trap (collisions with residual gas at 10 29 Torr produce a trap lifetime of 10 s).Here we report the successful coupling of a magnetooptic trap (MOT) to the copious production of an online isotope separator (TISOL). With this arrangement we have trapped approximately 6000 atoms of 38 K m or 2000 atoms of 37 K, sufficient to begin b-decay experiments. We have measured the isotope shifts of 37 K, 38 K m in the trapping transition: the deduced nuclear charge radii show an unusual lack of change at the major neutron number N 20 shell closure. We also wil...
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