An experimental study of the bombardment of isotopic silicon by 17 Mev protons
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Abstract
Energy levels are characterized by the following properties: excitation energy, Q, angular momentum, parity, level width, isobaric spin, magnetic moment and configuration. They may be studied experimentally by the following methods: (1) measurement of the energy spectra of nuclear reaction products, (2) measurement of peaks in the yield curve as a function of incident particle energy to obtain levels in the compound nucleus, (3) measurement of gamma ray intensity as a function of bombarding particle energy, (4) measurement of the maximum beta decay energy to locate levels in the residual nucleus, (5) measurement of input particle energy thresholds for production of neutrons, (6) measurement of angular distributions and angular correlations to find the spin and parity of a level.
The accurate measurement of neutron spectra is difficult because the uncharged neutron must be detected by indirect means. Cloud chambers, proportional counters, threshold detectors, time-of-flight methods, and nuclear emulsions have all been used successfully.
Nuclear emulsions detect neutrons by the track of developable silver halide crystals left by a recoil proton after being struck by a neutron. The range of the recoil proton and the angle it made with the incident neutron beam are measured. Knowledge of the range-energy curve for the emulsion and application of the laws of conservation of energy and momentum determines the energy of the incident neutron. The emulsion consists of a high concentration of silver halide crystals embedded in gelatin.
The following is a description of the nuclear emulsion method: A beam of particles strikes a target, neutrons are emitted, and they are detected by plates placed at several different angles with respect to the incident beam. Development of the plates must be done soon after exposure, since the latent image fades rapidly. In this experiment, a combination of the two-solution and low temperature development techniques was used.
The plates are scanned with a microscope and the following quantities are measured: (1) θ, the dip angle in the processed emulsion, (2) l, the projection in the x-y plane of the track length in the processed emulsion, and (3) φ the angle l makes with the x axis. φ and θ are measured at the beginning of the track. To avoid large errors, only tracks which fall within certain acceptance criteria are measured. This experiment required the following criteria to be satisfied: (1) if φ≤10°, tan θ≤0.075, (2) if 10°<φ≤l5°, tan θ=0, (3) if φ>15° it is not accepted, and (4) the track must lie wholly within the emulsion. Scanning is done in strips one field of view wide and about half the plate length long. At each x and y coordinate setting the entire depth of the emulsion is searched for tracks, then the x coordinate is changed by about half a field of view, and the process is repeated. Strips are separated by two or three fields of view so that overlapping does not occur. Length measurements are made in eyepiece scale divisions, which must be converted to microns by calibrating against a stage micrometer. The emulsion thickness must be measured daily since it varies with humidity.
Neutron energies are calculated by converting the measured track length to length in the unprocessed emulsion, and then applying conservation of energy and momentum. The number of tracks obtained per energy interval as a function of energy is tabulated. Two corrections are needed because: (1) the neutron-proton scattering cross section is strongly energy dependent, and (2) the probability of the recoil proton escaping from the emulsion increases with energy. Results are plotted in histogram form. The Q value for each neutron group is calculated; the highest neutron energy group usually represents formation of the ground state, the next highest represents the first excited state, etc.
The energy resolution of the nuclear emulsion method is limited by the following factors: (1) Range straggling of protons in the emulsion, (2) energy straggling of the beam in the target, (3) energy spread of the beam, (4) multiple scattering of the protons in the emulsion, (5) length and angle measurement errors, (6) finite target size and angular spread of neutrons incident on the emulsion.
This experiment was performed to measure the mass of p^28, by means of the reaction Si^28(p,n)p^28. The target, a suspension of natural silicon in polystyrene, was bombarded by protons from the Princeton cyclotron, with an energy of 17.45±0.15 Mev. The target thickness corresponded to an energy loss of 100 kev for 17 Mev protons, so the average proton energy reaching the plates was 17.4±0.2 Mev. Ilford C-2 plates, 400 microns thick, were placed 4.5 in. from the target in an evacuated scattering chamber. Plates were mounted at 30, 60, 90, 120, and 150 degrees. At each angle two plates were sandwiched together, emulsions facing, and wrapped in aluminun1 and lead foils. The beam was collimated by graphite apertures and was stopped by a graphite cup, relatively far from the plates.
Plates were scanned at 30, 60, and 90 degrees. The range-energy relation of Rotblat was used. Data was corrected for n-p scattering and geometry. Five hundred tracks were measured at 30° and 90°, and 250 tracks at 60°. The ground state of p^28 was resolved only at 30°, and actually the peak found in the 30° spectrum is probably due to both the ground and first excited states in p^28. Q for the ground state is -14.71±0.22 Mev; the mass defect of p^28 is 0.68±0.22 Mev; the mass of p^28 is 28.00073±0.00024 amu. The differential cross section at 30° for formation of the ground and first excited states of p^28 is 0.27±.018 mb/steradian. If one corrects for the Coulomb energy difference and the neutron-proton mass difference, the mass of p^28 is 9.39 Mev greater than that of Si^28. This means that the ground state of p^28 probably corresponds to the lower of two levels in Si^28, located at about 9.39 Mev. This is in excellent agreement with the hypothesis of charge independence of nuclear forces.
The reactions Si^29(p,n)p^29 and Si^30(p,n)p^30 were observed. No new levels were found in either p^29 or p^30.
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Thesis (M.A.)--Boston University
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