Delta Rays Research Articles

Thindown in Radiobiology

A new expression for the radial dose distribution, tested against available data, and yielding good agreement with enzyme and virus cross sections, is used to calculate cellular inactivation cross sections from track theory models and parameters. We use a cellular model and radiosensitivity parameters, fitted to HILAC data 15 years ago, to represent mammalian cells irradiated at the UNILAC. The observed branching with Z and the decline in cellular action cross sections with an increase in ion LET are attributed to thindown; that is, to the limits imposed by the maximum radial penetration of delta rays. Target size and structure (hence the model) also play a role. Similar effects are observed with nuclear emulsions, scintillation counters, and thermoluminescent crystals at ion speeds approaching the Bragg peak, where, in the track width regime, the cross sections depend more on the ion speed than on LET.

Thindown in Radiobiology

A new expression for the radial dose distribution, tested against available data, and yielding good agreement with enzyme and virus cross sections, is used to calculate cellular inactivation cross sections from track theory models and parameters. We use a cellular model and radiosensitivity parameters, fitted to HILAC data 15 years ago, to represent mammalian cells irradiated at the UNILAC. The observed branching with Z and the decline in cellular action cross sections with an increase in ion LET are attributed to thindown; that is, to the limits imposed by the maximum radial penetration of delta rays. Target size and structure (hence the model) also play a role. Similar effects are observed with nuclear emulsions, scintillation counters, and thermoluminescent crystals at ion speeds approaching the Bragg peak, where, in the track width regime, the cross sections depend more on the ion speed than on LET.

Restricted LET: Still a Good Parameter of Radiation Quality for Electrons and Photons

Monte Carlo simulated electron tracks in water vapour were used to prove that the characteristic of track structure fundamental for defining the distribution of the number of ionisations in a target is the mean primary ionisation density. This density can be equivalently represented by restricted linear energy transfer LΔ. Since the number of primary ionisations within short track segments fluctuates according to Poisson statistics, LΔ not only characterises the mean value but also the fluctuation in the number of primary ionisations, provided there is “crosser” geometry. For small sites, the fluctuation in number of ionisations due to delta rays is accounted for by a correction term. On the basis of these physical results, it appears worthwhile to investigate further whether mean primary ionisation density or LΔ has the properties of a generally applicable parameter of radiation quality in radiation biology.

Experimental Data from Irradiation of Physical Detectors Disclose Weaknesses in Basic Assumptions of the Delta Ray Theory of Track Structure

The applicability of track structure theory has been tested by comparing predictions based on the theory with experimental high-LET dose-response data for an amino acid alanine and a nylon based radiochromic dye film radiation detector. The linear energy transfer LET, has been varied from 28 to 20,200 MeV.cm2.g-1 using ion beams ranging from protons to sulphur ions. The low-LET reference radiations were beams of fast electrons and of 60Co γ rays. At doses well below saturation the two detectors act upon low-LET radiation in close accordance with the theoretical considerations, but at marginal high doses dose-rate effects are observed obscuring the evaluation of important parameters. Basic concepts in the theory have been investigated and are shown to be inappropriate in the present formulation.

Experimental Data from Irradiation of Physical Detectors Disclose Weaknesses in Basic Assumptions of the Delta Ray Theory of Track Structure

Experimental Data from Irradiation of Physical Detectors Disclose Weaknesses in Basic Assumptions of the Delta Ray Theory of Track Structure

Response of high resolution drift tubes to relativistic heavy ions

Abstract We report the results of various tests undertaken in a program to develop a drift tube hodoscope for a cosmic ray balloon experiment intended to search for extragalactic antimatter. Included are studies of mechanical integrity, electron drift velocity, tube gain, space charge saturation effects as measured for relativistic iron nuclei, and delta ray backgrounds associated with signals from iron projectiles. Implications of the results of these studies with regard to the use of drift tubes on baloon borne experiments are discussed. It is found that a spatial resolution of σ ∼ 300 μ m can be achieved over a dynamic range from Z = 20 to 30 with little degradation from delta ray effects for suitably chosen tube gains and discriminator threshold settings.

Dual Radiation Action and the Initial Slope of Survival Curves

The concepts and tools of the Theory of Dual Radiation Action (e.g., proximity functions and gamma distributions) are outlined, and their connection to single-event cell inactivation is exemplified by an analysis and interpretation of the cross-section data obtained by Todd. It is shown that the biological effect of individual charged particles is dominated by the combined action of a few delta rays.

Delta electron emission in superheavy quasiatoms with Z ? 137

Results of coupled channel calculations are presented for total and K-hole coincidentδ-electron spectra for the systems I-Pb, I-U and Au-U. Electron screening and vacancy sharing corrections are included. The computed cross sections are compared with the first coincidence measurements ofδ-electrons and K-shell vacancies in systems withZ ≳ 137.

A portable dose equivalent meter based on microdosimetry

It is generally accepted that the physical basis of the relative biological effectiveness of different radiations is the difference in the spatial distribution of ionization along the charged particle tracks. Thus it is possible to measure physical quantities which may be related to biological damage, and use them to estimate the radiation protection quantity “dose equivalent”. A prototype instrument utilizing a spherical proportional counter and a microcomputer to make such measurements has been developed and tested. The detector is filled with gas at low pressure in order to simulate micrometer diameter volumes of tissue. Energy deposition in these small volumes is a stochastic quantity which depends on charged particle stopping power, path length through the site, energy loss straggling, and energy transport by secondary charged particles (delta rays). It has been suggested that this energy deposition distribution be used as a basis for defining radiation quality factor ( Q) [1]. However, to the extent that the energy deposition in the site is proportional to the geometric chord length distribution, energy deposition can also be used to determine the linear energy transfer. The mean quality factor can then be calculated on the basis of the current definition. The prototype instrument uses two amplifiers, with gains of 50 and 1000, processing the pulses from a single detector in order to resolve the wide range of energy deposition events produced by neutron and gamma irradiation. Each amplifier is connected to a specially designed analog to digital converter and a 128 channel multichannel analyzer. A microcomputer controls the system and calculates dose and dose equivalent. Test results for a variety of accelerator produced neutron irradiations show that, from 0.1 to 15 MeV, system accuracy is ±0.5 for Q and ± 15% for dose equivalent.

Ionization of tetramethylsilane by alpha particles

The yield of electrons formed on exposure of liquid tetramethylsilane to 5.48 MeV alpha particles has been measured as a function of electric field. The number of electrons released per alpha was determined by pulse height analysis using a charge sensitive detector. The yields are low and increase with field. At 40 kV/cm the yield is 4100 electrons/alpha ( G = 0.075 electrons/100 eV). The results are shown to be consistent with the major source of electrons being delta rays.

Production of perhydroxyl (HO2) within the track core in the heavy particle radiolysis of water

The radiolysis of the ferrous sulfate-cupric sulfate system with /sup 12/C ions has been examined to provide a measure of HO/sub 2/ production within heavy particle tracks. The HO/sub 2/ yield decreases rapidly with increased /sup 12/C ion energy and at a given LET is considerably less than found for /sup 4/He ions. It is concluded that HO/sub 2/ production primarily reflects the local density of energy deposition within the track core and not the partition of energy between the core and delta rays. Substantial revision of currently existing models is required to take into account the increase in core size with increased particle Z. 19 references, 1 figure.

Delta electron emission as an atomic clock in deep-inelastic and intermediate energy heavy ion collisions

We investigate the spectra of electrons which are emitted during heavy ion collisions, traditionally denoted as δ-electrons. The slope in the kinetic energy distribution of ejected δ-electrons provides a finger-print of the occuring delay and deceleration times of the underlying deep-inelastic and intermediate energy collisions on the time scale 10 −21 - 10 −24s.

Delta -electron spectroscopy of multiply ionising proton-rate-gas collisions

Doubly differential cross sections for delta -electron ejection in proton-rare-gas collisions have been studied at ejection angles of 50, 90 and 120 degrees , electron energies of about 10 to 550 eV, and incident energies around 300 keV. Performing a charge-state analysis of the recoil ion in coincidence with the delta electrons we were able to distinguish between single and multiply ionising collisions. It was observed that the relative importance of multiple ionisation events is smaller at forward compared with backward ejection angles and increases with increasing energy of the delta electron. An analysis in terms of an ionisation model of independent electrons gives fair agreement with the experimental data.

Microdosimetric Distributions for Nanometer-Size Targets in Water Irradiated with 60 Co Gamma Rays: Frequency Distributions of Effective Primary Events by Individual Tracks of Electrons

A calculation method of frequency distributions for the microdosimetric quantity j (number of effective primary events per track per target), fj , is presented. fj distributions are calculated for nanometer-size targets in water irradiated with 60Co gamma rays, using degradation spectra of total electrons in water, phi t(E). Further, fractional total electron differential fluences effective to produce j effective primary events, phi t(E, j), are calculated. It is found that the frequency for occurrence of no effective primary event tracks (tracks traversing the target without any effective primary event inside the target) is very large for nanometer-size targets, and that the delta-ray effect on the fj distribution becomes more significant with the decrease of the target size and especially j effective primary events (j greater than or equal to 1) for very small targets are due mostly to delta rays.

THEORETICAL STUDY OF THE SPATIAL RESOLUTION OF X-RAY MICROANALYSIS IN ANALYTICAL ELECTRON MICROSCOPY

The spatial resolution of X-ray microanalysis has been investigated by Monte Carlo calculations, including knock-on secondary electron production. The results for AuMα in a l000A thin Au film at 100 keV showed an appreciable difference from the ones without knock-out events. Also calculations have been done with the Mott cross-section instead of the screened Rutherford cross-section for elastic scattering. A significant difference was found in the results among the three models.

Low noise ionization chambers for use in transmission measurements with medium and high energy beams

We have investigated requirements for an integrating-type ion chamber that performs with signal fluctuations (noise) near the 1/√N counting statistics level. A large noise contribution occurs when the chamber collection foils intercept a beam of particles whose energy exceeds spallation production threshold. Experimental results demonstrate the source of this noise to be the production of low-energy spallation products in the plates and gas of the chamber. Delta rays also contribute to the noise, but not as severely. We have developed hydrogen-filled ion chambers with longitudinal collection plates that successfully eliminate the spallation contribution. With such chambers, noise improvement is typically a factor of 15 over chambers of conventional design.

Changes produced by high doses of light charged particles in registration properties of mica track detectors

Sheets of muscovite and biotite mica track detectors were exposed to high doses of (a) electrons, (b) protons, and (c) alpha particles, in order to study the effect of these radiations on their track registration properties. Optical- and scanning-electron microscopes (SEM) were employed in these studies. In Muscovite mica, observable changes in the fission fragment etch-pit opening, the etched-track profile, and the general etching velocity occur around doses of (a) 3.5 × 106 Mrad of 1 MeV electrons, (b) 8.5 × 104 Mrad of 10 MeV protons, and (c) 2.2 × 104 Mrad of 40 MeV alpha particles. The values of these critical doses for biotite mica are (a) 2.7 × 105 Mrad of 1 MeV electrons, (b) 8.2 × 103 Mrad of 10 MeV protons, and (c) 1.6 × 103 Mrad of 40 MeV alpha particles. It has been concluded that the contribution of delta rays alone is not sufficient to account for the etchable track formation in mica solid state nuclear track detectors.

A nearly mono-energetic 6-7 MeV photon calibration source.

A photon source has been developed which delivers about 85% of its photon dose equivalent from photons with energies of 6.1,6.9 and 7.1 MeV produced in the 19F(p, alpha gamma)16O reaction. The source uses up to 50 muA of 2.7 MeV protons incident on a 6 mg/cm2 target of CaF2. It produces a photon field with a dose equivalent rate of up to 6 mSv/h (600 mrem/h) over a large area 100 cm from the target. The field can be calibrated in terms of photon fluence to within +/- 5%. In common with other high-energy photon sources, there is considerable contamination of the field by knock-on electrons and scattered photons. Experiments with various filter materials and detailed Monte-Carlo calculations with the EGS electron-photon transport code have been done to investigate the importance of these contaminants.

Inactivation of Ehrlich Ascites Tumor Cells by Heavy Ions

Exponentially growing and plateau-phase cultures of Ehrlich ascites tumor cells were irradiated with heavy ions (Z greater than or equal to 20) and assayed for loss of reproductive capacity either immediately or at delayed times after irradiation. The results indicated no modification of the exponential dose response due to conditions which usually favor the repair of potentially lethal damage at low ionization density. Postirradiation treatment of the cells with beta-arabinofuranosyladenine, a DNA synthesis inhibitor known to act on PLD repair, resulted in effects similar to those observed without this drug and confirmed the hypothesis that at such high values of ionization density only lethal, unmodifiable damage can be expressed. The inactivation cross-section values calculated from the slope of the measured survival curves showed no significant correlations with commonly used parameters of radiation quality such as LET or z2/beta 2. Instead, a functional dependence on the primary ion energy was indicated, being smaller by a factor of two at low energies (less than or equal to 2 MeV/amu) compared with values at energies above 4 MeV/amu, where agreement with the morphological nuclear cross section of the culture was found. This suggests that at higher specific ion energies energetic secondary electrons contribute to the induction of lethal damage, and that interaction of damaged sites between the primary track and the track ends of delta electrons may occur. The data are therefore also discussed in terms of the "penumbra model" which emphasizes the role of delta electrons in cell killing when radiations with very high ionization density are applied.

Deposited energy density for 100 MeV protons in lithium

The density of energy (LET) deposited in a lithium target by 100 MeV protons is estimated by two methods. The protons are slowed down by electronic stopping alone, nuclear stopping being negligible. Multiple scattering and transport of energy by recoil electrons (delta rays) are shown to be negligible. The continuous-slowing-down expression for the range straggling is used in a convolution with the no-straggling distribution in one method; in the other the appropriate transport integral equation is solved to first order in the straggling. (The corresponding solution for the range distribution is also given.) The results are nearly identical, numerically. The multiple-scattering estimate and the transport-equation calculation are thought to be novel. The peak energy density is about 4.8 times the mean, and the fwhm is about 5.4% of the mean range. In the corresponding problem for 200 MeV protons on lead, the numbers are 4.4 and 6.3%, respectively. Simple scaling relations for peak and fwhm are given in terms of a straggling parameter.