The COMBAS fragment separator

The basic ion-optical characteristics of the COMBAS fragment separator are analyzed. The momentum distributions of radioactive ⁶He, ⁸He, and ⁹Li nuclei obtained in the reaction ¹¹B (33 A MeV, where A is the mass number of a particle) + ⁹Be (332.6 mg/cm²) have been investigated in forward-angle measurements on the COMBAS fragment separator. The momentum and angular (horizontal) acceptances of the COMBAS separator have been measured using the ⁶He, ⁸He, and ⁹Li beams. It has been ascertained that the images of the ⁶He, ⁸He, and ⁹Li nuclear beams in final achromatic focus of the separator F _a approximately twofold exceed the size of the beam on a producing target (input focus F ₀), at which the primary beam has a diameter of 6 mm. The intensities of ⁶He, ⁸He, and ⁹Li beams obtained at a 5-μA intensity of the primary ¹¹B beam are 6.9 × 10⁵, 2 × 10⁴, and 4.7 × 10⁵ particles/s, respectively. These values are sufficient for use in spectroscopic measurements. It is proposed that time-of-flight analysis of nuclear reaction products at the exit from the COMBAS separator will be used not only to measure the energy of transported particles over the whole operating range of the momentum acceptance, but also to identify them by mass A and charge Z without loss of these particles. The problem of reducing the count rates of detectors and further improvement of their energy resolution for detected particles can be solved by placing a high-resolution magnetic spectrometer past the second target accepting the secondary radioactive nuclear beams.     

A method for determining the intensity of concomitant neutron source D(d, n)3He when studying the characteristics of delayed neutrons from nuclear fission induced by neutrons from reaction T(d, n)4He

A factor that may be responsible for the discrepancies in the total delay neutron yields, the relative yields of separate groups of delayed neutrons, and half-lives of their precursors has been investigated. These discrepancies are shown to be attributable to the effect of a concomitant neutron source—reaction D(d, n)³He. Such a source is unavoidably present in an experiment where reaction T(d, n)⁴He on a solid-state target is used as a neutron source. A method has been developed to calculate the contribution of neutrons from the reaction D(d, n)³He to the measured total and relative yields of delayed neutrons and the half-lives of their precursors in heavy nuclei fission induced by neutrons with energies of 14–18 MeV. The energy dependence of the parameters of delayed neutron groups from the neutron-induced fission of ²³⁸U nuclei in the energy range of 14.2–17.9 MeV is presented.     

Calibration of a multilayer semiconductor spectrometer using α sources

A method for calibrating a multilayer spectrometer using α particles is presented. This spectrometer is composed of successive semiconductor Si(Li) detectors and is used for precision detection of long-range charged particles (p, d, and t) with energies of ～100 MeV. The factors affecting the accuracy of calibration are analyzed. This method is shown to guarantee high accuracy in measuring charged-particle energies.     

Obtaining monochromatic beams of accelerated 6He ions with a near-Coulomb-barrier energy on the DRIB accelerator complex at the JINR

Two methods for obtaining monochromatic beams of accelerated ⁶He ions with energies around the Coulomb barrier on the DRIB accelerator complex at the Joint Institute for Nuclear Research (JINR) is described. In the first method, the MSP-144 magnetic spectrometer is used as a monochromator of low-energy ⁶He beam after its passing through an energy absorber. The energy resolution of the ⁶He beam, which is 500 keV, is governed by the linear dimension of the target in the focal plane (18 mm). In the other method, a special probe is used to investigate nuclear reactions with accelerated ⁶He beams without loss of the beam intensity. With this probe, it is possible to perform experiments on the internal beam of the postaccelerator (U-400 cyclotron) of the DRIB complex. As a result, a ⁶He beam with a required energy and a beam resolution of 150 keV or better is produced at the target.     

A magnetic transport electron spectrometer for in-beam measurements

A compact magnetic transport electron spectrometer has been built having an overall efficiency of 0.92%, a resolution Δp/p of 18% and a maximum transmitted energy of 3 MeV. Limitations to accuracy in intensity measurements due to non-isotropic electron angular distributions have been discussed.     

A Neutron Spectrometer for Experiments with Radioactive Beams on the ACCULINNA-2 Fragment Separator

A neutron spectrometer based on stilbene crystals has been developed by the Flerov Laboratory of Nuclear Reactions at the Joint Institute for Nuclear Research (Dubna, Russia). The timing resolution is determined as a function of the signal amplitude (σT = 0.18 ns at an amplitude of 1 MeV in the electron equivalent). The measured energy resolution of the detecting modules for γ rays is σE/E = 4.5% at E = 1 MeV. The quality of the n–γ discrimination is investigated. It is shown that reliable discrimination is possible, beginning with a deposited energy of 100 keV in the electron equivalent, which corresponds to the kinetic energy of recoil protons of ～700 keV. The neutron spectrometer helps to significantly expand the experimental capabilities and to carry out correlation experiments with radioactive beams on the ACCULINNA-2 fragment separator at a new level of quality.     

A method and setup for studying the energy dependence of delayed neutron characteristics in nuclear fission induced by neutrons from the T(p, n), D(d, n), and T(d, n) reactions

A method and a setup based on this method are described, with which it has for the first time become possible to measure the energy dependence of the absolute total and relative yields of delayed neutrons and the half-fives of their precursors from neutron-induced fission of heavy nuclei in the course of one experiment. T(p, n)³He, D(d, n)³He, and T(d, n)⁴He nuclear reactions induced by high-energy charged-particle beams from the KΓ-2.5 electrostatic accelerator at the Institute of Physics and Power Engineering are used as sources of monoenergetic neutrons. The measured total delayed neutron yields from neutron-induced fission of ²³³U and ²³⁹Pu nuclei in the energy range of 0.37–4.7 MeV, as well as the relative yields of delayed neutrons and the half-lives of their precursors from neutron-induced fission of ²³⁹Pu in the range of 15.8–17.9 MeV, are presented as an illustration of the method. The uncertainties of the data obtained by means of this method are shown to be significantly lower than the uncertainties of similar data measured using other techniques.     

A sectioned spectrometer of fast neutrons

Development of a capture gated spectrometer on the basis of a liquid organic scintillator doped with enriched ⁶Li is discussed. Particular interest is evoked by the good pulse height resolution of the spectrometer for 14-MeV neutrons, which is expected to be very high, ～10–15%. This resolution is attained by compensating for the nonlinearity of the light yield in the scintillator owing to the use of separate optically isolated sections, which independently detect scintillations from each recoil proton. The detector is sensitive to fluence rates ranging from 10^?4 to 10² cm^?2 s^?1 above a threshold of 500 keV under conditions of uncorrelated γ-ray background at a level of up to 10² s^?1 (E > 100 keV). A pilot model of the detector based on a scintillator without a lithium dopant has been produced and tested. The detector efficiency is governed by the scintillator volume (～1.2 l); for 3-MeV neutrons, its value is 0.2–0.5%. The response of the pilot detector to neutrons from a Pu-α-Be source with energies of up to 10 MeV has been measured. Initial testing indicates a low threshold at an ～600-keV energy of a recoil proton. A good spectral response is obtained using the criterion that three optical sections of the detector operate at a time. This spectrometer can find application in low-background experiments in basic physics research, as well as in space research and nuclear medicine for measuring the parameters of the neutron flux.     

An adaptive γ-ray spectrometer with a high event processing rate

A fast γ-ray spectrometer adaptable to the scintillator type is described. This spectrometer is capable of processing a γ-ray energy spectrum (with a resolution of <4% in the energy range from 20 keV to 10 MeV) in a sequence of time intervals in the real-time mode at a counting rate of up to 10⁶ cps. Digitization of the detector signals by a 14-bit ADC with a sampling rate of 64 MHz and real-time data stream processing are used to separate overlapping events and correctly generate energy spectra.     

A time-of-flight mass spectrometer for monitoring the inhalational anesthesia concentration in the real-time mode

The use of a medical mass spectrometer for measuring the concentration of the gas mixture components in the breathing circuit of an inhalational anesthesia machine in the real-time mode is described. The resolution of the mass spectrometer is M/ΔM = 200, and the detection threshold in terms of the partial pressure of the analyzed gases is 2 × 10^?12 mbar. The mass spectrometer is capable of measuring the volumetric content of CO₂, O₂, and inhalational anesthetic sevoflurane. The respiratory coefficient (CO₂/O₂) was measured during anesthesia to estimate the patient’s stress reaction to a surgical injury.     

Tagging of η mesons using the pd → 3Heη reaction near threshold

A zero-degree spectrometer for tagging η mesons on the CELSIUS/WASA facility is described, and its characteristics are presented. Tagging of η mesons is performed by detecting ³He ions produced by reaction pd → ³Heη close to the production threshold. The low background level (<2%) is in good agreement with the data obtained earlier on the SATURNE accelerator.     

Improving the timing resolution of an electromagnetic calorimeter based on lead tungstate crystals

Results of the beam tests of the prototype photon spectrometer PHOS for the ALICE experiment at the Large Hadron Collider (CERN) are presented. The spectrometer is based on detector elements composed of lead tungstate (PbWO₄) crystals with dimensions of 22 × 22 × 180 mm and Hamamatsu S8664-55 (S8148) avalanche photodiodes. The beam tests have been performed on the secondary T10 beamline of the PS proton synchrotron. The main emphasis has been placed on the possibility of improving the PHOS timing resolution. Introduction of an additional timing channel with a silicon photomultiplier (SiPM) used as a photodetector is shown to improve the timing resolution for 1-GeV deposited energy from current value σ _t = 3 to 0.3 ns. Silicon photomultipliers of the Hamamatsu MPPC S10362-33 family with an active area of 3 × 3 mm² are used in these measurements. Using fast photomultiplier tubes with an 8-mm-diameter photocathode, the timing resolution attainable in electromagnetic shower development in a lead tungstate crystal has been measured for a large-area photodetector. The timing resolution for a deposited energy of 1 GeV is 150 ps. The effect of the detector channel temperature on the timing resolution is investigated. Cooling the crystal results in an increase both in the scintillation intensity and in the decay time of the scintillator and fails to substantially improve the timing resolution.     

Accelerator energy calibration using nonresonant nuclear reactions

A new technique is presented for determining directly the beam energy of an accelerator to within 2 keV. This technique uses the conventional backscattering spectrometry (BS) setup, i.e. Si surface barrier detector and electronics along with a multichannel analyzer. Two measurements are required. The first is a BS measurement of a standard calibration sample, e.g. 5 Å of FeW alloy deposited on SiO₂. This data defines two linear equations that relate the energy per channel m and the energy intercept b of the system to the beam energy E₁. The second measurement is of some positive-Q nuclear reaction, e.g. ¹⁵N(p, α)¹²C,Q = 4.965 MeV. By writing the energy of the outgoing particle as a Taylor series about some initial energy guess and keeping terms to first order, we obtain a third linear equation relating m, b and E₁. A positive Q nuclear reaction is required to prevent these three equations from being homogeneous. These equations can be solved iteratively for E₁ along with m and b. This technique has the advantage that it can be readily applied at essentially any energy between 0.5 and 3 MeV. By contrast, only a small number of suitable resonances exist in this energy range.     

Measuring the nuclear charge of fission fragments using a large ionization chamber—part of the double-arm time-of-flight spectrometer

Identification of decay products is a relevant task in studying rare collinear decays of low excited heavy nuclei. A technique for measuring the nuclear charge of decay products detected by a wide-aperture ionization chamber—a part of the double-arm time-of-flight spectrometer—is described. Two versions of nuclear charge calibration using data of the reaction ²³⁵U(n _th, f) have been developed to determine the charge of the decay products. Testing with simulation data shows that the use of charge parameterization based on the Bohr-Willer empirical equation in the calibration procedure makes it possible to determine the nuclear charge of the fission fragments over a wide energy range. The charges of light ions from He to C, predicted on the basis of this approach, appear to be overestimated by two charge units.     

Basic questions related to electron-induced sputtering in the TEM

Although the theory of high-angle elastic scattering of fast electrons is well developed, accurate calculation of the incident-energy threshold and cross section for surface-atom sputtering is hampered by uncertainties in the value of the surface-displacement energy E_d and its angular dependence. We show that reasonable agreement with experiment is achieved by assuming a non-spherical escape potential with E_d=(5/3) E_sub, where E_sub is the sublimation energy. Since field-emission sources and aberration-corrected TEM lenses have become more widespread, sputtering has begun to impose a practical limit to the spatial resolution of microanalysis for some specimens. Sputtering can be delayed by coating the specimen with a thin layer of carbon, or prevented by reducing the incident energy; 60 keV should be sufficiently low for most materials.     

The effect of the composition of a Nd-loaded liquid organic scintillator on light yield

The light yield in samples of a Nd-loaded scintillator was measured. The scintillator composition differed in the solvent (pseudocumene (PC) and a mixture of PC with linear alkyl benzene), the scintillating dye type (РРО or ВРО), and the Nd concentration. The light yield in the PC (PPO, 1.5 g/L), which was approximately 11500 photons/MeV, was assumed to be the standard. The ionizing-radiation sources were ¹³⁷Cs, ²⁴¹Am, and ¹⁰⁹Cd radionuclides. It was shown that the light yield in the scintillator with a Nd concentration as high 30 g/L (~3%) in PC made it possible to attain energy resolution at a level of 3% at an energy of 3 MeV (neutrinoless 2β decay of ¹⁵⁰Nd).     

An Apparatus to Obtain a Pure Annihilation Peak

Abstract

A timing discrimination set‐up was developed and the discrimination method was applied to the annihilation gamma ray peak of the ²²Na radioisotope. The annihilation peak was discriminated from the superimposed photon spectrum generated by the Compton effect, backscattering, and background in the 0.5 MeV energy region. A NaI(Tl) inorganic scintillation detector was used to detect the annihilation gamma rays of the radioisotope. Energy resolution of 9.1% was obtained for the discriminated annihilation peak. The developed spectrometer has achieved the discrimination of the annihilation counts and the photo peak counts superimposed on them.     

High-resolution,energy-dispersive microcalorimeter spectrometer for X-ray microanalysis

We have developed a prototype X-ray microcalorimeter spectrometer with high energy resolution for use in X-ray microanalysis. The microcalorimeter spectrometer system consists of a superconducting transition-edge sensor X-ray microcalorimeter cooled to an operating temperature near 100 mK by a compact adiabatic demagnetization refrigerator, a superconducting quantum interference device current amplifier followed by pulse-shaping amplifiers and pileup rejection circuitry, and a multichannel analyser with computer interface for the real-time acquisition of X-ray spectra. With the spectrometer mounted on a scanning electron microscope, we have achieved an instrument response energy resolution of better than 10 eV full width at half-maximum (FWHM) over a broad energy range at real-time output count rates up to 150 s^?1. Careful analysis of digitized X-ray pulses yields an instrument-response energy resolution of 7.2 ± 0.4 eV FWHM at 5.89 keV for Mn Kα_1,2 X-rays from a radioactive ⁵⁵Fe source, the best reported energy resolution for any energy-dispersive detector.     

Two-arm semiconductor spectrometer for charged particles for the investigation of absorption by nuclei of stopped negative pions

A two-arm semiconductor spectrometer for the detection of secondary charged particles, such as p, d, t, ³He, ⁴He and of their correlations in the process of stopped pion absorption by nuclei is described. The spectrometer consists of two telescopes of Si-detectors with a sensitive surface of 800 mm², two semiconductor detectors as monitors and the “live” target, a silicon surface barrier detector. The number of semiconductor detectors is 19.A technique for pion stop selection in thin targets is described.The problem of particle identification and of measurements of their energy with the help of a multicrystal semiconductor telescope is discussed. The technique provides an absolute normalization of spectra. The “alive” target helps to obtain more information on the process of pion absorption by Si-nuclei.     

Measuring the neutron spectrum of the accelerator-based source using the time-of-flight method

The time-of-flight technique with a new method for generating short radiation bursts has been used to measure the neutron spectrum of the accelerator-based source with a stationary proton beam. Specific problems arising thereby and methods for solving them are described. The measured spectrum of neutrons in the reaction ⁷Li(p, n)⁷Be at a proton energy of 1.915 MeV is presented and compared to the calculation. This spectrum is shown to comply with the requirements for the neutron beam used in neutron capture therapy.