Frances Pleasonton

Evaluation of semiconductor detectors for fission fragment energy measurements

Abstract A practical guide for the evaluation of silicon semiconductor detectors for fission fragment energy measurements is given. The evaluation is based on shape parameters associated with the pulse amplitude spectrum for 252Cf spontaneous fission fragments. Reasonable limits for these parameters are suggested and the significance of each is discussed. Measurements of similar parameters for the 235U thermal-neutron fission fragment pulse amplitude spectrum are reported. A method for computer determination of the spectrum parameters is outlined.

Prompt γ-rays emitted in the thermal-neutron induced fission of 233U and 239Pu

Abstract The average number and average energy of γ-rays emitted within ≈ 5 nsec after fission have been determined as functions of fragment mass and as functions of total kinetic energy. They were obtained from a four-parameter experiment that recorded, event-by-event, correlated of γ-rays and of fission-fragment pairs and the time, relative to fission, at which a γ-ray was detected. For 233 U(n th , f) the average total number and energy emitted per fission were found to be 6.31 ± 0.3 and 6.69 ± 0.3 MeV, respectively, giving an average photon energy of 1.06 ± 0.07 MeV. The results for 239 Pu(n th , f) given in the same order, are 6.88 ± 0.35,6.73 ± 0.35 MeV, and 0.98 ± 0.07 MeV.

Ionization Following Internal Conversion in Xenon

The predominant internal conversion accompanying the decay of 12-day 131Xem from its metastable nuclear level to its stable ground state usually leaves the atoms in states of high multiple ionization, even though only a single primary electron vacancy is created in the initial process. The differential charge spectrum of the ions has been measured by the methods of magnetic analysis. Only 0.62 ± 0.08% of the decays leave the atom singly charged. The most probable state of ionization is 8, following 21.06 ± 0.21% of the nuclear transitions, and corresponding to the depopulation of the outer electron shell. Thereafter the probability of occurrence decreases toward the higher charges, twenty electrons being thrown off in 0.003 ± 0.002% of the decays, which presently is about as far as the sensitivity of the spectrometer carries the spectrum. The mechanism of this multiple ionization is discussed in terms of successive Auger events. An analysis of the intensity of the singly charged ions in particular, suggests that they result predominantly from a combination of direct internal conversion in the outer shell of the atom and of K-shell conversion followed by the (1s-5p) X -ray transition. The upper limit thereby set for the 5p shell conversion coefficient is 0.28, and the upper limit for the intensity of the (ls-5p) line is 0.011 in a total K-series intensity of 1.

Indirect measurement of neutron emission from fission fragments

Abstract An experiment to determine indirectly the average number of neutrons emitted by fission fragments as a function of fragment mass and total kinetic energy is described. The method is based on the measurement of both fragment kinetic energies and the velocity of one of the fragments, and is particularly applicable to medium or high excitation cases where direct neutron counting is difficult or impractical. Details of the analysis of such an experiment are given, and results for 252Cf spontaneous fission are shown.

Internal bremsstrahlung from 6He

Abstract The spectrum of internal bremsstrahlung from the superallowed beta emitter 6 He has been measured using a NaI scintillation spectrometer. The shape of the experimental spectrum agrees with the theory of Knipp, Uhlenbeck and Bloch for energies greater than 0.3 MeV. In this decay, Coulomb corrections are of little significance; thus the results can be interpreted as confirming the fundamental concept of internal bremsstrahlung as a process in which the electron exists first in a virtual state and then goes to its final state by emitting a photon.

An absolute measurement of the 6He decay energy

Abstract The end-point of the energy spectrum of recoil ions from the β-decay of 6 He was observed with a spectrometer which is calibrated in terms of a standard voltage cell. The average of two independent measurements gives a maximum β-energy of 3508±4 keV. This result is weighted with other measurements to give a maximum β-energy of 3508.4±3.8 keV. Addition of the 1.4 keV maximum recoil energy yields the decay energy 3509.8±3.8 keV.

The half-life of He6

Abstract The decay of He6 was measured, with a plastic scintillation counter, over a period of 10 half-lives. The data were obtained by photographing two scalers, one of which registered the β counts while the other, operating on a 60 Hz signal, served as the clock. After suitable corrections for background, dead-time of the counting system, and pulse height dependence on counting rate, 34 separate runs were combined. A least squares fit to the data gives a value of T 1 2 = (0.797 ±0.003) sec .

SYSTEM FOR CONTINUOUS GENERATION AND FAST TRANSFER OF RADIOACTIVE GASES

The system was developed to provide a stable and strong source of He6 for the Oak Ridge National Laboratory recoil spectrometer. A continuous flow of water‐vapor swept the He6 through about 80 ft of piping from the generator to the laboratory in 3 to 4 sec; the gas was then purified and delivered to the spectrometer in an additional 1 to 1.5 sec. Modifications in design for other experiments and for use in leak detecting are discussed.

Response of Solid-State Detectors to Fission Fragments

Pulse amplitude spectra for fission fragments from the spontaneous fission of Cf252 have been studied for more than 50 silicon surface-barrier detectors. Silicon resistivities ranged from 100 to 2000 Ohm-cm; spectra were obtained for several bias voltages for each detector. A search for correlations among shape parameters associated with Cf252 fragment pulse-amplitude spectra was carried out. Such correlations were, in fact, observed and are shown to be related to the pulse-height response characteristics of surface-barrier detectors for fission fragments. Some guide lines by which the quality of a fission-fragment detector may be determined from the spectrum parameters are given.