Lithium-drifted Detector Research Articles

Silicon Telescope Detectors

The results of research and development of special silicon detectors with a large active area (>8 cm2) for multilayer telescope spectrometers performed at the Laboratory of Nuclear Problems of the Joint Institute for Nuclear Research are reviewed. The results of studying their characteristics are presented. A method for manufacturing surface-barrier detectors (identifiers) operating in the total-depletion mode at a bias voltage two to three times higher than the depletion voltage is described. It is shown that lithium-drift detectors with thin (10–20 µm) entrance windows can be created on the side of the diffusion layer. The methods developed for manufacturing such detectors allowed “dead” regions to be minimized and the stable operation and high spectrometric parameters to be maintained during prolonged exposures to charged-particle beams.

Lithium drifted silicon detector fabrication on gettered floating-zone silicon

A gettering procedure using phosphorus doped glass is shown to remove lithium-ion precipitation sites from p-type floating-zone (FZ) silicon. A model involving interaction between grown-in vacancies and oxidation-injected silicon interstitials is proposed to explain the gettering procedure. Examples of silicon lithium-drifted detectors fabricated on ungettered and gettered FZ silicon are presented. >

Lithium-drift detectors based on the solid solutions Si1−x-Gex

Results of experiments to determine the effect of germanium content in single crystals of silicon-germanium solid solutions on the electrophysical, radiometric, and spectrometric characteristics of lithium drift detectors are presented. Surface and volume uniformity of the experimental semiconductor crystals was determined by measuring current, capacitance, noise, and spectrometric and radiometric parameters. Energy resolution and counting rates for beta particles as a function of external bias voltage at 300 K. The contribution of the variable parameters of the semiconductor detectors to the radiometric characteristics of silicon-germanium detectors was determined by establishing a correlation between electric, spectrometric, and radiometric parameters. Counting rates increased as germanium content increased, and it was determined that semiconductor structures with 2-6% germanium had the maximum structural uniformity in the presence of large-scale traps. For this optimal germanium content in lithium drift detectors, detection efficiencies were determined to be from 2.5 to 3 times higher than for silicon-lithium semiconductor detectors. 11 refs., 3 figs.

Transfer Coefficients of Radionuclides Secreted in Milk of Dairy Cows

This study simulated experimentally the transfer of radionuclides to milk of dairy cows on a worst-case situation using various radionuclides known to emanate from nuclear power stations and which have been detected on particulates. Two lactating Holstein cows were administered orally one gelatin capsule containing 10 radionuclides in water-soluble form per day for 14 consecutive days. Milk samples were collected and aliquots analyzed in a germanium lithium-drifted detector coupled to a 2048-multichannel gamma-ray analyzer to measure small amounts of complex mixtures of radionuclides. The transfer coefficients of the radionuclides were calculated when their secretion in milk reached or approached a plateau of concentration. The radionuclides and their transfer coefficients to milk were: chromium51 less than .01%; manganese54 .033±.005%; cobalt60 .01±.002%; iron59 .0048±.002%; zinc65 .31 ± .07%; selenium75 .29±.1%; antimony125 .011±.003%; iodine131 .88±.05%; and cesiium137 .79±.08%.

Long-time behaviour of Si(Li) detectors

The long-time behaviour of silicon lithium-drifted detectors was studied in the course of storage at 60°C with and without a bias voltage. Some observations on the behaviour of these detectors stored at room temperature for periods between one half and two years were also made.It has been found that the observed changes of detector parameters are due primarily to migration of lithium ions in silicon.

A Survey of Radiation Damage in Semiconductor Detectors

Examples of radiation damage in lithium-drifted detectors, lithium-drifted silicon detectors, and high-purity germanium detectors are discussed. The general patterns of damage, lithium-precipitation, annealing and recovery of detectors are outlined, and the observations are discussed.

SHIRX--A High-Resolution Rocket-Borne X-Ray Spectrometer

A rocket-borne x-ray spectrometer is described which used cooled lithium-drifted semiconductor detectors to measure the differential x-ray energy spectrum of Sco XR-1 in the range 0.6-20 keV.

High-Purity Germanium: Detector Fabrication and Performance

The availability of germanium with less than 1010 acceptors/cm3 has enabled us to fabricate and study the performance of germanium detectors of substantially larger volume than has hitherto been possible except by using lithium-drifting techniques. Detectors having a sensitive volume of up to 25 cm3 have been made. These detectors have a lithium diffused n+ contact and a metal barrier non-injecting back contact, and are totally depleted. The energy resolution of these detectors is similar to that of equivalent sized lithium-drifted detectors, but the high-purity detectors are easier to fabricate, and present considerably fewer handling problems.

A method to correct for charged-particle signal loss in lithium-drifted detectors

In semiconductor detectors, the charge collection time will vary with the physical point of deposition of energy within the intrinsic region of the detector. This leads to a geometrical dependence of the signal risetime so that if the signal risetime is not negligible relative to the amplifier differentiation time constant, varying signal loss will occur. For charged-particle spectroscopy, this loss will vary with incident particle kinetic energy. An energy-scale correction method is described for charged-particle spectroscopy in detectors of the p-i-n type. This method reduces energy scale distortions by about a factor of ten.

Transport of fallout radionuclides in the grass-to-milk food chain studied with a germanium lithium-drifted detector.

Transport of fallout radionuclides in the grass-to-milk food chain studied with a germanium lithium-drifted detector.

Decay of 134Cs

The gamma ray spectrum and the conversion line spectrum of 134Cs have been investigated using lithium-drifted detectors. The 563–569 and 796–802 keV doublets have been completely resolved for the first time. The energies in keV, relative intensities in percent per disintegration, and dominant multipolarities of the transitions observed are: 475.57(1.6%, E2); 563.06(8.6%, E2); 569.30(13.3 %, M1); 604.83(97.5%, E2); 796.02(87.0%, E2); 801.90(7.9%, E2); 1038.19(1%, M1 + E2); 1168.40(2%, E2); 1365.40(3.5%, E2); and 1580(0.03%, M1 or E2). The present results are consistent with the levels in 134Ba at 0(0+), 605(2+), 1168(2+), 1401(4+), 1580([Formula: see text]), 1643(3+), and 1970(4+) keV. There is no evidence for the previously reported levels at 923, 1062, 1374, 1770, and 1854 keV. The intensity of the beta-feeds to the levels at 1970, 1643, 1580, and 1401 keV respectively are 24.7%, 1.6%, 0.03%, and 73.7%, as computed from the intensity balance of levels in 134Ba. A modest attempt at comparison between the observed excited levels and theoretical predictions is made.

Gamma rays following the decay of 75Se

The gamma-ray spectrum of 75Se has been studied using lithium-drifted detectors. Seven new transitions have been observed for the first time. The energies in keV and the relative intensities (indicated in parentheses) of the new gamma rays observed are 335.14(0.18), 415.10 (2.76 × 10 −2), 485.58(3.68 × 10 −3), 528.55(1.46 × 10 −2), 535.79(9.34 × 10 −3), 543.03(1.40 × 10 −2) and 556.55(3.04 × 10 −3). The energies and relative intensities of the other gamma rays in the decay are also measured.

Decay Studies of Iodine-118 and -120

The radioactive decay of iodine-118 ground and isomeric state and iodine-120 ground and isomeric state from irradiations of silver with carbon-12 ions, palladium with nitrogen-14 ions, and cadmium with boron-11 ions has been studied with a germanium lithium-drifted detector, a NaI crystal, an anthracene crystal, and several 400-channel analyzers. The electron and $\ensuremath{\gamma}$ spectra, and singles and coincidence spectra, have been interpreted. Tentative decay schemes for the iodine-118 ground state and isomeric state as well as the iodine-120 ground state and isomeric state are given. The half-life of ${\mathrm{I}}^{118g}$ was found to be 13.0\ifmmode\pm\else\textpm\fi{}0.3 min ($\ensuremath{\gamma}$-ray energies: 542, 605, and 1147 keV, ${\ensuremath{\beta}}^{+}$ endpoint energy: 6.05 MeV). The half-life of the isomeric state was found to be 8.5\ifmmode\pm\else\textpm\fi{}0.5 min ($\ensuremath{\gamma}$-ray energies: 600 and 612 keV). The half-life of ${\mathrm{I}}^{120g}$ was found to be 83\ifmmode\pm\else\textpm\fi{}4 min ($\ensuremath{\gamma}$-ray energies: 560, 640, 1520, and 1540 keV, ${\ensuremath{\beta}}^{+}$ endpoint energies: 3.45 and 2.9 MeV). The half-life of the isomeric state was found to be 53\ifmmode\pm\else\textpm\fi{}4 min ($\ensuremath{\gamma}$-ray energies: 600 and 612 keV, ${\ensuremath{\beta}}^{+}$ endpoint energy: 3.85 MeV). A comparison of the levels of ${\mathrm{Te}}^{118,120,122,124,126}$ is given.

Accurate determination of the ionization energy in semiconductor detectors

The average energy g3 expended for electron-hole pair generation in silicon and germanium lithium-drifted detectors by gamma rays, electrons, and alpha particles has been measured as a function of temperature. These data indicate that the difference between ε α and ε e− in silicon is considerably less than previously reported, and in germanium ε α ∼ ε e−.

Calcul des distributions d'amplitude dans les mesures de spectrometrie gamma faites a l'aide de detecteurs au germanium

Taking advantage of the good energy resolution and linearity of germanium lithium drift detectors, we describe a method for the calculation of pulse height distributions observed with these spectrometers for monoenergetic γ rays. This discussion includes the main types of interactions: photoelectric, single and double Compton effects for incident photons and for radiations scattered from surrounding materials. The resulting calculated gamma ray shapes are compared with experimental spectra, the detector considered was a coaxial device with a sensitive volume of 20 cm 3.

Photopeak and double-escape peak efficiencies of germanium lithium drift detectors

This paper presents theoretical calculations by the Monte-Carlo method of the intrinsic efficiencies, photo and double-escape fractions of Ge(Li) drift detectors for parallel beams of gamma rays in the range of energy from 100 keV to 10 MeV. For the photopeak efficiencies the calculations were made from 100 keV to 1.5 MeV and for the double-escape peak efficiencies, they were carried for energies greater than 1.5 MeV. The detectors considered were cylinders having 18 mm dia. and depletion depths of 2, 3.5, 4, 5, 6, 7, 8, 9 and 10 mm. The results are compared with the experimental values obtained by Ewan et al. 1) with a 18 × 3.5 mm detector, for both the photopeak and the double-escape peak efficiencies and with a 5 cm 2 × 8 mm for the full energy peak efficiency.

A Vacuum Encapsulated Lithium-Drift Detector Telescope

Fabrication techniques for producing lithium-drifted telescopes for space experiments have been developed. High resistance grooves have been obtained in lithium-drifted silicon detectors which can separate the detector into independent regions having small cross-talk as well as sharp spatial definition. This indicates the possibility of making integrated arrays of lithium-drifted detectors in a single substrate. The origin of leakage current and noise is not yet clear. The surface condition of the n-i junctions can be a contributing factor. In addition, there are indications that the surface barrier plays a prominent role in this problem. Results of environmental life tests show that the methods of detector fabrication and vacuum encapsulation are capable of providing detectors with the stability required for space experiments.

Design Criteria for Wide Range Gamma Radiacs Using Semiconductor Diode Detectors

The report covers the application of silicon semiconductor detectors to a wide-range, gamma-ray doserate meter using the detectors in a pulse counting mode. It reviews pertinent circuit and detector characteristics that establish the doserate ranges that can be covered, the energy dependence to be expected from both large lithium-drifted detectors and very small silicon junction diodes, and finally the stability requirements imposed on the electronics by the pulse mode of operation. Methods of tailoring the photon energy response of semiconductor detectors are reviewed. Particular attention is given to the effect on design of amplifier and detector noise and variations in the noise level. One example of a combination of detectors to cover the doserate range from 0.2mR/h to 2000 R/h is shown. The report supports the conclusion that it is practical to make a survey meter, using silicon semiconductor detectors, that covers both the health physics and high doserate ranges of interest in military radiac design.

Correlation between Lithium Drift Mobility and Minority-Carrier Drift Mobility in Germanium

The minority-carrier drift mobility of driftable and nondriftable germanium was measured in an effort to determine the presence of an impurity which reduced the lithium drift mobility but did not affect the resistivity and lifetime of some of the material used to make lithium-drift detectors. The measurements were made at 77°K where the mobility is limited by impurity scattering rather than lattice scattering. The mobility was measured as a function of temperature for five samples of Ge which had varying lithium drift mobilities. Good correlation was found between the driftability of the material and the minority-carrier drift mobility at 77°K. Estimated impurity levels in the range of 1014-1015/cm3 apparently reduce the effective lithium mobility, while impurity concentrations greater than 5 X 1015 make the Ge unsuitable for making lithium-drift detectors. A mass-spectrometer analysis indicated that the impurity apparently has a mass number less than Mg; oxygen is a likely possibility due to its known behavior with Li in silicon.

Thermal-vacuum behavior of lithium-drifted silicon detectors

The potential advantages of lithium-drifted silicon detectors compared with conventional silicon surface-barrier detectors under conditions of a thermal-vacuum environment are discussed. Both surface-barrier and n-i-p lithium-drifted detectors have been fabricated which will operate at elevated temperatures and high vacuum for extended periods of time with no evidence of noise instability. The operating characteristics of several litjium-drifted silicon detectors have been studied before, during and after environmental testing to investigate the effects of such testing on detector properties. The results of these studies are reported.