G.A. Needham

Excitation of analogue isovector resonances via the (n, p) reaction at 60 MeV?

The (n,p) charge-exchange reaction is used as a selective tool for studying T+1 isovector excitations. Here, 60 MeV neutrons have been used to measure (n,p) spectra from a range of nuclei. For 6Li and 7Li large components of l=1 dipole strength, not seen in photoneutron data, have been found at higher excitation. In the cases of 6Li, 12C and 16O comparison with photonuclear and (d,2He) data indicates that most of the l=1 strength appears to be s=0. Gamow-Teller (GT)-type transitions have been measured in 6Li, 12C and 28Si and a measure of nu sigma tau , the isovector spin-dependent component of the effective nucleon-nucleon interaction, has been obtained at 60 MeV. In the Ni isotopes, increasing blocking of the GDR with N-Z is seen. A structure of substantial strength in 209Bi(n,p)209Pb, originally thought to be isovector quadrupole, may more probably be evidence of a 2h(cross) omega sigma tau -type excitation.

Excitation of analog isovector giant resonances via the (n, p) reaction on 14N and 16O at 60MeV

Abstract Data were accumulated for the (n, p) reaction on 14N and 16O using 60MeV neutrons. Angular distributions of the cross sections for eight proton groups were extracted and compared to distorted-waves Born approximation (DWBA) calculations using a simple macroscopic model. Enhancements in the spectra were seen in each case in the region of excitation corresponding to the giant dipole resonance (GDR) of the target nucleus. DWBA calculations show that these excitations are primarily dipole and favor the use of the Goldhaber-Teller form factor. Comparison with photo-absorption measurements suggests that there may also be significant contributions to the (n, p) reaction from “spin-flip” dipole excitations. The other proton groups could be correlated with the results of other experiments and include excitations of analogs of the giant isovector “magnetic” dipole mode of 14N.

The response of NaI(T1) to 30−60 MeV Z = 1 particles

Abstract Measurements are reported for the light output response on a NaI(T1) detector for protons, deuterons, and tritons with energies between 30 and 60 MeV. The stopping power region covered by this data overlaps previous data taken with electrons and protons. The new data around 60 MeV is interesting in that the light output is greater for deuterons (and tritons) than that for protons by the equivalent of ≈ 1 MeV. A comparison is made with the model of Murray and Meyer for light output response in inorganic crystals.

Analog giant dipole Ni(n, p) at 59.6 MeV

A broad enhancement above the continuum observed in 58, 60, 62, 64Ni(n, p) at 59.6 MeV is interpreted as the parent analog of the T> component of the giant dipole resonance. A continuum parameterization is used to extract the resonance cross section. The angular distributions for the state are most consistent with the Steinwedel-Jensen model for the giant dipole. The sum-rule strength is somewhat greater than observed with photonuclear measurements. A measurement of the blocking of the cross section with neutron excess is made, and is consistent with theoretical predictions.

A multiwire chamber system for measurements of charged particle spectra

A large solid angle, high counting rate detection system for p, d t identification and energy measurement in the range of 15 to 65 MeV has been developed. The detector consists of two position-sensitive delay-line multiwire chambers, a thin plastic scintillator ΔE, and two large area NaI E detectors. Measurements of energy spectra of protons from 12C(n, p) are presented.

A multiwire chamber system for measurements of neutron spectra

Abstract A new type of detector for measuring neutron flux and energy over a wide range of angles and energies is being developed. Measurements of neutron elastic and inelastic scattering as well as neutron energy continua are possible. Time-of-flight is not used for measuring outgoing neutron energy, and so for continuum measurements this system has some distinct advantages over conventional neutron detectors. Neutron energy measurement is carried out by measuring the energy and angle of the recoil proton produced by the neutron in a CH2 converter. Spectra from 7Li(p, n)7Be at 62 MeV and 40Ca(n, n′χ) at 65 MeV are presented.

Electron capture by protons in a neutron beam facility

Abstract Relative measurements of electron capture probabilities for protons of 42.5 and 61.8 MeV are presented. Attention is drawn to some uses of the resulting beam in a 0° neutron beam facility.

A New Type of Neutron Detector Using Multiwire Chambers

A new type of neutron flux and energy-measuring detector, which uses large-area ΔE and E detectors, and multiwire chambers (MWC) is being developed. The purpose is to allow one to measure neutron elastic and inelastic scattering and in particular to measure (n,xn) neutron continuum over a wide energy range. The detector offers many distinct advantages over the conventional neutron time-of-flight (TOF) method. The system was tested using a large area CH2 converter to produce recoil protons, which are tracked in the two MWC’s and registered in the AE and E detectors. A 7Li(p,n) spectrum from 62 MeV protons was measured and a 40Ca(n,n′) spectrum from 65 MeV neutrons was also measured.

A method for measuring neutron total non-elastic cross sections

Abstract A method for measuring neutron non-elastic cross sections at medium energies is described. The method uses a relatively thick target and a large solid angle neutron detector which intercepts most of the elastically scattered neutrons. The non-elastic cross section is determined from attenuation measurements and a knowledge of the total cross section, the relative elastic cross section (as given by optical model predictions) and the relative efficiency of the detector. Measurements for C, O, Ca and Fe at 40 and 50 MeV have been carried out using this method.