Edward J. Dowdy

Efficiency of a Bismuth-Germanate Scintillator: Comparison of Monte Carlo Calculations with Measurements

Monte Carlo calculations of a bismuth-germanate scintillators efficiency agree closely with experimental measurements. For this comparison, we studied the absolute gamma-ray photopeak efficiency of a scintillator (7.62 cm long by 7.62 cm in diameter) at several gamma-ray energies from 166 to 2615 keV at distances from 30.5 to 152.4 cm. Computer calculations were done in a three-dimensional cylindrical geometry with the Monte Carlo coupled photon-electron code CYLTRAN. For the experiment we measured 11 sources with simple spectra and precisely known strengths. The average deviation between the calculations and the measurements is 3%. Our calculated results also closely agree with recently published calculated results.

Neutron multiplication measurements using moments of the neutron counting distribution

Abstract We demonstrate an improved technique for determining the multiplication of highly subcritical systems that uses the moments of the counting distribution from a neutron detector. In the investigation, the multiplications of each of 2, 3, and 4 kg parts of a plutonium sphere were measured and compared with values calculated using three-dimensional Monte Carlo techniques. The measured and calculated neutron multiplications agree within the statistical uncertainties of the measurements and calculations.

Unfolding bismuth-germanate pulse-height distributions to determine gamma-ray flux spectra and dose rates

Abstract We report a new procedure for unfolding gamma-ray pulse-height distributions acquired with bismuth-germanate detectors. The equipment used for acquiring the distributions consists of a LeCroy 3500 data acquisition and analysis system and eight bismuth-germanate scintillation detectors 7.62 cm in diameter and 7.62 cm long. The system was calibrated and characterized from 0.12 to 8.28 MeV by using gamma-ray spectra from a variety of radioactive sources and from the 14 N(p,γ) 15 O reaction. By fitting these pulse-height distributions with a function containing 17 parameters, we determined theoretical response functions and used them to obtain the gamma-ray flux spectra at multiple space points from a variety of radioactive objects of interest to nuclear safeguards. We used two flux-spectrum-to-dose-rate conversion curves to obtain dose rates. For a composite source, consisting of several sources with accurately known strengths, the result of our procedure agreed with the expected value to within less than 10%. Direct use of measured spectra and the flux-spectrum-to-dose-rate curves to obtain dose rates avoids the errors that can arise because of spectrum dependence in simple gamma-ray dosimeter instruments.

Determination of moisture and trace impurities in helium by he(23s) flowing afterglow

Abstract The He(2 3 s ) afterglow technique for the determination of trace impurities in helium utilizes the bimolecular, energy-transfer reactions of electronically metastable atoms with impurities and spectroscopic detection of the resulting emissions from molecules and transient fragments. The method has been successfully applied to the monitoring of H 2 O, CO 2 , CO, CH 4 , N 2 , and O 2 in the helium coolant of the high-temperature gas-cooled reactor (HTGR). The lower detection limits for these impurities are in the 0.1–10 ppm (cm 3 m -3 ) range using only photomultiplier tube current measurements, with a dynamic range of detection of 2–4 orders of magnitude, depending on the impurity. The apparatus, operating parameters, and methods of calibration are discussed.

An interpolation technique for gamma-ray attenuation coefficients from 40 keV to 15 MeV

Abstract A fast an accurate interpolation method for determining gamma-ray attenuation coefficients has been developed. Based on a simple program that can be stored in a hand calculator, the method fits the attenuation coefficients as a function of gamma-ray energy. For materials whose atomic number is in the low to medium range, a single 8- or 9-term polynomial suffices over an energy range of 40 keV to 15 MeV. For materials of high atomic number, discontinuities in the attenuation coefficients at the K- and L-absorption edges require the use of more than one polynomial.

Neutron Multiplication Measurement Instrument

The Advanced Nuclear Technology Group of the Los Alamos National Laboratory is now using intelligent data-acquisition and analysis instrumentation for determining the multiplication of nuclear material. Earlier instrumentation, such as the large NIM-crate systems, depended on house power and required additional computation to determine multiplication or to estimate error. The portable, battery-powered multiplication measurement unit, with advanced computational power, acquires data, calculates multiplication, and completes error analysis automatically. Thus, the multiplication is determined easily and an available error estimate enables the user to judge the significance of results.

Radiation dosimetry through spectral definition

Abstract We have developed a fieldable instrumentation system for determining from measured flux spectra, both the neutron and gamma ray dose rate distributions associated with radioactive sources. This system includes the sensors, the computer-based data acquisition and analysis hardware, and the requisite software for unfolding the sensor response functions to obtain the flux spectra, and for folding the resultant flux spectra with appropriate flux spectrum-to-dose conversion factors. We use bismuth germanate scintillators that have experimentally measured and analytically interpolated response functions to determine the gamma ray flux spectra, and a suite of neutron sensors, based on proton recoil and 3 He capture, to determine the neutron flux spectra. In addition, gamma ray peak identification is done using HPGe sensors. We describe the equipment and procedures and present some recent results.

Bismuth germanate scintillators: Applications in nuclear safeguards and health physics

Abstract Bismuth germanate (BGO) scintillators are preferable to NaI(Tl) scintillators or germanium detectors for some applications. We describe two systems based on BGO scintillators for applications in nuclear safeguards and health physics. The first system, which consists of eight scintillators and a computer-based data acquisition system, is very efficient. The second, which consists of one scintillator and a small analyzer, is less efficient but portable. A computer code that uses measured response functions and photopeak efficiencies, unfolds the BGO distributions measured with these systems to determine gamma-ray flux spectra and dose rates. One application of these systems is the accurate determination of flux spectra and dose rates from containers of uranium or plutonium. A second application determined these quantities from a replica of Little Boy, the device exploded over Hiroshima.

A Safeguards Instrument to Monitur Spent Reactor Fuel

A hand-held instrument for monitoring irradiated nuclear fuel inventories located in water-filled storage ponds has been developed. This instrument provides sufficient precise qualitative and quantitative information to be useful as a confirmatory technique for International Atomic Energy Agency inspectors, and is believed to be of potential use to nuclear fuel managers and to operators of spent-fuel storage facilities, both at reactor and away-from-reactor, and to operators of nuclear fuel reprocessing plants. Because the Cerenkov radiation glow can barely be seen by the unaided eye under darkened conditions, a night vision device is incorporated to aid the operator in locating the fuel assembly to be measured. Beam splitting optics placed in front of the image intensifier and a preset aperture select a predetermined portion of the observed scene for measurement of the light intensity using a photomultiplier (PM) tube and digital readout. The PM tube gain is adjusted by use of an internal optical reference source, providing long terrm repeatability and instrumerit-to-instrument consistency. Interchangeable lenses accommodate various viewing and measuring conditions.