I. S. Sherman

Response of NaI, anthracene and plastic scintillators to electrons and the problems of detecting low energy electrons with scintillation counters☆

Abstract The electron response of NaI(Tl) has been measured with magnetic-spectrometer-energy-selected electrons incident on cleaved crystal surfaces, in the energy range 5–1000 keV. The light output/keV (≡ response) shows a maximum in the neighborhood of 15–20 keV which is some 20% larger than that at 1 MeV; below 15 keV the response falls sharply reaching 80% of the peak value at ∼ 5 keV. The electron response of anthracene and a plastic scintillator (Pilot B) were measured under the same experimental conditions for comparison. Both anthracene and Pilot B show nonproportional response in the low energy region. At 500 keV the ratio of the responses NaI(Tl)/anthracene/Pilot B are 1.6/1.0/0.38; at 20 keV the ratios are 2.5/1.0/0.41. Decay curve analysis yields for some of the long lived states (excited by incident electrons) in NaI(Tl), 200–300 μsec, 7–9 msec, 40–80 msec, 4 sec, 1 min and 12 min; for anthracene 36 μsec, 120 μsec and 570 μsec. Relatively intense long lived states and more backscattering make NaI(Tl) not generally useful as a low energy electron detector. Uncovered anthracene mounted directly on a photomultiplier and the combination cooled to prevent sublimation of the phosphor in the high vacuum of the beta spectrometer provides a counter which has efficiencies measurable from the pulse height distributions down to ∼6 keV. At backgrounds of 300–500 cpm it has usable efficiencies down to 1 keV.

A Low Walk, High Resolution Timing System for Silicon Detectors

Utilizing times of flight on the order of a nanosecond for heavy particle identification with silicon detectors, it is desirable to reliably distinguish time differences of approximately 100 picoseconds or less. In addition, the required timing circuitry must not substantially deteriorate amplitude resolution capability. In the system designed for this purpose the detector charge is first collected on the capacitance of the detector and fast, low noise, preamplifier and is later transferred to the charge sensitive preamplifier. The fast signal is processed to obtain good time resolution and low walk. The system has a FWHM time resolution of 40 picoseconds, referred to 5.8 MeV deposited in a 100 pF silicon detector. An actual peak width from 3 MeV deposited in an 80 pF silicon by He3 particles was 165 psec FWHM, which is about twice that expected if more optimum pulse shaping had been employed. The instrumental walk for a 100 to 1 amplitude range is less than 150 picoseconds, and is approximately 100 picoseconds for a 30 to 1 range. The noise added to the slow system is equal to that caused by an additional 17 pF of capacitance at its input, plus an overall increase of 5% or less for commonly used slow pulse shaping.

CCD sensors in synchrotron X-ray detectors

Abstract The intense photon flux from advanced synchrotron light sources, such as the 7-GeV synchrotron being designed at Argonne, require integrating-type detectors. Charge-coupled devices (CCDs) are well suited as synchrotron X-ray detectors. When irradiated indirectly via a phosphor followed by reducing optics, diffraction patterns of 100 cm 2 can be imaged on a 2 cm 2 CCD. With a conversion efficiency of ∼ 1 CCD electron/X-ray photon, a peak saturation capacity of > 10 6 X-rays can be obtained. A programmable CCD controller operating at a clock frequency of 20 MHz has been developed. The readout rate is 5 × 10 6 pixels/s and the shift rate in the parallel registers is 10 6 lines/s. The test detector was evaluated in two experiments. In protein crystallography diffraction patterns have been obtained from a lysozyme crystal using a conventional rotating anode X-ray generator. Based on these results we expect to obtain at a synchrotron diffraction images at a rate of ∼ 1 frame/s or a complete 3-dimensional data set from a single crystal in ∼ 2 min. In electron energy-loss spectroscopy (EELS), the CCD was used in a parallel detection mode which is similar to the mode array detectors are used in dispersive EXAFS. With a beam current corresponding to 3 × 10 9 electron/s on the detector, a series of 64 spectra were recorded on the CCD in a continuous sequence without interruption due to readout. The frame-to-frame pixel signal fluctuations had σ = 0.4% from which DQE = 0.4 was obtained, where the detector conversion efficiency was 2.6 CCD electrons/X-ray photon. These multiple frame series also showed the time-resolved modulation of the electron microscope optics by stray magnetic fields.

CCD-based synchrotron x-ray detector for protein crystallograph-performance projected from an experiment

The intense x radiation from a synchrotron source could, with a suitable detector, provide a complete set of diffraction images from a protein crystal before the crystal is damaged by radiation (2-3 min). An area detector consisting of a 40 mm dia. x-ray fluorescing phosphor, coupled with an image intensifier and lens to a CCD image sensor, was developed to determine the effectiveness of such a detector in protein crystallography. The detector was used in an experiment with a rotating anode x-ray generator. Diffraction patterns from a lysozyme crystal obtained with this detector are compared to those obtained with film. The two images appear to be virtually identical. The flux of 10/4 x-ray photons/s was observed on the detector at the rotating anode generator. At the 6-GeV synchrotron being designed at Argonne, the flux on an 80×80 mm2 detector is expected to be ≫ 109 photons/s. The projected design of such a synchrotron detector shows that a diffraction-peak count ≫ 106 could be obtained in ˜0.5 s. With an additional ˜0.5 s readout time of a 512×512 pixel CCD, the data acquisition time per frame would be ˜1 s so that ninety 1o diffraction images could be obtained, with approximately 1% precision, in less than 3 min.

High Resolution Ge (Li) Spectrometer for High Input Rates

A system was developed for obtaining γ‐ray spectra with high resolution at high input rates. For rates up to 100 000 γ‐ray events/sec in a 3.5 cm3 (13 mm thick) Ge(Li) detector, the FWHM at 1.33 MeV is 2–3½ keV and the spectral shift is no more than 0.1%. The amplifying chain, consisting of a cooled FET preamplifier and a simple main amplifier, is pole‐zero compensated throughout. The amplifier produces unipolar pulses shaped with equal integrating and differentiating time constants of 1.2 μsec. The amplifier is followed by a two‐diode baseline restorer, a pileup rejector, a linear gate, and an analog to digital converter. The salient features of the system are described and performance data are presented and discussed.

Observations on the energy resolution of germanium detectors for 0.1-10 MeV gamma rays

The ratio of variance to yield was studied for γ-rays at energies up to 10.080 MeV in lithium-drifted germanium detectors. With detectors of thickness up to 14 mm this ratio was found to be as low as 0.13 /spl I.plusmn/ 0.02 at 122 and 356 keV. At energies between 1 and 10 MeV the ratio was found to be 0.16 ± 0.01, corresponding to a detector contribution of 4.8 /spl I.plusmn/ 0.1 keV (FWHM) at 10 MeV. Investigation of the dependence of the ratio of variance to yield on electric field intensity resulted in acceptance of F = 0.13 /spl I.plusmn/ 0.02 as the best experimental estimate of the Fano factor, and F = 0.16 is interpreted as an upper limit for γ-rays at energies between 1 and 10 MeV.

Measurement of Trace Radionuclides in Soil by L X-Ray Spectrometry

L x-ray spectrometry is shown to be a promising cost efficient technique for assaying several radionuclides in soil simultaneously without prior chemical separation. Analysis is performed with a unique Si(Li)-NaI(Tl) spectrometer. The Si(Li) detector, with thin windows on both surfaces, is mounted on its edge to permit counting x rays from two samples simultaneously. With a thin soil sample on each of its sides this detector is sandwiched between two large NaI(Tl) scintillators. When the Si(Li) detector is operating in anticoincidence with the NaI(Tl) crystals the ¿ background in the x-ray spectrum is reduced by 50%. Thus, this spectrometer has four times the sensitivity of a conventional Si(Li) spectrometer. Initial measurements were made with a prototype spectrometer consisting of a 3 cm2 Si(Li) detector having a resolution of 350-400 eV and two 5¿×4¿ naI(Tl) scintillators. A 30 cm2 Si(Li) detector array with two 12¿×6¿ NaI(Tl) crystals is under development. Measurements of Pu in soil containing 241Am, from Rocky Flats, and Pu in soil containing 137Cs, from a waste-discharge site, are in good agreement with radiochemical measurements. Activity as low as 1 pCi/g of the natural U and Th in soil has been measured. With the array the minimum detectable Pu activity is expected to be 1 pCi/g which corresponds to a concentration of 10-7 - 10-5 ppm by weight.

Considerations in Measuring Trace Radionuclides in Soil Samples by L X-Ray Detection

Several aspects pertaining to the measurements of trace radionuclides (such as plutonium) in soil samples by L x-ray detection have been considered. A 3 × 3 multiplexed array of edge-mounted Si detectors with a total detection area of 50 cm2 and FWHM resolution of 300 eV at 17 keV has been conceived. The fractional transmission-weighted solid angle for a distributed, self-absorbing source was calculated to be 0.25, which is one half the value of that for a lossless sheet source. To determine the minimum detectable activity, intensities of 59 L x-ray lines from the decay chains of 238U and 232Th have been calculated. A total of 130 x rays ranging from 9-20 keV emanate from soil per 100 alpha decays each of 232Th and 238U, compared with 4.6 for 239Pu. The minimum Pu activity detectable with the Si array in such contaminants as worldwide fallout, weapons material, and Rocky Flats soil is expected to approach 1 pCi in 1 g of soil. This concentration is comparable to the natural activity in soil. The minimum Pu activity detectable with NaI detectors is 100 times higher.

Detector With Charge-Coupled-Device Sensor For Protein Crystallography With Synchrotron X Rays

A two-dimensional detector consisting of a 40-mm-diameter x-ray fluorescing phosphor, coupled with an image intensifier and lens to a CCD image sensor, was developed for protein crystallography with synchrotron x rays. The intense x radiation from a synchrotron source could, with a suitable detector, provide a complete set of diffraction images from a protein crystal before the crystal is damaged by radiation (2 to 3 min). The CCD-based detector was evaluated in an experiment with a rotating anode x-ray generator to determine its suitability in this application. Diffraction patterns from a lysozyme crystal obtained with this detector were compared to those obtained with film. The two images appear to be virtually identical. The flux of 104 x-ray photons/s was observed on the detector at the rotating anode generator. At the 6-GeV synchrotron being designed at Argonne National Laboratory, the flux on an 80 X80 mm2 detector is expected to be >109 x-ray photons/s. The projected design of such a synchrotron detector shows that with a CCD pixel full-well capacity of 7 X105 diffraction peaks containing 3 X 106 x rays could be recorded. With an exposure time of 0.5 s and an additional 0.5 s readout time of a 512 X512 pixel CCD, the data acquisition time per frame would be 1 s so that ninety 1° diffraction images could be obtained, with approximately 1% precision, in less than 3 min.

Local efficiency variations in coaxial Ge(Li) detectors

Abstract Substantial variations in full energies peak efficiency were observed when a 1 mm γ-ray beam of 122 keV from 57 Co was directed at a series of points across the face and along the axis of an open-ended true coaxial detector. The full energy peak efficiency near the n-layer was measured to be less than 50% of that near the p-core. As the peak efficiency decreases across the intrinsic region, the continuum below the peak (0–117 keV) increases, so that the total counts in the spectrum remain essentially constant over the sensitive area. This non-uniform response is thought to be due primarily to imperfect compensation resulting in recombination and therefore incomplete charge collection.