N.A. Orr

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.

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 production of isotopic 36S targets by ion implantation into carbon foils

Abstract The production of thin (5−10 μg/cm 2 ) targets of 36 S by the implantation of 50 keV negative ions into very clean carbon foils is described. Enrichments of > 96% in 36 S have been achieved.

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.

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.

Heavy-ion reaction studies of 35,36P

Abstract The spectroscopy of the T z ⩾ 3 2 nuclei 35 P and 36 P has been studied using a variety of multi-nucleon heavy-ion transfer reactions. This has led to the observation of a number of new levels in both nuclei. The probable structure of these and previously reported levels are deduced through comparison with DWBA and shell-model calculations.

Studies of the Tz = 52 nuclei 31Al and 29Mg

Abstract The multi-nucleon transfer reactions 30Si(18O,17F)31Al and 30Si(13C, 14O)29Mg have been investigated at incident energies of 100 and 93 MeV respectively and a mean reaction angle of 8°. Excited states of the T z = 5 2 nuclei 31Al and 29Mg have been studied and a mass excess of −14.967 ± 0.025 MeV obtained for the ground state of 31Al. The 28Si(18O,17F)29Al and 28Si(13C, 14O)27Mg reactions have also been studied, enabling conclusions to be drawn concerning the probable structure of many of the states observed in 31Al and 29Mg. Several 0ħω levels predicted in recent complete sd-shell calculations have been assigned experimental counterparts for the first time. The results also support a previous suggestion that negative-parity states occur in 29Mg at 1.10 and 1.46 MeV.

Study of 51Ca via three-neutron transfer

Abstract Low-lying levels in 51 Ca have been studied via the reaction 48 Ca( 18 O, 15 O) 51 Ca at a beam energy of 108 MeV. Recoiling 51 Ca nuclei were detected in coincidence with 15 O ejectiles, largely removing the substantial background from target contaminants that was encountered in previous work. The lowest 51 Ca level identified is 1.01 ±0.11 MeV less bound than the ground state, according to a recent measurement of the ground state mass using the 48 Ca( 14 C, 11 C) 51 Ca reaction. Consider- ations of the difficulty in identifying the ground state peak suggest that published values of the mass excess should be viewed with caution, and that a definitive mass measurement is still required in order to resolve questions on the structure of the neutron-rich calcium isotopes. A total of six energy levels in 51 Ca have been identified, and their mass excesses measured. Only two of these levels have been observed previously, via the ( 14 C, 11 C) reaction. Striking differences in selectivity between the ( 18 O, 15 O) and ( 14 C, 11 C) transfer reactions are discussed.

Observations of 20n via the heavy-ion transfer reaction 48Ca(18O, 20N)46Sc

Abstract The Q -values for groups observed in the multinucleon transfer reaction 48 Ca( 18 O, 20 N) 46 Sc have been measured using a variety of heavy-ion spectrographic techniques. The group at lowest excitation energy has a Q -value of −25.87 ± 0.02 MeV, corresponding to a mass excess of 22.63 ± 0.06 MeV for 20 N. This is in significant disagreement with earlier determinations, being ∼1 MeV less bound. Possible explanations are discussed.