O. Tengblad

The beta decay of 9Li to levels in 9Be: A new look

Beta-delayed neutrons and alpha particles from the decay of 9Li have been studied with a 3He neutron spectrometer and silicon surface barrier detectors. The data show that the (50 ± 3)% feeding to particle unbound states mainly proceeds to the four excited states in 9Be at 2.43 MeV (30 ± 3)%, 2.78 MeV (16 ± 3)%, and 11.28 and 11.81 MeV (3.8 ± 0.2)%. The strong feeding to the 11.81 MeV state observed corresponds to a log ft value of 2.84. The beta-decay rates are strongly affected by the broad levels populated in the decay. Corrections involving R-matrix calculations have been applied for the extraction of the reduced transition probabilities. We confirm the recently determined strong asymmetry in the beta decays of the case32− ground states of 9C and 9Li to the excited case52− states in the daughters 9B and 9Be.

First observation of beta-delayed deuteron emission

Abstract Beta-delayed deuteron emission has been observed in the decay of 6 He with a branching ratio of (2.8±0.5)×10 −6 . Simple theoretical estimates predict an intensity that is significantly higher.

Response function for 3He neutron spectrometers

Abstract The response to monoenergetic neutrons in the energy range 130–3030 keV for 3 He-filled ionization chambers of type FNS-1 is experimentally determined. The experiment is performed using beams of monoenergetic neutrons produced in proton induced reactions in thin LiF targets at a 5 MeV Van de Graaff accelerator. The measured pulse height spectra are fitted with regard to the full energy neutron peak, wall effects, and recoil distributions of elastically scattered hydrogen and helium nuclei. The absolute efficiency of the detectors in determined. Effects of neutron scattering in the stainless steel housing of the detector are discussed.

Integral \̄gn-spectra derived from experimental β-spectra of individual fission products

Abstract Individual β-spectra of 111 fission products, most of them with high Q β values, have been measured and converted into the corresponding \ gn-spectra. Combined with abundance data for typical reactor conditions they contribute close to 90% of the total number of antineutrinos above 6 MeV. The addition of the contributions from the remaining fission products yields the integral \ gn-spectrum around a nuclear reactor. Using this spectrum the average charged- to neutral-current cross-section ratio for reactor neutrinos interaction with deuterium is determined to be g s cc g s nc = 0.363 ± 0.011. This is compatible with the experimental result 0.282 ± 0.089 obtained by Reines et al .

11Be(beta-p), a quasi-free neutron decay?

We have observed beta(-)-delayed proton emission from the neutron-rich nucleus Be-11 by analyzing a sample collected at the ISOLDE facility at CERN with accelerator mass spectrometry (AMS). With a branching ratio of (8.3 +/- 0.9). 10(-6) the strength of this decay mode, as measured by the B-GT-value, is unexpectedly high. The result is discussed within a simple single-particle model and could be interpreted as a quasi-free decay of the Be-11 halo neutron into a single-proton state.

Multiparticle emission in the decay of Ar-31

A multihit capacity setup was used to study the decay of the dripline nucleus Ar-31, produced at the ISOLDE facility at CERN. A spectroscopic analysis of the beta-delayed three-proton decay of Ar-31 is presented for the first time together with a quantitative analysis of the beta-delayed 2p gamma decay. A new method for determination of the spin of low-lying levels in the beta p daughter 30S using proton-proton angular correlations is presented and used to determine that the spin of the 5.2-MeV level is most likely 3(+) with 4(+) also possible. The half-life of Ar-31 is found to be 15.1(3) ms. An improved analysis of the Fermi beta strength including the beta 3p-decay mode gives a total measured branching ratio of 3.60(44)%, which is lower than the theoretical value found to be 4.24(43)%. Finally, a previously unidentified. transition from the isobaric analog state in the decay of Ar-33 has been found.

Relative proton and γ widths of astrophysically important states in 30S studied in the β-delayed decay of 31Ar

Resonances just above the proton threshold in S-30 affect the P-29(p, gamma)S-30 reaction under astrophysical conditions. The (p,gamma)-reaction rate is currently determined indirectly and depends on the properties of the relevant resonances. We present here a method for finding the ratio between the proton and gamma partial widths of resonances in S-30. The widths are determined from the beta 2p- and beta p gamma-decay of Ar-31, which is produced at the ISOLDE radioactive ion beam facility at the European research organization CERN. Experimental limits on the ratio between the proton and gamma partial widths for astrophysical relevant levels in S-30 have been found for the first time. A level at 4689.2(24) keV is identified in the gamma spectrum, and an upper limit on the Gamma(p)/ Gamma gamma ratio of 0.26 (95% C.L.) is found. In the two-proton spectrum two levels at 5227(3) keV and 5847(4) keV are identified. These levels were previously seen to gamma decay and upper limits on the Gamma(gamma)/Gamma(p) ratio of 0.5 and 9, respectively, (95% C.L.) are found, where the latter differs from previous calculations.

The 2p decay mechanism of Ar

Abstract We have measured the beta-decay of 31 Ar with a high granularity setup sensitive to multiparticle decay branches. Two-proton emission is observed from the isobaric analog state in 31 Cl to the four lowest states in 29 P and furthermore from a large number of states fed in Gamow–Teller transitions. The mechanism of two-proton emission is studied via energy and angular correlations between the two protons. In all cases the mechanism is found to be sequential yielding information about states in 30 S up to 8 MeV excitation energy. Improved data on the β -delayed one-proton branches together with the two-proton data provide precise information about the beta-strength distribution up to 15 MeV excitation energy.

The Beta Decay of

Beta-delayed neutrons and alpha particles from the decay of 9Li have been studied with a 3He neutron spectrometer and silicon surface barrier detectors. The data show that the (50 ± 3)% feeding to particle unbound states mainly proceeds to the four excited states in 9Be at 2.43 MeV (30 ± 3)%, 2.78 MeV (16 ± 3)%, and 11.28 and 11.81 MeV (3.8 ± 0.2)%. The strong feeding to the 11.81 MeV state observed corresponds to a log ft value of 2.84. The beta-decay rates are strongly affected by the broad levels populated in the decay. Corrections involving R-matrix calculations have been applied for the extraction of the reduced transition probabilities. We confirm the recently determined strong asymmetry in the beta decays of the case32− ground states of 9C and 9Li to the excited case52− states in the daughters 9B and 9Be.

^{9}

It is demonstrated that neutron scattering in the stainless steel housing of a 3He spectrometer gives rise to spurious peaks in the spectra. Calculations show that the observed peak structure in the β-delayed neutron spectrum of 9Li is explained by this effect. Distortion of the response function for monoenergetic neutrons is discussed with regard to scattering by iron.