C.L. Woods

The identification and rejection of energy-degraded events in gas ionization counters

Abstract A common feature of the measurement of charged particles with gas detectors is the presence of a small fraction of events (∼ 0.1–0.2%) for which significantly less than the total ionization is recorded. These events cause an energy tailing that can seriously impair the identification functions of gas detectors used either to instrument the focal plane of magnetic spectrometers or for accelerator mass spectrometry. The anomalous, energy-degraded events are shown to arise both from reactions between the incident ions and the detector gas and, more importantly, from the scattering of ions by the gas. It is demonstrated that an appropriate detector configuration provides the means to reject most of the anomalous events, allowing the measurement of very low cross section reactions without significant background interference from such events.

Masses and level schemes of 33Si and 34Si

Abstract The mass excesses of 33Si and 34Si have been measured using the 36S(11B, 14, 13N)33, 34Si; reactions. Values of −20.550 ± 0.030 MeV ( 33 Si) and −20.017 ± 0.025 MeV ( 34 Si) were obtained. In addition, an excited state of 34Si was observed at 5.33 ± 0.05 MeV, and excited levels of 33Si were observed at 1.06 ± 0.02 and 4.32 ± 0.03 MeV. These results are compared with recent shell-model calculations.

Study of 19N and 21O by multinucleon transfer

Abstract The multinucleon transfer reactions 18 O( 18 O, 17 F) 19 N and 18 O( 18 O, 15 O) 21 O have been studied at a beam energy of 117 MeV and a mean reaction angle of 10°. The same reactions, but detecting the neutron-rich ejectiles, have also been studied at a mean angle of 4.5°. Excitation energies for low-lying levels in the two T z = 5 2 nuclei 19 N and 21 O have been derived. The correspondence between the observed levels and those predicted by 0kh ω shell-model calculations is discussed. The proposed level assignments are deduced by analogy with results from the 16 O( 18 O, 17 F) 17 N and 16 O( 18 O, 15 O) 19 O reactions, which were also studied in the present work, and are supported by calculations employing a semi-classical reaction model.

The 26Mg(18O, 17F)27Na and 26Mg(18O, 15O)29Mg reactions and the level schemes of 27Na and 29Mg

Abstract The 26 Mg( 18 O, 17 F) 27 Na and 26 Mg( 18 O, 15 O) 29 Mg reactions have been used to determine the ground-state masses and partial level schemes of 27 Na and 29 Mg. Values of −5514 ± 60 keV and −10600 ± 45 keV were determined for the mass excesses of 27 Na and 29 Mg, respectively. The probable structure of some of the levels observed was deduced from a comparison with shell-model calculations and with previous experimental information. A number of other 18 O- and 16 O-induced reactions on magnesium targets were also studied in order to obtain information on the reaction mechanisms of the ( 18 O, 17 F) and ( 18 O, 15 O) reactions.

Ground-state mass excesses and low-lying levels of the neutron-rich nuclei 36P, 35P and 34P

Abstract The ground-state masses and the excitation energies of low-lying levels of the nuclei 36P, 35P and 34P have been measured using the 36,34S(7Li,7Be)36,34P charge-exchange reactions, and the one-nucleon-transfer reaction 36S(6Li, 7Be)35P. Mass-excess values of −20.251 ± 0.027 MeV ( 36 P ), −24.828 ± 0.017 MeV ( 35 P ) , and −24.569 ± 0.040 MeV ( 34 P ) were obtained. A single excited state is observed at 252 ± 10 keV in 36 P , and a number of new levels are observed in the nuclei 35P and 34P. The 36,34S(11B, 11C)36,34P reactions were also studied in order to remove ambiguities associated with the 429 keV level in 7Be.

Reaction studies of the neutron-rich nuclei 22, 23F

Abstract The masses and excited states of the neutron-rich nuclei 22 F and 23 F have been measured using the heavy-ion transfer reactions 22 Ne( 7 Li, 7 Be) 22 F and 22 Ne( 18 O, 17 F) 23 F. Mass excesses of 2.80 ± 0.02 MeV and 3.32 ± 0.09 MeV, respectively, were deduced from the reaction Q -values. The value for 22 F differs from that previously accepted due to the existence of a low-lying doublet. A number of excited states have been identified in both nuclei, and the results are compared with shell-model calculations.

The mass of 35Si

Abstract The mass excess of 35 Si has been determined as −14.427 ± 0.060 MeV from a measurement employing the 36 S( 13 C, 14 O) 35 Si reaction. No excited states of 35 Si were observed. The 34 S( 13 C, 14 O) 33 Si reaction was studied simultaneously, and previously unreported excited states in 33 Si were observed at 1.47, 2.00, 3.19, 4.13 and 5.48 MeV.

A measurement of the mass of 39Sc

Abstract The mass of the proton-rich nucleus 39 Sc has been derived from a measurement of the Q -value of the 40 Ca( 14 N, 15 O) 39 Sc reaction at a beam energy of 102.5 MeV and mean reaction angle of 6°. A value of −27.67 ±0.03 MeV was obtained for the Q -value, implying a mass excess of −14.19 ± 0.03 MeV for the ground state of 39 Sc, which is therefore unbound to proton emission by an energy of 580 ±30 keV. There is some evidence for an excited state in 39 Sc at an energy of 950 ±keV. This state is tentatively identified as the analog of the 1.27 MeV 3 2 − state in 39 Ar. The cross sections to the 1 2 + and 5 2 + states in 15 C confirm an earlier suggestion that this reaction is well suited to the observation of proton-rich nuclei via direct transfer reactions having very negative Q -values.

Time-of-flight system for a heavy ion magnetic spectrometer

Abstract A system to measure the time of flight for heavy ions around an Enge split-pole magnetic spectrometer has been developed. The system consists of a parallel plate avalanche counter at the focal plane and either a microchannel plate detector or a silicon recoil detector in the target chamber. In-beam tests under experimental conditions have demonstrated a typical resolution of 1.0 to 1.5 ns, compared to a total flight time of ∼ 100 ns. Resolution figures approaching these have been recovered, when using the 4.5° angular acceptance of the spectrometer, by applying calculated corrections to the raw data according to the angle and focal plane position. A description of the detectors is followed by the results of the in-beam tests. An example of an application to the study of a heavy ion reaction is also described.

Shell-model calculations for neutron-rich nuclei with A = 35–41

Shell-model calculations are presented for neutron-rich nuclei in the mass region A = 35–41 using two new interactions. The usefulness and reliability of the interactions are evaluated with particular emphasis on their predictions for neutron-rich isotopes. The calculations are performed in a 0 kh ω basis space with active protons in the 1d52, 2s12 and 1d32 orbitals and active neutrons i the 1f72 and 2p32 orbitals.