Ronald G. Johnson

Design and calibration of an absolute flux detector for 1–15 MeV neutrons

Abstract An absolute neutron flux monitor having fast timing and a calculable response has been developed for use in a collimated beam of 1–15 MeV neutrons. The detector consists of dual thin plastic scintillators in which the proton recoil spectrum distortion caused by the escape of protons from the first scintillator is eliminated experimentally. The absolute detector efficiency was measured at 2.45 and 1.40 MeV neutron energies using the associated-particle technique at the NBS Positive-Ion Van de Graaff facility. The efficiency and pulse height distributions were calculated using a Monte Carlo based program in order to extent the efficiency throughout the 1–15 MeV interval. The uncertainty in the efficiency is 1–2% (standard deviation).

Detectors for neutron scattering in the eV region

Abstract With the advent of high-current high-energy proton accelerators used as pulsed neutron sources for condensed matter studies, interest in extending neutron scattering measurements to the eV region has increased. Among the instruments proposed is the resonance-detector spectrometer (RDS) which uses the sharp low-energy nuclear resonances (usually capture) to define the energy of the scattered neutron. These spectrometers require detectors with high efficiency and good background rejection. Two promising candidates for such detectors, a bismuth germanate (BGO) scintillator and a high-purity germanium (HPGe) detector have been tested at the NBS neutron time-of-flight facility. These tests used simple recoil scattering at high scattering angles and several different resonance foils. Efficiencies and signal to background ratios were measured.

The hybrid undulator for the NIST-NRL free-electron laser

Abstract The NIST-NRL FEL will use a 3.64 m hybrid undulator that is being constructed at Brobeck Division of Maxwell Laboratories. The undulator has a period of 2.8 cm, a variable gap with a 1.0 cm minimum, a peak magnetic field of 0.54 T, and 130 total periods. The magnetic design uses SmCo permanent magnets and vanadium permendur pole pieces. Provision has been made to operate the undulator at half its length to enhance lasing at longer wavelengths. Remote control of gap and taper will permit on-line tuning of the wavelength and extraction efficiency. Vacuum chambers for both full- and half-length configurations are available. The oval aperture of the vacuum chambers is 0.86 cm by 1.6 cm. A system to measure the magnetic field along the undulator is included in the structure. A full-scale, one-period model of the magnetic design has been tested and the results exceed specifications. The mechanical structure of the undulator is nearly complete as is the control system. Construction of the magnet assemblies is under way.

An optical resonator for the NIST-NRL free electron laser

Abstract A 9 m linear optical cavity will be used in the National Institute of Standards and Technology-Naval Research Laboratory free electron laser (NIST-NRL FEL). This FEL, which is driven by a racetrack microtron, is designed to lase from 0.2 to 10 μm. The resonator must accomodate the large dynamic range in wavelength, low gain, high average power and coherent harmonic emission of the laser. The laser can be configured for maximum gain, as is necessary in the ultraviolet, or to minimize diffraction losses, as is necessary in the infrared. This is done by changing the length of the undulator and the radius of curvature of the cavity mirrors.

The NIST/NRL Free-Electron Laser Facility

A free-electron laser (FEL) user facility is being constructed at the National Institute of Standards and Technology (NIST) in collaboration with the Naval Research Laboratory. The FEL, which will be operated as an oscillator, will be driven by the electron beam of the racetrack microtron (RTM) that is nearing completion. Variation of the electron kinetic energy from 17 MeV to 185 MeV will permit the FEL wavelength to be tuned from 200 nm to 10 pm. Performance will be enhanced by the high brightness, low energy spread, and continuous-pulse nature of the RTM electron beam. We are designing a new injector to increase the peak current of the RTM. A 3.6-m undulator is under construction, and the 9-m optical cavity is under design. The FEL will emit a continuous train of 3-ps pulses at 66 MHz with an average power of 10-200 W, depending on the wavelength, and a peak power of up to several hundred kW. An experimental area is being prepared with up to five stations for research using the FEL beam. Initial operation is scheduled for 1991.

An Absolute Neutron Flux Detector for the 1-20 MeV Energy Region

A neutron flux detector for use in the 1–20 MeV energy range is described. A dual thin scintillator configuration yields a proton recoil spectrum which approaches the ideal thin scintillator response. Corrections due to multiple neutron scattering and escape of recoil protons from the scintillator are small. The detector efficiency was experimentally calibrated at 2.47 and 14.1 MeV using the associated particle technique and was extended to other energies by means of a Monte Carlo calculation. The efficiency uncertainty is estimated to be 1–2% in the 1–20 MeV region.

The Hybrid Undulator For The Nist-NRL Free-electron Laser

The NIST-NRL FEL will use a 3.64 m hybrid undulator that is being constructed at Brobeck Division of Maxwell Laboratories. The undulator has a period of 2.8 cm, a variable gap with a 1.0 cm minimum, a peak magnetic field of 0.54 T, and 130 total periods. The magnetic design uses SmCo permanent magnets and vanadium permendur pole pieces. Provision has been made to operate the undulator at half its length to enhance lasing at longer wavelengths. Remote control of gap and taper will permit on-line tuning of the wavelength and extraction efficiency. Vacuum chambers for both full- and half-length configurations are available. The oval aperture of the vacuum chambers is 0.86 cm by 1.6 cm. A system to measure the magnetic field along the undulator is included in the structure. A full-scale, one-period model of the magnetic design has been tested and the results exceed specifications. The mechanical structure of the undulator is nearly complete as is the control system. Construction of the magnet assemblies is under way.

Carlo DTS: A Monte Carlo code for the dual thin scintillator neutron detector

Abstract A Monte Carlo code called CARLO DTS, developed for the efficiency and proton recoil spectra calculation of the Dual Thin Scintillator (DTS) neutron detector is described. The code CARLO DTS covers the neutron energy range between 1 and 20 MeV. The cross sections and angular distributions were taken from the ENDF/B-V data file for the nuclear reactions involved: H(n.n)H, C(n,n)C and inelastic scattering, (n,α), (n,n′)3α reactions on carbon-12. The theoretical calculations are compared to experimental results at two neutron energies, namely:2.446 and 14.04 MeV, obtained by means of the Time Correlated Associated Particle Technique.

NIST-NRL free-electron laser facility

A free-electron laser facility (FEL) is being constructed at the National Institute of Standards and Technology (NIST) in collaboration with the Naval Research Laboratory (NRL) . The FEL will be driven by the electron beam from the NIST racetrack microtron (RTM). The anticipated performance of the FEL is: (1) wavelength variability from 200 run to 10 tim; (2) continuous train of 3-ps pulses at 66 MHz; and (3) average power of 10 W to 200 W. This excellent performance will be achieved primarily because of the unique characteristics of the RTM. This accelerator will provide a continuously pulsed electron beam with high brightness and low energy spread at energies from 17 MeV to 185 MeV. For FEL operation high peak current is required and a new injector for this purpose has been designed. The undulator for the project is 3.64-m long with 130 periods and a peak field of 0.54 T. The construction of the undulator is nearly complete and delivery is expected shortly. The 9-m optical cavity has been designed and is under construction. An experimental area is being prepared for FEL users which will have up to six stations. Initial operation of the FEL is scheduled for 1991. The NIST-NRL FEL will provide a powerful, tunable light source for research in biomedicine, materials science , physics , and chemistry.

The NIST-NRL Free-Electron-laser Facility

A free-electron-laser facility (FEL) is being constructed at the National Institute of Standards and Technology (NIST) in collaboration with the Naval Research Laboratory (NRL). The NIST-NRL FEL will provide a powerful, tunable light source for research in biomedicine, materials science, physics and chemistry. Anticipated performance of the FEL is: (1) wavelength variability from 200 nm to 10 /spl mu/m, (2) continuous train of 3 ps pulses at 66 MHz, and (3) average power of 10 to 200 W. This excellent performance will be achieved primarily because of the unique characteristics of the electron source, the NIST-LANL racetrack microtron (RTM). The RTM will provide a continuously pulsed electron beam with high brightness and low energy spread at energies from 17 to 185 MeV. Construction of the RTM is nearing completion, and a new injector to increase the peak current has been designed. A 3.64 m undulator is under construction, and the 9 m optical cavity is being designed. An experimental area is being prepared for FEL users, which will have up to five stations. Initial operation of the FEL is scheduled for 1991.