Unpolarized sources that generate highly polarized fields outside the source

Abstract

It is demonstrated that an unpolarized primary electromagnetic source may, under special conditions, produce a field outside the source domain that is almost completely polarized in nearly all directions. This result demonstrates that the polarization statistics of a random electromagnetic field may differ significantly from the polarization statistics of the source distribution that generates it, and may in fact be quite different in different directions of observation. An example of such a source is given.     

Dose dependences of radiation induced yield in mixed radiation fields

A theoretical model that describes dose dependences of trap filling (radiation yield) in mixed radiation fields consisting of two components is proposed. The model consists of one type of electron traps and one type of hole traps and assumes as an initial step the creation of two types of tracks, each represented by some volume with a uniform electron-hole pair density, different for each track. The relaxation process that follows comprises interband recombination, trapping of electrons and holes, and recombination of electrons with trapped holes and of holes with trapped electrons. These processes result in filled traps in amounts depending on the absorbed dose in the track and the number and types of tracks created in a given region of irradiated matter. The summation over the matter with areas of different degrees of overlapping (assuming poisson distribution of the created tracks), gives expressions for the dependences of trap filling as a function of doses for separated and simultaneous irradiation. It is shown that the key parameters determining the behaviour of the dose dependences are the ratios between the doses in the separated tracks and the average doses delivered on the irradiated matter by the separated components of the mixed field. If the ratios of the average dose to the track dose are low, the dose dependences will be linear. In the opposite limiting case the dose dependencies go to saturation. The linear and additive approximations of dose dependences in a mixed field are valid at low doses only.     

Temperature fields in thermal radiation detectors

Results of analysis of thermal processes in a superconducting thermal radiation detector, formulated as a boundary-value problem of complicated heat transfer in a three-dimensional region of complex geometry, are reported. Variants of the problem for simplification are given; the limitations of these assumptions are pointed out.Khar'kov Aviation Institute. Translated from Inzhenerno-Fizicheskii zhurnal, Vol. 64, No. 2, pp. 228–232, February, 1993.     

Further progress in the characterisation of complex radiation fields

One of the topics which forms part of CONRAD project addresses the problems related to the dosimetry of complex-mixed radiation fields at workplaces. This topic was included in work package (WP) 6. WP 6 was established to co-ordinate research activities in two areas:the development of new techniques and the improvement of current techniques for characterisation of complex workplace fields (including high-energy fields and pulsed fields): measurement and calculation of particle energy and direction distributions (Subgroup A); and model improvements for dose assessment of solar particle events (Subgroup B). In both cases in order to aid the research, WP 6 increases the efficiency of resource utilisation, and facilitates the technology transfer to practical application and for the development of standards. This contribution presents a general overview of activities of SG A; specific results related to the benchmark experiment at GSI Darmstadt are presented separately, and will be published in other way. As far as the results acquired in the frame of the SG B activities, these are presented in the meeting held as part of EURADOS AM 2008.     

Physics of cosmic radiation fields.

This paper glances at the knowledge of composition and energy spectra of galactic cosmic rays and briefly discusses the mechanism of solar modulation and of shielding against these particles by the earth's magnetic field. A short review of the properties of solar particle events is given, in which particles emitted from the sun enter the atmosphere. Particle production in the earth's atmosphere in hadronic and electromagnetic cascades is described and the altitude variations of the different particle components are investigated. Typical energy spectra in the atmosphere are presented for some types of particles.     

Time-to-rupture and creep of polymers in radiation fields

The influence of electron bombardment on the time-to-rupture and creep of standard capron fibers in nitrogen and air was studied. An effect of the reversible increase in the creep rate was observed. It was shown that the time-to-rupture and creep rate of capron were substantially dependent on the radiation intensity, load, temperature, and ambient atmosphere. A formula describing the time-to-rupture and creep rate of polymers in radiation fields was derived, and it was shown that this formula correlates with a formula for the electrical conductivity of irradiated dielectrics.     

Response of radiation protection dosemeters in mixed high-energy photon and electron radiation fields

The response of radiation protection dosemeters in terms of the phantom-related operational quantities Hp(10) and H'(10.0 degrees) was measured for personal and area monitoring systems in mixed high-energy electron and photon radiation fields with energies up to 7 MeV. Using mixed radiation fields composed of different fractions of charged particle and photon fluence, three conditions were produced at the point of measurement: charged particle equilibrium (CPE) (a), a lack (b) and an excess (c) of charged particles relative to the conditions of CPE. Personal and area dosemeters of different types were investigated under conditions (a)-(c). A large variability of the response of the different dosemeter types was observed. The results are presented and discussed.     

Quantum interference in optical fields and atomic radiation

We discuss the connection between quantum interference effects in optical beams and radiation fields emitted from atomic systems. We illustrate this connection by a study of the first- and second-order correlation functions of optical fields and atomic dipole moments. We explore the role of correlations between the emitting systems and present examples of practical methods to implement two systems with non-orthogonal dipole moments. We also derive general conditions for quantum interference in a two-atom system and for a control of spontaneous emission. The relation between population trapping and dark states is also discussed. Moreover, we present quantum dressed-atom models of cancellation of spontaneous emission, amplification on dark transitions, fluorescence quenching and coherent population trapping.     

Determination of personal dose equivalents in accelerator radiation fields

Values for the dose equivalent are required for radiation protection purposes, but determination of such values can be quite difficult for high energy radiations. The accurate determination of personal dose equivalents in accelerator radiation fields requires the propel use of appropriate radiological quantities and units, knowledge of the dose equivalent response of the personal dosemeters used, measurement or calculation of the fluence spectrum in the workplace and the fluence spectrum of the reference radiation used to calibrate the dosemeters, in addition to knowledge of the appropriate fluence-to-dose equivalent conversion coefficients. This information can then be used to select the appropriate dosemeters, set up the optimum calibration conditions, or to establish correction factors that account for differences in the calibration and workplace fluence spectra. High energy neutrons account for a significant fraction of the dose equivalent received by workers at accelerator facilities, and this work discusses the procedures and methods needed to determine dose equivalent produced by neutrons in the vicinity of high energy particle accclerators.     

Dose build up correction for radiation monitors in high-energy bremsstrahlung photon radiation fields

Conventional radiation monitors have been found to underestimate the personal dose equivalent in the high-energy bremsstrahlung photon radiation fields encountered near electron storage rings. Depth-dose measurements in a water phantom were carried out with a radiation survey meter in the bremsstrahlung photon radiation fields from a 450 MeV electron storage ring to find out the magnitude of the underestimation. Dose equivalent indicated by the survey meter was found to build up with increase in thickness of water placed in front of the meter up to certain depth and then reduce with further increase in thickness. A dose equivalent build up factor was estimated from the measurements. An absorbed dose build up factor in a water phantom was also estimated from calculations performed using the Monte Carlo codes, EGS-4 and EGSnrc. The calculations are found to be in very good agreement with the measurements. The studies indicate inadequacy of commercially available radiation monitors for radiation monitoring within shielded enclosures and in streaming high-energy photon radiation fields from electron storage rings, and the need for proper correction for use in such radiation fields.     

Investigation of turbo-molecular pumps in strong magnetic fields

In modern particle and accelerator physics as well as in nuclear fusion experiments turbo-molecular pumps (TMP) are used in close proximity to super-conducting magnets. This can cause considerable heating of the fast moving rotor by eddy currents, which can ultimately lead to the destruction of the pump. Motivated by the KATRIN neutrino experiment, where TMPs are operated close to super-conducting magnets, a measurement programme has been elaborated to investigate the effect of magnetic fields on TMPs. An infra-red pyrometer has been used to measure the temperature of the revolving rotor. In addition the effect of different gas loads on the temperature was investigated. With these data a simplified model has been developed to predict the evolution of the rotor temperature over time, using easy to measure parameters. Here we introduce the new model and present first measurements and their application in predicting the rotor temperature in the pulsed field of a nuclear fusion experiment.     

Investigation of turbo-molecular pumps in strong magnetic fields

In modern particle and accelerator physics as well as in nuclear fusion experiments turbo-molecular pumps (TMP) are used in close proximity to super-conducting magnets. This can cause considerable heating of the fast moving rotor by eddy currents, which can ultimately lead to the destruction of the pump. Motivated by the KATRIN neutrino experiment, where TMPs are operated close to super-conducting magnets, a measurement programme has been elaborated to investigate the effect of magnetic fields on TMPs. An infra-red pyrometer has been used to measure the temperature of the revolving rotor. In addition the effect of different gas loads on the temperature was investigated. With these data a simplified model has been developed to predict the evolution of the rotor temperature over time, using easy to measure parameters. Here we introduce the new model and present first measurements and their application in predicting the rotor temperature in the pulsed field of a nuclear fusion experiment.     

Measurements of radiation fields around high-energy proton accelerators

Monitoring of ionising radiation around high-energy particle accelerators is a difficult task due to the complexity of the radiation field, which is made up of neutrons, charged hadrons, muons, photons and electrons, with energy spectra extending over a wide energy range. The dose-equivalent outside a thick shield is mainly owing to neutrons, with some contribution from photons and, to a minor extent, the other particles. Neutron dosimetry and spectrometry are thus of primary importance to correctly evaluate the exposure of personnel. This paper reviews the relevant techniques and instrumentation employed for monitoring radiation fields around high-energy proton accelerators, with particular emphasis on the recent development to increase the response of neutron measuring devices > 20 MeV. Rem-counters, pressurised ionisation chambers, superheated emulsions, tissue-equivalent proportional counters and Bonner sphere spectrometers are discussed.     

In-flight measurements of radiation fields and doses.

The cosmic ray field at civil aviation altitudes consists of low-ionising radiations and neutrons with different energies. At the meeting on radiation exposure of civil air-crew held in Luxembourg in 1991, it was pointed out that there was a need to obtain a better evaluation of the dose with special regard to that of high energy neutrons. This problem has been tackled within the multinational research programmes of the Commission of the European Communities. These research activities made it possible to set up and calibrate the most advanced systems for the assessment of the occupational exposure of aircraft crew. In particular, two advanced neutron spectrometers have been developed, which cover the entire energy range of neutrons. Under the CEC contracts, a large variety of data has been gathered on many longhaul flights, using different dosimetric systems. Since most of the data have been gathered during the period of solar minimum (1994-1997) they represent an upper limit of the dose due to galactic cosmic rays.     

A method for calculation of radiation quantities at all points in gamma radiation calibration fields

Radiation quantity values at all points of a known distance range of gamma radiation calibration fields were calculated using a mathematical method. The method is based on interpolation using Chi-square test on a set of experimental data at optional points of both collimated and un-collimated calibration set-ups by means of a reference instrument. In comparison with Monte Carlo calculations, the values that were calculated by this method differ by <1% for collimated and 2% for un-collimated calibration set-ups. Consequently, the radiation quantities at all points of gamma radiation calibration field set-ups can accurately be formulated and determined by this method. In addition, all points in the radiation fields can be regarded as test points.     

Evaluation of temperature fields in corrosion-resistant steels under the action of laser radiation

We construct a mathematical model for the theoretical investigation of nonstationary three-dimensional temperature fields in bodies with plane boundary surfaces subjected to the surface laser treatment. The model takes into account the local character and the mode of action of laser beams, the shape of the spot of heating, the path of its translational motion, the sizes of a product, and its heat exchange with the ambient medium. A procedure aimed at the prediction of the modes of laser treatment is proposed. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 6, pp. 43–48, November–December, 2007.     

Microdosimetric characteristics of the clinical proton beams at JINR,Dubna

High-energy proton radiotherapy beams give rise to secondary heavy charged particles with elevated linear energy transfer (LET), which contribute to the dose in a patient. This contribution to the characteristics of radiotherapy proton beams was experimentally studied by means of a LET spectrometer based on a track detector. The spectrometer permits LET spectra to be established in the region above 10 keV.micron-1 in tissue. Sets of track detectors were exposed in the various depths of a phantom irradiated with protons of two energies, 150 and 205 MeV. It was observed that the contribution of particles with the values of LET mentioned increases with the depth, representing from about 2 (at the surface) up to few tens% close to Bragg peak region of the total dose. There, some of primary protons contribute also above 10 keV.micron-1. Using the 'biological weighted function' proposed, the clinical RBE was calculated, it could approach 1.3. This effect has to be taken into account during the clinical beam production and the radiotherapy.     

Measurement of neutron dose equivalent to proton therapy patients outside of the proton radiation field

Measurements of neutron dose equivalent values and neutron spectral fluences close to but outside of the therapeutic proton radiation field are presented. The neutron spectral fluences were determined at five locations with Bonner sphere measurements and established by unfolding techniques. More than 50 additional neutron dose equivalent values were measured with LiI and BF₃ thermal neutron detectors surrounded by a 25 cm polyethylene moderating sphere. For a large-field treatment, typical values of neutron dose equivalent per therapeutic proton absorbed dose, H/D, at 50 cm distance from isocenter, range from 1 mSv/Gy (at 0° with respect to the proton beam axis) to 5 mSv/Gy (at 90°). Experiments reveal that H/D varies significantly with the treatment technique, e.g., patient orientation, proton beam energy, and range-modulation. The relative uncertainty in H/D values is approximately 40% (one standard deviation).