Nuclear Instruments and Methods

Calculations of the transport of high-energy photomuons through a very thick iron shield have been carried out. The results presented differ from those given previously in that the angular distribution of the photons which produce the photomuons is included in the calculations. It is shown that the effect of this angular distribution is small and does not account for the previously reported discrepancy between calculated and experimental results.

A time analysis system is described for measurement of lifetimes of nuclear states, including limiters for photo-multiplier outputs, time-to-pulse height converter and fast coincidence system. The intrinsic resolving time for the time-to-pulse height converter is 40 psec and the resolving time of the whole system is about a nsec.

A facility has been constructed at Gulf Energy & Environmental Systems, Inc., to measure neutron-capture cross sections throughout the full neutron-energy region of interest in reactor design. Neutron-flux measurement techniques have been developed that yield absolute values with greater precision than has been previously achieved in time-of-flight measurements. The 10B(n, α)7Li and the hydrogen-scattering cross sections are used as standard cross sections for flux measurements below and above 80 keV, respectively. The 3He(n,p)T cross section was also measured to allow detectors based on this cross section to be used as secondary flux standards.Two neutron flight paths with lengths of 20 and 230 m are used in a complementary fashion to optimize data accumulation, and large liquid scintillators are used at both flight paths to detect the capture gamma rays. These scintillators have been designed to achieve good gamma-ray energy resolution and timing characteristics while preserving a maximum of experimental flexibility.

A new second order focusing, n = 1.5 cylindrical mirror electron spectrometer has been built. It enables us to measure the electrons emitted from the radioactive sources in the 0.1–20 keV region with an instrumental energy resolution of 0.1-1%. The ring-shaped input slit serving as a virtual electron optical object together with the large dimensions of the analyser (the total focal length equals 525 mm) allow us to utilize sources up to 1.5 cm2 area. The spectrometer was adjusted using electron guns and tested also by 57Co and 169Yb radioactive sources prepared by vacuum evaporation and mass separation. The instrument operates automatically at oil-free vacuum of 4 × 10−6 Pa and the electron spectra are scanned in cycles. The background of the channeltron is about 1.5 counts/min.

By means of measurements as well as Monte Carlo calculations response functions have been obtained which describe the pulse-height distributions produced by monoenergetic electrons incident perpendicularly on silicon detectors. Generally good agreement has been found between experimental and Monte Carlo results at energies of 0.25, 0.50, 0.75 and 1.00 MeV for detectors with thicknesses of 0.061, 0.105, 0.191, 0.530, 1.0 and 3.0 mm. Additional calculated results have been obtained as follows: (1) response functions at fifteen energies between 0.15 and 5.0 MeV for detectors with ten thicknesses between 0.05 and 10.0 mm; (2) response functions for arrangements in which the silicon detector is shielded partially or completely by other detectors operating in anticoincidence; (3) reflection, transmission and absorption coefficients of electrons for plane-parallel silicon targets.

High resolution studies have been made of the energy loss distributions of protons after passage through thin foils for the energy range 0.36–4.5 MeV. Although the measurements obtain to a region considered beyond the range of validity, the predictions of Symon and Vavilov are found to be applicable to the observed distributions.

To assist in the evaluation of the hazard associated with exposure to high-energy neutrons, a Monte Carlo computer program was used to calculate the energy deposition as a function of depth in a 30-cm-thick infinite slab of tissue resulting from neutrons incident on the slab at energies up to 60 MeV. The program treated non-elastic and elastic interactions, including evaporation processes and nuclear recoils. Cases of both normal and isotropic incidence were calculated for neutrons of 0.5, 2, 10, 18, 30 and 60 MeV. From these data, current-to-dose conversion factors were extracted for the average whole-body dose, the dose at a 5-cm depth, and the maximum dose. A set of quality factors (QF's) was adopted for tranforming rad dose to rem dose.

Dynamic proton polarizations of +98−4+2% and −100−0+4% have been obtained in 1,2-propanediol doped with Crv-complexes, at a sample temperature of about 0.37 K in a 2.5 T magnetic field. Short polarization time constants indicate that the mechanism for obtaining these high polarizations is the cooling of the electron spin-spin interaction reservoir. The temperature and field dependence of the proton spin-lattice relaxation time were measured down to 55 mK and 0.2 T.

We have measured the absolute photpeak efficiency of a Si(Li) semiconductor detector by a new technique. Proton- and helium-ion bombardments of 18 target materials (8⩽Z2⩽29) were used to produce X-rays at photon energies spanning the range 0.52–8.04 keV. The scattered projectile (1H, 4He) yields into known solid angles at 160° were measured simultaneously with the X-ray data. The detector efficiency was deduced with the help of the recent theoretical K-ionization cross section results of Basbas et al. Our results are in good agreement with a calculated efficiency curve, and also with a measurement using a thin 57Co source. Elastic scattering cross sections have also been measured for protons on a few elements (Mg, Al, Si, Ca, Ni) at 0.5 and 1.0 MeV.

Stopping powers of 0.8–3.9 MeV deuterons and 2.9–6.0 MeV protons have been measured for Havar foil. An Enge spectrograph was used in the energy loss measurements. Results are consistent with the additivity of stopping effects of the constituent elements of the alloy, a conclusion which differs from that of another group2).

Absolute cross sections for the 27Al(p, 3pn)24Na reaction have been determined to be (7.92±0.18) mb at 28 GeV and (10.94 ± 0.24) mb at 0.8 GeV. In this work, proton beam intensities in the AGS fast external beam were measured with current transformer and integrator systems which were calibrated electrically. The 24Na induced in aluminum foils was assayed by comparing the intensity of its 1368 keV gamma ray with that of the 1332 keV line from calibrated 60Co standards.

A multiparticle spectrometer at the CERN Intersecting Storage Rings is equipped with proportional chambers. In this paper we describe the construction of 20 modular 100 × 200 cm2 wire chambers, the electronics for a 50K wire system, and the performance of the chambers.

A multiparticle spectrometer at the CERN Intersecting Storage Rings is equipped with a multiwire proportional chamber detector. In this paper we describe trigger, data acquisition, and monitor system of the detector.

We describe a study of the Si(001) surface structure using 0.1 − 2.0 MeV He+ ion scattering. The experimental configuration required for this work is discussed in detail. The measured intensities of the Si surface peak as a function of energy are presented for the clean Si(001) surface and two different states of the hydrogen covered Si(001) surface. The hydrogen (deuterium) coverage is determined via the 3He(d, p)4He reaction. The experimental results are compared to computer simulations; various aspects of the model of thermal vibrations used in the simulation are discussed. The results indicate that there is extensive sub-surface stain in the reconstructed Si(001) surface.

Total energy spectra of backscattered electrons for 1.0-MeV electrons with perpendicular incidence on targets of saturation thicknesses of Al, Fe, Sn and Au are shown. The variation of the shape of the energy distributions with angle observed for Al and Fe is illustrated. Total backscatter coefficients of 0.08, 0.22, 0.37, and 0.51 were obtained for Al, Fe, Sn, and Au. Mapping of electrons backscattered from a saturation thickness of Al for an angle of incidence of 30° from the target plane was also carried out at 1.0 MeV. Angular distributions as a function of two angles and typical energy distributions are included.

The design and performance of an array of plastic scintillator neutron counters are discussed. The array uses counters of three sizes to detect neutrons of 1.0 to 450 MeV, measuring their energy by time-of-flight and their production angles by counter identification and end-to-end timing. Information from 180 000 neutron-proton charge exchange events is used to investigate the resolution stability and internal consistency of the system. Neutron pulse height distributions for narrow bins of neutron energy between 1.25 and 313 MeV are presented and compared with the predictions of a Monte Carlo program.

Spectra of carbon, nitrogen, oxygen and neon, obtained at an energy of 1.15 MeV/nucleon, have been studied in the 30–1200 Å wavelenght range. By comparison of these different spectra, identification of numerous new lines belonging to the hydrogen-, helium-, or lithium-like isoelectronic sequence could be carried out. For the two- and three-electron ions, mean lives of the more intense transitions have been measured.

Elastic and Compton scattering of 1.17 and 1.33 Mev gamma rays through angles between 4.5° and 8° have been studied with a Ge(Li) detector for the first time. The corresponding photopeaks have been resolved in the case of each energy. A technique has been developed for the determination of scattering cross sections of lead from ratios of counts obtained in the case of aluminium and lead scatterers. The errors in the elastic and the Compton scattering cross sections of lead vary between approximately four and six percent. The results for lead agree surprisingly well with predictions based on non-relativistic HF calculations of Hubbell et al. concerning form factors for Rayleigh scattering, and incoherent scattering functions for Compton scattering. However, small differences exist between experimental results and the calculations and indicate the desirability of accurate relativistic calculations of Compton, Rayleigh and Delbrück scattering processes.

A simple theory concerning the photon induced electron emission and absorption is derived, based on the diffusion model proposed by Bethe, Rose and Smith. Analytical expressions are presented for emission efficiency and energy deposition of electrons produced in a slab exposed to 1.25 MeV gamma rays and compared to experimental measurements and Monte Carlo calculations.Although the agreement between them is generally good, in view of the simplified model, a considerable difference is shown for the forward emission efficiency in the case of a very thick slab compared to the electron range and for the backward emission efficiency in the case of a relatively thin slab or of very low atomic number material.

A 14-stage accelerating tube is supplied by a 7-stage Cockcroft-Walton generator with selenium rectifying units. The potential distribution across the accelerating tube is ensured by a parallel resistor. Special care has been devoted to the electron-optical part and this makes it possible to control a 3 mA electron beam with a current of 110 μA through the parallel resistor at a generator load of 3.2 mA. Focussing and intensity of the beam are independent of the high tension. X-ray intensities of 300 r/min at a distance of one metre from the target are readily attainable.

The associated particle technique, with a gas target, has been used to measure the absolute central neutron detection efficiency of two scintillators, (NE213 and NE102A) with an uncertainty of less than ±2%, over the energy range 1.5–25 MeV. A commercial n/γ discrimination system was used with NE213. Efficiencies for various discrimination levels were determined simultaneously by two parameter computer storage. The average efficiency of each detector was measured by scanning the neutron cone across the front face.The measurements have been compared with two Monte Carlo efficiency programs (Stanton's and 05S), without artifically fitting any parameter. When the discrimination level (in terms of proton energy) is determined from the measured light output relationship, very good agreement (to about 3%) is obtained between the measurements and the predictions. The agreement of a simple analytical expression is also found to be good over the energy range where n-p scattering dominates.

A 1.5 m × 1.0 m × 0.6 m NeHe streamer chamber and high voltage pulsing system capable of 1.0 MV pulses with widths continuously variable from 10 to 20 ns has been developed by a University of Illinois - ANL group. The design and construction of the dc fast charge regulator, Marx generator, Blumlein pulse shaping network, transmission line, and chamber are described. Studies on the effects of increasing field emission and injection of free electrons into the pulse shaping network gap on pulse amplitude stability are reported. Under normal operating conditions, pulse amplitude stability of 2–3% rms has been achieved. Photographic and pulse monitoring systems are described and operating experience over two experimental runs is summarized.

This report presents the design of a xenon gas ionization chamber. Corrections to energy fluence measurements were considered. The correction for escape of xenon fluorescent L X rays was measured with a xenon-methane proportional counter. The chamber was compared to a Si(Li) detector and an argon-methane proportional counter over the energy range from 1.7–10.5 keV. The chamber was shown to be accurate to ±2%, independent of uncertainties in entrance window transmissivity. Overall chamber accuracy is ±3%. The useful energy fluence rate range for the chamber was shown to be from below 10−11 to 5 × 10−5 W/cm2. A value of 21.5±0.4 eV for the mean energy per ion pair in xenon was obtained from the measurements. A chamber useful up to 35 keV could be constructed without accuracy degradation by an increase in chamber size.

An apparatus consisting of two magnetic spectrometers has been constructed at the CERN Proton Synchrotron (PS) in a missing-mass experiment to identify and select the beam particle and the outgoing one, and to measure their momenta. This paper gives details of the two spectrometers, which are comprised of multiwire proportional chambers, high resolution time-of-flight counters, and aerogel and water Cherenkov detectors.

A 1.6 GeV/c spectrometer has been constructed at SLAC incorporating an n = 0, 90° bend, 254 cm radius magnet with second-order corrections. The magnet is of the window frame type allowing invariant focal properties up to 21 kG. It has a momentum resolution of ± 0.08% and an angular resolution of ± 0.4 mrad. It simultaneously focusses production angle and momentum from a 20 cm long target onto a single focal plane orthogonal to the beam direction allowing a considerable simplification in detecting a high energy scattering process.