Search

The number (AN), energy (AE) and dose (AD) albedos were measured at the scattering angle of 168° for 59.54 keV gamma-rays emitted from a radioactive source of Am-241. The experiments were carried o...

Number, energy and dose albedos are measured at a scattering angle of 180° for a broad beam of 662 keV gamma rays obtained from a radioactive source of 137Cs (having strength in µCi; 1 Ci = 3.7 × 1010 disintegrations per second). The gamma beam is incident on semi-infinite thick targets of variable atomic numbers. The scattering media is divided into three sets, which are pure elements, alloys and composite materials. Experiments are carried out using a 3″ × 3″ NaI(Tl) scintillation detector. To obtain precision in data, the response unfolding of a scintillation detector is used, which converts the observed pulse-height distribution to a true photon spectrum over the energy range of 2.5 to 640 keV. The detector response unfolding results in the true intensity of back-scattered gamma flux by shifting the events resulting from partial absorption of photons to the full energy peak of the spectrum. In the present study, albedo factors are studied as a function of target thickness and their atomic numb...

The effective atomic numbers of some transition and rare earth compounds have been determined by measuring the ratio of Rayleigh to Compton scattering signal using 59.5 keV gamma radiation from americium-241 radioactive source. The scattered gamma photons from the elements and compounds at an angle of 90° were detected using an ORTEC high-purity germanium detector coupled with 16K multi-channel analyzer. By measuring the ratio of Rayleigh to Compton scattered signal (momentum transfer 3.38 A−1), the effective atomic numbers of the transition and rare earth compounds have been determined and compared with theoretical values predicted by AutoZeff, power law, and direct method.

A densitometer system, operating in a non-destructive and non-invasive way, is presented to investigate the density and moisture variation in wood samples of Pinus roxburghii (known as chir pine). The inherent ability of Compton scattering can ascertain quantitative information of electron density of the target material. A well collimated beam of 662 keV gamma rays from 137Cs radioactive source has been used to extract the information of density from interior of sample by recording scattered spectra with NaI(Tl) scintillation detector. An inverse matrix approach for unfolding of observed pulse-height distribution to a true photon spectrum is used, by choosing bin mesh (E1/2) of 0.025 (MeV)1/2, for the measurements of scattered spectra. The regression lines, obtained from data of scattered spectra, provide the density and moisture content of selected wood sample. The adjusted R2 value 0.984 for fitted curve, justifies the linear dependence of scattered intensity upon wood density. Also, Compton sca...

A method has been presented for calculation of effective atomic number (Zeff) of composite materials, by using back-scattering of 662 keV gamma photons obtained from a 137Cs mono-energetic radioactive source. The present technique is a non-destructive approach, and is employed to evaluate Zeff of different composite materials, by interacting gamma photons with semi-infinite material in a back-scattering geometry, using a 3″ × 3″ NaI(Tl) scintillation detector. The present work is undertaken to study the effect of target thickness on intensity distribution of gamma photons which are multiply back-scattered from targets (pure elements) and composites (mixtures of different elements). The intensity of multiply back-scattered events increases with increasing target thickness and finally saturates. The saturation thickness for multiply back-scattered events is used to assign a number (Zeff) for multi-element materials. Response function of the 3″ × 3″ NaI(Tl) scintillation detector is applied on observ...

In this study, the semi-empirical and empirical calculations of K X-ray intensity ratios, K-shell fluorescence yields, and vacancy transfer probabilities have been performed for 3d transition elements. Also, σKα, σKβ production cross-sections, Kβ/Kα intensity ratios, ωK fluorescence yields, and ηKL vacancy transfer probabilities of 3d transition elements have been measured. The samples were excited by 59.5 keV γ-rays from a 241Am annular radioactive source. K X-rays emitted by samples were counted by an Ultra-LEGe detector with a resolution of 150 eV at 5.9 keV.

In this study, we measured the mass attenuation coefficient of n-type GaAs, p-type GaAs, n-type Si and Au/n-Si/n+Si/Al samples with and without external electric field. Samples were set in perpendicular direction to the 100 mCi 241Am radioactive source and counts were made with a NaI(Tl) detector. It was observed that when an external electric field was applied onto the samples, mass attenuation coefficients were increased. Results showed that this study is consistent with previous studies.

The article presents an in situ clouds-powered radioactive source detection and localization approach, namely color-depth-radiation mapping, using 3-D land mapping within hazardous indoor environment and incorporating sensor fusion between a RGB-D camera and a portable radiation detector. In the approach, to achieve fast and robust image registration, color images detected by the camera are initially employed to extract crucial visual features and establish pairs of matched image features between successive scanned images. Following this, matched features are incorporated with the corresponding calibrated depth information to generate 3-D keypoint cloud pairs. To remove potential noises in the acquired data-sets, a novel geometric-based filtering algorithm is developed to reject incorrect keypoint pairs prior to iterative closest point-based image registration. Most importantly, an algorithm to determine the radioactive sources’ parameters including strength and 3-D position is developed for accurate radi...

The performance of a new X-ray spectrometer is presented. The device was designed originally to be employed in environmental sciences, and allows the use of different types of primary radiation sources, such as a radioactive source (241Am) or tubes with anodes of different metals (Rh or W). Among the advantages of this spectrometer are the possibility of exchanging detectors to improve efficiency at different X-ray energy ranges [such as Si (Li), Si-PIN, hyperpure Ge, or CdTe detectors], to attach it to a wavelength dispersive system, and its use in vacuum or atmospheric pressure. Furthermore, it is feasible to modify the geometry so a secondary target may be installed for using polarized X-rays as exciting radiation.

Abstract Two nuclear methodologies were used for porosity and pore size distribution determination of SiC ceramic foams. Thirty samples were analysed, six of each one of the following pore densities: 30, 45, 60, 80 and 100 pores per inch (ppi). The two nuclear techniques employed were X-ray microtomography and gamma ray transmission. For the microtomography technique, the spatial resolution of the images was 32 μm. For the gamma ray transmission methodology, a NaI (Tl) scintillation detector and Am-241 radioactive source were used. The gamma transmission technique was precise for porosity determination in relationship to the nominal values supplied for the sample manufacturer. The 30 and 45 ppi samples analysed by the microtomography technique present average porosities equivalent to the nominal porosity, and the other samples present an average of 4·6% smaller values. The 30 and 45 ppi sample two-dimensional images show voids inside the structural solid material of the ceramic.

The rapid development of the nuclear power plant (NPP) in China leads to increasing attention to the minimization of radioactive waste. The primary coolant is one of the sources of low-level radioactive wastewater and must be decontaminated before its discharge to the environment. One of the possibilities is by means of continuous electrodeionization (CEDI) technology. In this paper the lab-scale experiments demonstrate that CEDI can offer favorable decontamination of primary coolant in NPP, with minimized radioactive spent resin production. Displacement of the anion exchanger by weak base anion exchangers in a CEDI module can improve the Co 2+ and Sr 2+ removal. In the dilute effluent of the modified module, Co 2+ and Sr 2+ concentrations are below 2 ng·l -1 and 58 to 114 ng·l -1 , respectively, which is much lower than the commercial one of 205 to 289 ng · l -1 and 268 to 326 ng·l -1 . This displacement has a negligible influence on the electrical resistance of the module.

Within the homeland security and emergency response communities, there is a need for a low-profile system to detect and locate radioactive sources. RadTrac has been developed at Argonne National La...

In our earlier papers, we developed a theory of reactor noise for accelerator-driven systems (ADSs). It was shown that reactor noise in ADSs is different from that in critical or radioactive source-driven subcritical systems because of the periodically pulsed source and its non-Poisson character. Various noise descriptors, such as Rossi alpha, Feynman alpha (or variance to mean), power spectral density, and cross-power spectral density, were derived, for a periodically pulsed source, including correlation between different pulses and finite pulses of different shapes. Throughout the work we restricted ourselves to the case of prompt neutrons only. In the present paper, we extend the theory to the delayed neutron case. Feynman-alpha and Rossi-alpha formulas are derived by considering the source to be a periodically pulsed non-Poisson source, without correlations between different pulses. Each pulse is assumed to be a delta function. The calculations are carried out in the time domain that leads to closed-form expressions for these descriptors.

In the presented work, the leading edge timing and constant fraction discrimination timing techniques were applied to beta rays emitted from a 137Cs radioactive source through the developed spectrometers in order to improve the beta energy spectrum of the source. The energy spectra, with and without the techniques, were obtained with a BC‐400 plastic scintillation detector. The obtained spectra were compared with each other for the elimination of spurious pulses, which rise from background, backscattering, scattering, and any radiations other than the source, on the multichannel analyzer output.

In order to obtain pure beta and gamma ray energy spectra, two spectrometers were developed to eliminate the background and the spurious counts not from a radiation source. Anticoincidence (active shielding) and timing methods were used in the beta and gamma spectrometers, separately. The results obtained from the spectrometers from both methods were compared with each other. The timing method was more successful in rejection of background and spurious counts in the energy spectrum of a radioactive source.

In the USA and other countries large numbers of vehicles pass through border crossings each day. The illicit movement of radioactive sources is a concern that has resulted in the installation of radiation detection and identification instruments at border crossing points. This activity is judged to be necessary because of the possibility of an act of terrorism involving a radioactive source that may include any number of dangerous radionuclides. The problem of detecting, identifying and interdicting illicit radioactive sources is complicated by the fact that many materials present in cargo are somewhat radioactive. Some cargo contains naturally occurring radioactive material that may trigger radiation portal monitor alarms. Such nuisance alarms can be an operational limiting factor for screening of cargo at border crossings. Information about the nature of the radioactive materials in cargo that can interfere with the detection of radionuclides of concern is necessary to help anticipate and recognise likely sources of these nuisance alarms.

A nonlinear wavelet method was designed for smoothing low-level gamma-ray spectra. The spectra of a 60Co graduated radioactive source and a mixed soil sample were smoothed respectively according to this method and a 5 point smoothing method. The FWHM of 1,332 keV peak of 60Co source and the absolute activities of 238U of soil sample were calculated. The results show that the nonlinear wavelet method is better than the traditional method, with less loss of spectral peak and a more complete reduction of statistical fluctuation.

An anti-Compton spectrometer with semi-2π Compton suppression is designed to identify the photons emitted from low-level radioactive wastes from radioisotope usage and nuclear research laboratory. Since the objective sample is massive and large, the system has a full opening toward the sample position. The characteristics and features of the system concerning Compton suppression and reduction of the background component due to natural radioactive source are estimated by the Monte Carlo simulations. The anti-Compton technique is shown to be quite advantageous for the reduction of the surrounding natural background radiation, as well as the suppression of the background for the higher energy photons.

Anomalous gamma emission was rarely observed during direct-current glow discharge in {approx}3 Torrs of deuterium gas using a deuterated palladium foil cathode. Autoradiography after the discharge experiment showed that isotopes with low- and high-energy radiation components were produced before or during the discharge. The palladium foil after the anomalous gamma-ray emission was analyzed by secondary ion mass spectroscopy, which revealed a considerable increase in the content of iron and copper on the surface.

The monitoring of Phosdrin (mevinphos; insecticide) from foliage and foliage extracts was achieved by an aspiration-type ion mobility spectrometer. This technique is based on ion mobility, which is proportional to the molecular weight, shape, and charge. The operation principle of the ion mobility spectrometer is to measure mobility distribution changes of product and reactant ions. This technique can measure positive and negative ion clusters at the same time in six different measuring electrodes. Each measuring electrode detects a different portion of the ion mobility distribution formed within the cell's radioactive source. The pattern recognition used is based on differences in the gas profiles for different compounds. This study shows that an ion mobility spectrometer can be used to monitor Phosdrin from foliage without the need for any time-consuming extraction procedure. The responses for Phosdrin-containing and background (control) samples were easily separated from each other. The responses declined as a function of time in the positive and sum response channels. In addition, the sum of the absolute values of signals at six measuring channels (sum response) were linearly proportional to the concentration of Phosdrin. Just before application (i.e., in background), this value was 41 bits, whereas these values were 10-fold, 11-fold, 8-fold, 6-fold, 5-fold, and 3.5-fold at the time points 4, 8, 11, 24, 50, and 72 hours after the spraying of Phosdrin.

Interstitial heating and brachytherapy are often combined in the treatment of cancer. In such instances, a needle-type internal electrode is inserted into the RALS (remotely controlled afterloading system) catheter instead of a radioactive source. A problem with this approach, however, is that the temperature distribution pattern generated by the inserted electrode varies at any given target region in each heating treatment, which makes it difficult to accurately replicate the heating treatment protocol. This variation is suspected to be caused by non-uniformity in the small gap between the internal electrode and the inner surface of the surrounding catheter, causing electric currents to flow between the electrode and the heating material, which differs from procedure to procedure. To solve this problem, the gap was filled with water of high permittivity, and the temperature distribution was investigated using phantoms. With this method, a stable and reproducible temperature rise distribution was obtained in the phantom experiment.

Radioactive afterheat is an important source term for the release of radionuclides in fusion systems under accident conditions. Heat transfer calculations are used to determine time-temperature histories in regions of interest, but the true source term needs to be the effective afterheat, which considers the transport of penetrating gamma rays. Without consideration of photon transport, accident temperatures may be overestimated in some regions while being underestimated in others. The importance of this effect is demonstrated for a simple, one-dimensional problem. The significance of this effect depends strongly on the accident scenario being analyzed.

An ion generator was developed to neutralize concentrated streams of large, highly charged particles in a low-velocity wind tunnel. The aerosol stream tested consisted of 30 mu m aluminum oxide particles (aerodynamic diameter 52 mu m) at a flow rate of 9.6 m3/h (160 L min) and a mass concentration of 43 g/m3. The average number of excess charges per particle was 240,000 (positive), which corresponds to a neutralizing current requirement of 0.11 mu A. Neutralization to < +/- 10,000 charges per particle was necessary to prevent electrostatic sampling artifacts. Neutralization with radioactive sources would have required an impractically large source. The ion generator, constructed from 21 and 32 mm PVC pipe, has 4 peripheral radial electrodes of 0.5 mm tungsten wire and a 2.0 mm diameter central electrode. The aerosol flowed through the ion generator along its axis. The ion generator was powered by an adjustable (0-8.5 kV) power supply. Performance of the ion generator was monitored with an isokinetic Farad...

The method of multinodal treatment of nuclear transition and spatial migration of radioactive materials in complex nuclear technological systems is described. The modeling of such processes is established in a very flexible way, enabling the user to investigate a wide range of problems related to nuclear safety, i.e., to predict the radioactive source term in normal operation and under accidental conditions of any nuclear plant. The migration of radioactive material can be connected either with the flow of reactor coolant or with any of the other physicochemical processes (filtration, deposition, etc.). On the basis of this method, a modular code system - TIBSO - has been developed.

A new aerosol monitor rapidly measures the mass concentration of individual elements with atomic number Z>20 in dust- or smoke-laden air. The device separates the aerosol's nongaseous components by pumping the air through a section of a filter paper tape. After gathering particles for a fixed period of time, pumping stops and the tape advances, forcing a new section to intercept the airstream and bringing the exposed section in front of a 25 mCi (milli Curies) Cd109 radioactive source. The source bombards the tape's contents with 22 keV (kilo-electron-Volts) X- rays. These, in turn, induce fluorescence in the trapped atoms; that is, they cause the atoms to emit X-rays of their own at lower but still easily measurable (if Z>20), characteristic energies. An analysis of the latter's X-ray energy spectrum provides raw data revealing the identity and abundance of each corresponding element in the aerosol simultaneously. Note that the X-ray fluorescence method is independent of the state of chemical combination...

Performances of aerosol chargers employing alpha- or beta-radioactive sources to ionize carrier gas and using applied electric fields up to 4 kV/cm collinear to the aerosol flow have been investigated. The study was done for fine particles with radii from 0.027 to 1.5 μm acquiring an electrical charge in the non-uniform bipolar atmosphere of ions in air and of electrons and N4 + ions in nitrogen. The charging process of aerosol particles of different concentrations has been analyzed by means of computer simulation, and the charge of the particles as they leave the charger has been calculated in two cases in which the velocity vector of the aerosol stream is either parallel or anti-parallel to the strength vector of the applied electric field. A comparison has been made between calculations and experimental results of particle charging in air, as well as between calculation results in air and ultrapure nitrogen under the same conditions. Two types of chargers with the charging zones irradiated totally or partially by ionizing radiation have been examined. The conditions for obtaining the maximum particle charges both in air and nitrogen as well as the conditions for the initial particle charge to be unaffected in air have been found. The experimental results of fine particle charging in air are in agreement with the theoretical calculations.

A Monte Carlo (MC) computer program for the calculation of the distribution of characteristic X-ray intensities as a function of the counting geometry is described. The program evaluates the contribution to the intensities of a fluorescent line from each point of the source, sample and detector. The main considerations of the calculation are based on the critical nature of the internal geometry of the main analyzer components within an energy dispersive X-ray fluorescence (EDXRF) spectrometer. The K XRF intensities for the inner and outer radius of the annular-sample and collimator radius were measured. An optimum geometry can be obtained by varying the collimator radius and the detector-sample distance. The results of the calculations show that the intensity distribution of the radioisotope excited fluorescent radiation through annular sample area is strongly dependent on the collimator radius, with a maximum intensity effect with a smaller sample area and with decreasing collimator radius. An i...

This paper discusses the effect of the distance between an a-ray source and a stream of ultrafine aerosol particles on the fraction of charged particles. A simple geometrical model is derived which takes into account the effect of charger walls on (i) the effective extent of the radiation cone, and (ii) the ion losses by Brownian diffusion. The model predicts with reasonable accuracy the relative concentration of ions leaving the charger when no aerosol is present. The experiments demonstrated that the relative concentrations of charged particles at the charger outlet can also be predicted by using the geometrical model developed for ions alone. In spite that the specific ionization of α rays has a maximum at about 3.7 cm from the source, the highest concentrations of charged particles were obtained when the aerosol was made to flow just above the radioactive source (zero separation distance).

The SOS response to alpha-radiation was measured using two Escherichia coli strains with differential repair capabilities. Monolayers of exponentially growing cells, placed on cellulose acetate filters, were exposed to an electrodeposited 252Cf alpha source to total doses from 46 to 165 cGy at a dose-rate of 0.8 cGy/min. A mean alpha energy of 2.9 MeV, with a corresponding LET of 133 keV/micrometer (for water) was attained by means of a filter placed between the bacterial culture and the radioactive source. Owing to the low fluences ranging from 0.013 to 0.037 alpha-particles/cell and short irradiation times, calculations of the estimated number of irradiated cells were done to obtain the total absorbed doses by the culture. The results show that in spite of the low doses, the damage was sufficient to induce the SOS function in one of the strains used (PQ37). Nevertheless in another strain IN88, lacking double strand break repair and which should be more sensitive, SOS induction could not be observed, probably due to the killing effect of the high LET radiation.

An optimization artificial neural networks model is developed for solving the ill-posed inverse transport problem associated with localizing radioactive sources in a medium with known properties and dimensions. The model is based on the recurrent (or feedback) Hopfield network with fixed weights. The source distribution is determined based on the response of a limited number of external detectors of known spatial deployment in conjunction with a radiation transport model. The algorithm is tested and evaluated for a large number of simulated two-dimensional cases. Computations are carried out at different noise levels to account for statistical errors encountered in engineering applications. The sensitivity to noise is found to depend on the number of detectors and on their spatial deployment. A pretest empirical procedure is, therefore, suggested for determining an effective arrangement of detectors for a given problem.

A procedure to correct Compton profiles for geometrical effects, related to the solid angles subtended for an X-ray source and detector, has been obtained. The geometrical response function for a given experimental set up is found by a Monte Carlo simulation program and is meant to be used as a histogram for convolution purposes. The main features of this function show that it is not peaked at the angle between collimator axes because the maximum is shifted towards greater values and also increasingly broadens as this angle decreases. This procedure corrects the profile shape as a whole, not only its width as was done in previous methods. We have tested this correction with data taken from an experiment in which 60 keV photons from a 241Am radioactive source were scattered in a triple-distilled water sample and detected with an intrinsic germanium detector. A Compton profile, measured at an angle of 135°, was compared with a tabulated one convoluted with the distribution function, and thus the re...

Experiments of unipolar diffusion charging of ultrafine aerosol particles (10–50 nm) at low pressure (0.2–0.5 atm) were performed. Aerosol charging was achieved using a novel unipolar charger which made use of a low-intensity α-ray radioactive source. The results of the experiments were compared with numerical solutions to the fundamental equations of charging. The Fuchs' limiting-sphere charging theory in the transition regime was used for the calculation of the combination coefficients between ions and particles. Good agreement was found between experimental results and numerical predictions of the charged fraction as a function of particle diameter. This is Particle Technology Laboratory Publication No. 767.

The fabrication of a miniature spark counter for public communication and education using naturally occurring radon as a radioactive source without involving any man-made radioactivity is described. The battery-powered miniature spark counter weighs 2.07 kg with a volume of 4.844 x 10/sup -4/ m/sup 3/. The circuitry consists of seven major components: timer, high-voltage power supply, attenuator, noninverting amplifier, low-pass filter, one-shot generator, and counter. Cellulose nitrate films irradiated with alpha particles from radon emanating from soil were etched and counted. The visible sparks during counting are rather heuristic, which can be used to demonstrate naturally occurring radioactivity in classrooms or showplaces.

Several double-wall stainless steel cylindrical capsules containing 37Cs (CsCl salt) are used as a source of gamma rays in a proposed food irradiation facility. The capsules, normally stored in a water pool, are withdrawn into air when in use. The radioactive decay of the 37Cs is a heat source (approximately 213 W/capsule). A key element in the facility design is to ensure that the temperature of the radioactive source capsules, when in air, is maintained below specified limits. The thermal analysis performed to ensure that the cooling provided is adequate, in spite of the capsule stacking arrangement, is presented in this paper. A thermal model is presented for the heat transfer through a single capsule and the thermal interaction among the stacked capsules and with the surrounding air. Also presented is an analysis of the transient heat transfer phenomenon associated with the potential loss of the forced-convection cooling employed.

1. A cheap, robust, radioactive source in sheet form has been designed and constructed from commercially available material. Log-related isotopic dilutions of the source have been used to prepare a Standard Radioactive Scale. 2. The Standard Radioactive Scale has been developed as a method of internal standardization of whole-body autoradiography and its use as an aid to quantitative determination is discussed. 3. The distribution of radioactivity in the sheet source is sufficiently uniform to allow its use for high resolution radiography of thin specimens. This radiographic technique has been applied to whole body sections to demonstrate differences in quenching by tissues and to examine calcification in the rat foetus.

A study of Si detector response has been carried out using beams of monoenergetic (0.5–2.5 MeV) H and 4He ions backscattered from a thick gold target, in an attempt to determine if the resulting pulse height could be used as an energy calibration technique. At each chosen energy, the accelerator had been calibrated to within 0.5 kV by means of a suitable nuclear resonance reaction. After applying small corrections for the nuclear (i.e. non-ionizing) energy loss, and for the energy loss in traversing the surface dead layer of the detector, the observed pulse heights are found to be exactly proportional to the energy of the incident H or He ion, within the experimental reproducibility of 0.2%. In the case of 4He ions, the linearity of detector response was verified up to 5.48 MeV by means of a standard 241Am radioactive source. One strange anomaly is that the energy e to create an electron-hole pair in silicon is significantly smaller for He (and 6Li) than for H ions: namely, eHe/eH = 0.989 and eLi...

Since the early 1900s, ionizing radiation sources have been used to treat cancers in both children and adults. Radiation is a local treatment that plays an important role in the cure, control, and palliation of pediatric cancers. Irradiation may be the preferred treatment, but it is usually combined with surgery or chemotherapy. In St. Luke’s Hospital, 40% of pediatric patients with cancer have been treated with radiation over the past 5 years. RADIATION AND ITS BIOLOGICAL INTERACTIONS Soon after Dr. Roentgen successfully produced X rays and Marie and Pierre Curie discovered radium, these entities were found to have an effect on cancers. However, the complexities of the biophysical interactions continue to be unraveled. Radiation treatments can be placed in two major categories: (a) teletherapy with external beam treatments and (b) brachytherapy with implants and mold techniques. The latter is rarely used in children and almost never used in palliative situations. External radiation from megavoltage machines is used in the treatment of childhood cancer because a maximum dosage can be delivered to the tumor while sparing adjacent normal tissue from serious damage. Modem day external beam treatments are generally delivered with either Cobalt-60 gamma rays or linear accelerator x-rays. (See Table 1 for machine comparison.) Cobalt is a radioactive source produced as a by-product from nuclear reactors. It is always radioactive and slowly decays over the course of several years. In contrast, linear accelerators bombard an X ray target with high en

A study of the effect of radioactivity on the breakdown threshold in waveguides is presented. The radioactive source is considered to be a ‘slug’ of material terminating the waveguide, this being dictated by the application: microwave drying of fission product materials. It is shown that if the gas within the waveguide is air, the breakdown behaviour will be attachment-dominated, provided that the pressure and temperature do not deviate greatly from N TP. The result of this is that the limiting value of specific activity is largely independent of pressure, temperature, and waveguide size. The minimum predicted limiting value is shown to be sufficiently large to make microwave drying of radioactive waste materials feasible.

Summary An account is given of a magnetic spectrometer of large solid angle and good resolution which should find applications in the study of complex beta-ray spectra by the method of beta-gamma coincidences. The original apparatus, consisting of a segmented annular G.M. counter set round a radioactive source in a magnetic field is described, the method of use being to select from the output of the counter, large pulses which correspond to the simultaneous triggering of several sections by the passage of an electron tangentially to the annular counter. The performance of the instrument is discussed and modifications are described which rectify the defects in the earlier design and give a solid angle of 2·5 per cent of 4π with a resolution of 5 per cent. The geometry of this type of spectrometer is considered and the performance which might be obtained with good design is discussed. It is concluded that collecting angles up to the order of 50 per cent of 4φ are possible with good resolution.

A small vacuum pinhole-camera incorporating a diffuse radioactive source and a track-registering film has been used to observe channelling phenomena in thin single crystals. This simple technique provides a striking demonstration of axial and planar channelling without the need for a well-collimated beam of charged particles in high vacuum. The resulting pinhole-channelling pattern is analogous to a blocking pattern in so far as it is a projection of the crystal geometry in real space. As an example a pattern obtained with a (111)-oriented silicon crystal is shown.

The radioactive source ytterbium-169 emits a 52-kev characteristic x-ray and various gamma rays with energies from 65 kev to 310 kev. Ytterbium sources in small exposure units weighing about 20 pounds are useful for radiography of castings, weldments, assemblies, and other forms of various materials, such as aluminum, magnesium, iron, plastics, and wood. Radiography with Yb169 produces no hazard from electrical shock or explosion. Radiation scattered from air, the object, and surrounding materials is less with Yb169 than with conventional x-ray sources, Ir192, or Co60. Therefore, portable radiographic applications can be made with less difficulty in protection of personnel.

The properties of an experimental grid-controlled photo-multiplier tube were investigated and, under pulsed operation, a transfer conductance of 60 mA/volt was achieved. The primary electron current was produced by placing a suitable scintillator near the photocathode and energizing it by means of a radioactive source of 90Sr. This resulted in a substantially constant illumination of the photo-cathode and, because of the long half life of the 90Sr, high long-term stability for the production of the primary electron current.

To me, the most exciting application of computers in the classroom is simulation. The computer allows students to perform experiments that they could not possibly perform in real life because of restrictions of time, safety, and expense. Students can try their hand at managing a herd of buffalo for 50 years (1), or see the influence of the mutation rate on the evolution of cryptic coloration in Peppered Moths (2). I recently used a public domain program called "Geiger Counter X" to simulate the radiation counts of radium, strontium, and plutonium (3). The program allows the student to choose among the radioactive sources, the distance from the source to the Geiger tube, and whether lead shields will be inserted between the source and the Geiger tube. I had only one computer, so I had most of the class work on an assignment in the text while the others took turns gathering data from the program. "Geiger Counter X" is flexible so that students can design different experiments with it. For example, they might find the change in the scintillation rate with a change in the radioactive source; they might find the change in scintillation rate with a change of distance from the source to the Geiger Tube for a single type of radioactive substance; or they might find the change in the scintillation rate of a single source with the tube shielded by different thicknesses of lead. These are just a few of the most obvious experiments that can be simulated by the program. Once the data are gathered from the computer, graphs can be plotted, either as histograms or line plots, and the results can be compared and contrasted by the whole class. The simulation program augmented a hands-on unit where students measured low-level sources of radiation with an actual Geiger counter. "Geiger Counter X" stimulated interest and expanded hands-on activity by allowing students to simulate working with extremely

In paraffin sectioning static electricity is often induced by friction between the knife and the block. The sections can be discharged by ionizing the air in the vicinity of the paraffin. This can be achieved by mounting a strip containing radioactive polonium near the microtome knife. This approach implies a certain radiological risk. An alternative solution is to apply a high electric field close to the knife. The latter method may offer hazards related to the high tension and production of ozone. In comparing the risks and benefits of the two methods we have concluded that the application of a high electric field offers less risks to people in the working area than the installation of a radioactive source. The costs of the two methods are of the same order.