H. W. Schmitt

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.

Correlated Energy and Time‐of‐Flight Measurements of Fission Fragments with Semiconductor Detectors: System Design and Performance

A system for the measurement of correlated energies and times‐of‐flight of fission fragments has been developed. This system, adaptable for use with other charged particles as well, includes a fast‐response transformer coupling scheme in which the charge originating in a solid‐state detector passes through the transformer primary to a low noise charge‐sensitive amplifier; the fast timing signal is obtained from the transformer secondary. Noise thereby added to the linear energy signal is negligible. The measured time resolution of the system for particles in a narrow band of energies was ≲0.4 nsec, full‐width at half‐maximum. The design of the system is described, and the detailed results of performance tests, including tests with coincident 252Cf spontaneous fission fragments and energy‐time correlation measurements for bromine and iodine ions (artificial fission fragments), are given.

Thin film detector response to the passage of accelerated heavy ions

Abstract The response of a thin film detector (TFD) to the passage of heavy-mass ions (16O, 35, 37Cl, 40Ar, 79, 81Br, 127I) as a function of incident energy, velocity and energy loss is reported. Over the range of energies studied a basic dependence of the TFD response on the atomic number and velocity of the transiting heavy ion is observed. The sensitivity of the TFD response to atomic number appears to increase with higher velocities. The observed trends are consistent with those expected on the basis of transiting light-mass ion response studies. Useful applications of this novel detector to heavy-ion reaction studies now appear to be feasible.

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.

NEUTRON MULTIPLICITY COUNTER.

Abstract A neutron multiplicity counter assembly has been constructed to enable a search for superheavy elements in natural samples and in accelerator targets. The detector consists of twenty 3 He counters placed in a paraffin moderator. These counters surround a central sample cavity with a capacity of about 20 1, capable of accomodating samples up to 100 kg in weight, depending on the density. A particular feature of this counter is its relative insensitivity to gamma rays. The efficiency for detecting a single neutron is ≈ 30%. An estimate of v , the number of neutrons emitted per fission in a sample, may be obtained from the observed multiplicity distribution P ( n ), where n is the number of neutron counts in an event. More accurate values of v may be obtained for small, isolated samples of spontaneously fissioning isotopes, where the neutron counters may be gated by a fission fragment detector. The design of the counter and an analysis of its properties are presented.

Heavy Ion Energy Accumulation in the Tandem Van de Graaff Accelerator

A new energy‐accumulation method has been found for accelerating very small but usable beams of artificial fission fragments such as bromine and iodine ions to energies three times higher than those previously available from tandem Van de Graaff accelerators. Energies up to 120 MeV now available cover completely the energy range found for the fission particles from uranium. The ions have been used to study the behavior of silicon solid‐state fission‐particle detectors of many types. For the first time direct calibration methods are available to aid in experimental studies of the fission process. Further work will be directed toward an experimental study of the stopping power of various elements for heavy ions.

Instrumentation for fission fragment energy correlation experiments

Abstract Experiments have been performed at the Oak Ridge National Laboratory in which the kinetic energies of correlated fragment pairs from thermal- and resonance-neutron-induced fission have been measured. In addition, a three-parameter ternary fission experiments has been performed in which the energies of correlated fragments were measured in coincidence with the energy of a third emitted particle, usually a longrange alpha particle. The detectors were large-area silicon surface barrier detectors. The instrumentation associated with these experiments is discussed in detail. The complete system is described, with attention given to the problems of background reduction (fast-coincidence requirements), stability, linearity and resolution. Particular attention is given to the reduction of spectrum distortion by pile-up pulses, e.g., alpha-on-fission pile-up within the amplifier resolving time. Methods and limitations of pile-up detection are discussed. A new method for inspection and removal of pile-up pulses, which may be useful in a wide variety of applications, is presented.

Cylinder Radiation and Activation

In the calculation of the interaction of radiation from a point source with a body (or conversely, the intensity of radiation from an emitting body at a point in space), a somewhat complex exponential integral must be evaluated. If the interaction in the body is small, however, the integral may be approximated by I=∫V dv/R2, where V is the volume of the body and R is the distance from the point to the volume element dv in the body. This integral is discussed and applied to the case of a cylindrical body. Extensive tables (>12 000 entries) and formulas have been prepared which will enable the reader to determine the value of this integral for a cylinder of any size and any point in space. A discussion of non‐negligible absorption and multiple interactions is also included.