D. L. Friesel

Temperature sensitivity of surface channel effects on high-purity germanium detectors

The temperature sensitivity of surface channel effects on planar high-purity germanium detectors was measured using 60-keV gamma-ray scanning techniques, as part of a radiation damage study. When measured in this manner, the surface effects on most detectors showed extreme temperature sensitivity in the 72–95 K region. The effect of the surface channel increased with increasing temperature to such an extent that the efficiency, as measured by the count rate in the 1332-keV peak from a 60Co source, decreased by a factor of over two in some cases. Since the peak efficiency for the 1332-keV gamma ray decreased as the temperature increased throughout the operating range (72–120 K) the effect of the surface channel must continue to increase beyond the temperature (95 K) at which the 60-keV scan loses its sensitivity because of the strong attenuation of these much lower energy gamma rays. Radiation damage had no measurable effect on the surface characteristics. No correlation between the surface effects and the resolution changes of the 1332-keV peak was observed.

A variable geometry high-purity germanium detector telescope system for use with intermediate energy charged particles

Abstract A versatile variable geometry detector telescope system consisting of silicon surface barrier and high-purity germanium planar detector has been developed and used at intermediate light ion energies at IUCF for several years. Detailed descriptions of the telescope system and its operational properties are given. The practical experiences of using detectors fabricated from both n- and p-type high-purity germanium for a variety of particles and energies are described along with details of the effects of long-term storage, radiation damage, and radiation damage annealing procedures on detector properties. Diffusion of the Li n+ contact after many radiation damage and anneal cycles is also discussed.

High-purity germanium charged-particle detectors: A LBL-IUCF update☆

Abstract During the continued use and corresponding analysis of the performance of high-purity germanium charged-particle detectors, two important conclusions were reached: (1) Germanium transmission detectors should not be made from material that is “too” pure. It is desirable to have the largest possible net donor concentration consistent with maintaining a reasonable value for the depletion bias. (2) It is very desirable to use detectors fabricated from n-type germanium in a high-radiation environment.

Gamma-ray escape peak characteristics of radiation-damaged reverse-electrode germanium coaxial detectors

Abstract A comparison of the characteristics of full-energy gamma-ray peaks and their corresponding escape peaks when high energy photons interact in radiation damaged reverse-electrode (n-type) germanium coaxial detectors is presented. Coaxial detector geometry is the dominant factor, causing charge collection to be dramatically better for interactions occurring near the outer periphery of the detector as well as increasing of the probability of escape events occurring in this region. It follows that the resolution of escape peaks is better than that of ordinary gamma-ray peaks. This is experimentally verified. A nearly identical but undamaged detector exhibited significant Doppler broadening of single escape peaks. Because double escape events preferentially occur at outer radii, energy shifts of double escape reflect extremely small amounts of charge trapping in undamaged detectors.

Status report on the IUCF cooler-storage ring

Commissioning activities on the IUCF Cooler, a storage ring with internal targets and electron cooling, were begun in November, 1987. By June 1988, proton beam accumulation, storage, acceleration, and cooling (both in the absence and in the presence of internal targets) had been demonstrated. Commissioning studies in the summer of 1988 were devoted to measuring the lattice parameters and exploring the operating characteristics of the cooler and optimizing its performance for use as an intermediate-energy nuclear physics research instrument. Both stripping injection of 90 MeV H+2 ions and direct, fast kicker injection of 148 and 179 MeV polarized proton beams from the IUCF cyclotrons were demonstrated. The initial performance characteristics of the ring for beams stored, accelerated, and cooled in this new facility are discussed here. Results from preliminary measurements of the equilibrium properties of stored, cooled beams in the presence of thin internal targets (014atoms/cm2) are also reported.

A 16N gamma-ray facility

Abstract A practical 16N gamma-ray source is created in a medium-energy cyclotron environment. A 16N source emits 6129 and 7115 keV gamma rays. The viability of this several μCi source for detector calibration and studying detector physics is established.

Field Mapping Results of the IUCF 200 MeV Cyclotron

The Indiana University 200 MeV Isochronous Cyclotron is a separated sector (N = 4) machine whose design goals include the acceleration of both protons and heavy ions over a variable energy range up to a maximum energy of approximately 220 Z2/A MeV. The large range in energy and particle mass requires that the radial profile of the magnetic fields be adjustable to match the relativistic mass increase of the accelerated particle. For the acceleration of protons to 200 MeV for example, an increase in the field from injection to extraction radius of about 22% is needed, whereas for heavier ions the field profile must rise approximately 2%.

A variable temperature cryostat that produces in situ clean-up of germanium. detector surfaces

Variable-temperature cryostats that can maintain germanium detectors at temperatures from 82 K to about 400 K while the thermal shield surrounding the detectors remains much colder have been developed. Cryostats such as these offer the possibility of cryopumping material from the surface of detectors to the colder thermal shield. The diode characteristics of several detectors have shown very significant improvement following thermal cycles up to about 150 K in these cryostats. The possible applications of such cryostats are discussed. >

Electron Gun Design Study for the IUCF Beam Cooling System

The design of a low temperature electron beam cooling system for the Indiana University electron-cooled storage ring is in progress. The storage ring, which will accept the light ion beams from the existing k-200, multi-stage cyclotron facility, requires an electron beam variable in energy from about 7 to 275 keV. The electron beam system consists of a high perveance electron gun with Pierce geometry and a flat cathode. The gun and a 28 element accelerating column are immersed in a uniform longitudinal magnetic guide field. A computer modeling study of the system was conducted to determine electron beam density and transverse temperature variations as a function of anode region and accelerator column design parameters. Transverse electron beam temperatures (Et = mc2ß2¿(¿H+¿v)) of less than a few tenths of an electron volt at a maximum current density of 0.4 A/cm2 are desired over the full energy range. This was achieved in the calculations without the use of resonant focusing for a 2 Amp, 275 keV electron beam. Some systematics of the electron beam temperature variations with system design parameters are presented. A short discussion of the mechanical design of the proposed electron beam system is also given.

High Voltage System Design for the IUCF 300 KV Electron Cooling System

A summary of the electron beam high voltage system design for the IUCF Cooler1,2,3, now under construction, is presented. There are extremely stringent regulation requirements (~10ppm) on the main high voltage power supply (-300 kVDC, 15 mA), and less stringent requirements on the gun anode power supply, in order to achieve the regulation needed to store beams in the IUCF Cooler with very low momentum spreads (¿p/p ¿ 2xl0-5). An overview of the main high voltage power supply (HVPS) specifications and design, as well as provisions and plans to improve the regulation are discussed. The electron collection system, modeled after the FNAL collector which was able to collect between 99.9% and 99.99% of the electron beam, is discussed along with the requirements of the associated power supplies. The designs of the high voltage acceleration structures and high voltage platform are discussed, as well as practical design considerations based upon experience with the Fermilab 120 keV electron cooling system.