S.C. Roy

Elastic scattering of γ-rays and X-rays by atoms

Abstract A survey is given of the current status of theoretical and experimental studies of elastic photon-atom scattering, with particular emphasis on the soft γ-ray energy regime 59.5 keV–1.33 MeV. Basic assumptions of theory, simple approximations, and best available calculations are described. The focus is on the Rayleigh scattering amplitudes for elastic scattering off the bound atomic electrons, but it is also necessary to discuss Delbruck and nuclear amplitudes. The numerical code of Kissel is central to the results which are obtained. A new reference data set of theoretical predictions is presented for a grid of 10 elements (Z = 13–103) and 7 energies (59.5 keV–1.33 MeV) at 55 scattering angles (0–180°). Basic considerations of the experiment are described, including sources and detectors and experimental arrangements. All modern experiments (using Ge detectors) are summarized, and older experiments are listed for reference. Finally, the status of comparison between theory and experiment is examined. All experiments available on the reference grid of theoretical calculation are compared with the predictions; agreement appears generally satisfactory. The situations at higher energies (focusing on Delbruck scattering) and lower energy X-ray scattering (anomalous scattering) are also discussed.

Elastic scattering of photons

Abstract Photon scattering from bound electrons of atoms, a component of the elastic photon-atom scattering amplitude called Rayleigh scattering, is discussed. General features of the many-body scattering amplitude and its partitioning into Rayleigh and Delbruck (and nuclear) single-electron transition scattering amplitudes are examined. The use of the state-of-the-art precise second-order S-matrix calculations of Rayleigh scattering in terms of single-electron transition-amplitudes has led to significant progress in our understanding of the scattering process. The importance of relativistic, higher multipole, and bound-bound contributions in calculating anomalous scattering factor deviations from form factor amplitudes must be emphasized. Accurate interpolation of cross sections in the three-dimensional space of scattering angle, photon energy, and atomic number, utilizing the available published S-matrix data, has permitted extensive tabulation of differential scattering cross sections. S-matrix results may be compared with experiments to assess their validity; they may be compared with simpler but more approximate approaches, to identify the extent of the utility of such approaches and to develop simpler prescription schemes which can give results comparable to the S-matrix results. In spite of their many successes, the present second order S-matrix methods also have limitations and shortcomings, observed in certain recent experiments. We describe the explanation of these experiments in terms of a composite theory which also incorporates non-local exchange and correlation effects. We end with a discussion of other future issues.

Superheated drop as a neutron spectrometer

Abstract Superheated drops are known to vaporise when exposed to energetic nuclear radiation since the discovery of bubble chamber. The application of superheated drops in neutron research especially in neutron dosimetry is a subject of intense research for quite sometime. As the degree of superheat increases in a given liquid, less and less energetic neutrons are required to cause nucleation. This property of superheated liquid makes it possible to use it as a neutron spectrometer. Neutron detection efficiency of superheated drops made of R12 exposed to Am–Be neutron source has been measured over a wide range of temperature −17–60°C and the results have been utilised to construct the energy spectrum of the neutron source. This paper demonstrates that a suitable neutron spectrometer may be constructed by using a single liquid and varying the temperature of the liquid suitably at a closer grid.

Superheated drop detector: A potential tool in neutron research

Abstract The Superheated Drop Detector (SDD) developed in recent years is a potential tool in radiation spectrometry, dosimetry, area monitoring and in many other applications. Its operation under neutron irradiation and possible applications are presented. Among the possible applications of SDD in neutron research, its use in neutron dosimetry has been explored recently and is discussed.

Superheated liquid and its place in radiation physics

Abstract Superheated liquid drops suspended in a visco-elastic gel (known as a superheated drop detector) or in a more rigid polymer matrix (known as a bubble detector) are known to be a useful tool in radiation physics. Superheated liquids have been used as radiation detectors in health physics, medical physics, space physics, nuclear and high energy physics. In addition, the physics of nucleation is not fully understood and requires further investigation. The present paper discusses the special features of a superheated drop detector which has made its place in almost all branches of radiation physics within 20 years of its discovery.

How high can the temperature of a liquid be raised without boiling

How high the temperature of a liquid can be raised beyond its boiling point without vaporizing (known as the limit of superheat) is an interesting subject of investigation. A different method of finding the limit of superheat of liquids is presented here. The superheated liquids are taken in the form of drops suspended in a dust free gel. The temperature of the superheated liquid is increased very slowly from room temperature to the temperature at which the liquid nucleates to boiling. The nucleation is detected acoustically by a sensitive piezoelectric transducer, coupled to a multichannel scaler, and the nucleation rate is observed as a function of time. The limit of superheat measured by the present method supersedes other measurements and theoretical predictions in reaching the temperature closest to the critical temperature of the liquids.

Use of basic principle of nucleation in determining temperature-threshold neutron energy relationship in superheated emulsions

Detection of neutrons through use of superheated emulsions has been known for about two decades. The minimum neutron energy (threshold) required to nucleate drops of a given liquid has a dependence on the temperature of the liquid. The basic principle of nucleation has been utilized to find the relationship between the operating temperature and threshold neutron energy for superheated emulsions made of R-114 liquid. The threshold energy thus determined for different temperatures has been compared with accurate experimental results obtained using monoenergetic neutron sources. The agreement is found to be satisfactory and confirms the applicability of the present simple method to other liquids.

Efficiency of neutron detection of superheated drops of Freon-22

Abstract Neutron detection efficiency of superheated drops of Freon-22 for neutrons obtained from a 3 Curie Am-Be neutron source has been reported in this paper. Although Freon-22 having lower boiling point than many other similar liquids (e.g., Freon-12, Freon-114, Isobutane) is expected to be more sensitive to neutrons, it has not been reported so far and therefore this paper constitutes the first report on the subject. Neutron detection efficiency of both Freon-22 and Freon-12 have been determined from the measured nucleation rate using the volumetric method developed in our laboratory. The result shows that the neutron detection efficiency of Freon-22 for the neutron energy spectrum obtained from an Am–Be source, is almost double, while the life time is 58.6% smaller than that of Freon-12, for a particular neutron flux of that source.

Anomalous scattering effects in elastic photon-atom scattering from biomedically important elements

Abstract In a variety of biomedical applications it has been demonstrated that elastic photon-atom scattering can be important. These include: solving for the crystal structure of a macromolecule, imaging, and radiation dose calculations. To the extent that scattering is significant, it is important to remember that there are a number of effects which go beyond the form-factor approximation. In this paper, we discuss what is presently known about the validity of form-factor approximations for predictions of elastic photon scattering by atoms. We quantify the uncertainty in form-factor theories for sample atoms of biomedical interest, including: carbon, oxygen, aluminum, calcium, iron, iodine and lead. The importance of effects that go beyond form-factor approximation is illustrated by demonstrating that errors can be introduced in using a simple form-factor-based scaling technique using scattering factors measured at one energy to predict scattering at another energy. These anomalous dispersion have been used extensively in recent years in studying structures of biological macromolecules, either in combination with or as an alternative to isomorphous replacement techniques. We present accurate, high precision anomalous scattering factors for a range of elements commonly used in macro-molecular structure studies, tabulated on a fixed grid interval in the energy range 1- to 100 keV.

A neutron sensor based on superheated droplets

Abstract The neutron sensor based on superheated droplets, developed in the U.S.A. and U.K., is one of the most attractive techniques at present for neutron radiation dosimetry. Limited shelf life, i.e. six months (BTI, U.K.), and the dependence of neutron response on climatic conditions are some of the problems associated with these sensors. The development of the above type of sensor suitable for tropical conditions is therefore required. The authors have begun to develop such sensors. The preliminary results show a lower limit of detection of 10 μSv for an 241 Am-Be neutron source and a linear response from 10 μSv to 1 mSv has been reported.