Linear Energy Transfer Spectra Research Articles

Linear energy transfer (LET) spectra of cosmic radiation in low Earth orbit.

Integral linear energy transfer (LET) spectra of cosmic radiation (CR) particles were measured on five Cosmos series spacecraft in low Earth orbit (LEO). Particular emphasis is placed on results of the Cosmos 1887 biosatellite which carried a set of joint U.S.S.R.-U.S.A. radiation experiments involving passive detectors that included thermoluminescent detectors (TLDs), plastic nuclear track detectors (PNTDs), fission foils, nuclear photo-emulsions, etc. which were located both inside and outside the spacecraft. Measured LET spectra are compared with those theoretically calculated. Results show that there is some dependence of LET spectra on orbital parameters. The results are used to estimate the CR quality factor (QF) for the Cosmos 1887 mission.

Model analysis of space shuttle dosimetry data

An extensive model analysis of plastic track detector measurements of high-LET particles on the Space Shuttle has been performed. Three shuttle flights: STS-51F (low-altitude, high-inclination), STS-51J (high-altitude, low-inclination), and STS-61C (low-altitude, low-inclination) are considered. The model includes contributions from trapped protons and galactic cosmic radiation, as well as target secondary particles. Target secondaries, expected to be of importance in thickly shielded space environments, are found to be a significant component of the measured LET (linear energy transfer) spectra.

Spectra of linear energy transfer of neutrons in the reactor range

Spectra of linear energy transfer of neutrons in the reactor range

An Investigation of CR-39 Nuclear Track Detector Material as a Fast Neutron Microdosemeter

This work describes the evaluation of CR-39 for use as a fast neutron microdosemeter, and provides some insight as to the feasibility of such use. The material was calibrated by irradiation with protons and alpha particles accelerated by a tandem electrostatic accelerator. Proton energies of 10, 6, and 3 MeV and alpha particle energies of 15, 10, and 7 MeV were used, at angles of incidence from 0 to 85 deg. The critical angle, normal incidence diameter, and track etching rates versus linear energy transfer (LET) were obtained from these measurements. A method for extracting a LET spectrum from CR-39 irradiated in-phantom with the fast neutron beam was developed and these results are compared with LET spectra measured by a 1.27 cm diameter Rossi chamber.

An Investigation of CR-39 Nuclear Track Detector Material as a Fast Neutron Microdosemeter

Abstract This work describes the evaluation of CR-39 for use as a fast neutron microdosemeter, and provides some insight as to the feasibility of such use. The material was calibrated by irradiation with protons and alpha particles accelerated by a tandem electrostatic accelerator. Proton energies of 10, 6, and 3 MeV and alpha particle energies of 15, 10, and 7 MeV were used, at angles of incidence from 0 to 85 deg. The critical angle, normal incidence diameter, and track etching rates versus linear energy transfer (LET) were obtained from these measurements. A method for extracting a LET spectrum from CR-39 irradiated in-phantom with the fast neutron beam was developed and these results are compared with LET spectra measured by a 1.27 cm diameter Rossi chamber.

Particle LET spectra from microelectronics packaging materials subjected to neutron and proton irradiation

Cumulative fractions for linear energy transfer spectra were measured for particles ejected from microelectronics packaging materials subjected to neutron and proton irradiations. The measurements for the neutron irradiations compare well with Monte Carlo theoretical calculations. The spectra can be used to access microelectronics vulnerabilities in strategic-nuclear-weapon, space-trapped, and neutral-beam directed-energy particle environments. >

The Relative Biological Effectiveness of 670 MeV/A Neon as a Function of Depth in Water for a Tissue Model

Linear energy transfer (LET infinity) spectra of identified charge fragments and primaries, produced by nuclear interactions of 670 MeV/A neon in water, were measured along the unmodulated Bragg curve of the neon beam. The relative biological effectiveness (RBE) values for spermatogonial cell killing, as reported on the basis of weight loss assay of mouse testes irradiated with beams of approximately constant single LET infinity, were summed over the particle LET infinity spectra to obtain an effective RBE for each charged-particle species, as a function of water absorber thickness. The resultant values of effective RBE were combined to obtain an effective RBE for the mixed radiation field. The RBE calculated in this way was compared with experimental RBEs obtained for spermatogonial cell killing in the mixed radiation field produced by neon ions traversing a thick water absorber. Discrepancies of 10-40% were observed between the calculated RBE and the RBE measured in the mixed radiation field. Part of this discrepancy can be attributed to undetected low-Z fragments, whose contribution is not included in the calculation, leading to an overestimated value for the calculated RBE. On the other hand, calculated values 10% greater than the measured RBE are explained as track structure effects due to the higher radial ionization density near neon tracks relative to the ionization density near the silicon tracks used to fit the RBE vs LET infinity data.

Linear Energy Transfer Spectra of Cosmic Radiation Aboard the Cosmos-1129 Satellite

The most important characteristic of the hazard due to cosmic radiation is the spectrum of linear energy transfer (LET), which enables one to estimate the dose equivalent. This has prompted us to study LET spectra of cosmic radiation aboard Cosmos-1129 using nuclear emulsions as a threshold detector.

LET Spectra in Low Earth Orbit

The Trapped Ions in Space (TRIS) experiment was flown on the Space Shuttle in October, 1984. It contained a stack of plastic track detectors that recorded linear energy transfer (LET) spectra behind various thicknesses of shielding in a 57 degree inclination orbit. A preliminary analysis of the data shows that behind shielding of at least 40 mils Al equivalent, the LET spectrum is dominated by galactic cosmic rays and the direct ionization of trapped protons and that it is reasonably well predicted by the CREME model. For a minimum shielding of approximately 4 mils Al equivalent, the measured LET flux exceeds the CREME predictions, especially at high LET. This is probably due to contributions from the products of proton-induced nuclear reactions in the detector and helium ions trapped in the inner radiation belt.

Animal studies of life shortening and cancer risk from space radiation

The U. S. Air Force study of the delayed effects of single, total body exposures to simulated space radiation in rhesus monkeys is now in its 21st year. Observations on 301 irradiated and 57 age-matched control animals indicate that life expectancy loss from exposure to protons in the energy range encountered in the Van Allen belts and solar proton events can be expressed as a logarithmic function of the dose. The primary causes of life shortening are cancer and endometriosis (an abnormal proliferation of the lining of the uterus in females). Life shortening estimates permit comparison of the risk associated with space radiation exposures to be compared with that of other occupational and environmental hazards, thereby facilitating risk/benefit decisions in the planning and operational phases of manned space missions. Calculations of the relative risk of fatal cancers in the irradiated subjects reveal that the total body surface dose required to double the risk of death from cancer over a 20-year post exposure period varies with the linear energy transfer (LET) of the radiation. The ability to determine the integrated dose and LET spectrum in space radiation exposures of human is, therefore, critical to the assessment of life-time cancer risk.

Applications of nuclear track detectors in space radiation dosimetry

Abstract Over the past twenty years, the radiation environment of manned spacecraft has been measured by a variety of detector types. The high-LET (linear energy transfer) spectrum produced by HZE particles has been measured by plastic nuclear track detectors which included different combinations of CR-39, polycarbonate and cellulose nitrate sheets. These detectors are light- weight, compact, and have provided an effective means of measuring the high LET on all the space shuttle flights to date, including Spacelab-1, as well as the past missions of Apollo and the Skylab series. For low LET, absorbed dose and dose rates as a function of such parameters as inclination, altitude, spacecraft type and shielding have been measured with thermoluminescent detectors (TLD's). For low Earth-orbit missions the dose encountered was found to be strongly altitude-dependent, with a smaller dependence on orbit inclination. This paper presents radiation data gathered on the last several space shuttle flights and discusses further experiments which are being planned.

Summary of current radiation dosimetry results on manned spacecraft

Measurements of radiation exposures aboard manned space flights of various altitudes, orbital inclinations and durations were performed by means of passive radiation detectors, thermoluminescent detectors (TLD's), and in some cases by active electronic counters. The TLD's and electronic counters covered the lower portion of the LET (linear energy transfer) spectra, while the nuclear track detectors measured high-LET produced by HZE particles. In Spacelab (SL-1), TLD's recorded a range of 102 to 190 millirad, yielding an average low-LET dose rate of 11.2 mrad per day inside the module, about twice the dose rate measured on previous space shuttle flights. Because of a higher inclination of the SL-1 orbit (57° versus 28.5° for previous shuttle flights), substantial fluxes of highly ionizing HZE particles were also observed, yielding an overall average mission dose-equivalent of about 135 millirem, about three times higher than measured on previous shuttle missions. A dose rate more than an order of magnitude higher than for any other space shuttle flight was obtained for mission STS-41C, reflecting the highest orbital altitude to date of 519 km.

Cosmic-Ray-Induced Errors in MOS Devices

The results are reported for a comprehensive analytical and experimental study of galactic cosmic-ray-induced errors in MOS devices. An error rate model is described which utilizes exact expressions for a path-length distribution function and a Linear Energy Transfer (LET) spectrum for the cosmic ray environment to calculate the expected cosmic-ray-induced error rate in space for a given parallel-piped-shaped sensitive volume. The model validity is confirmed by comparison of predictions to bit-error data from devices in orbiting satellites, and to cosmic ray simulation measurements on the same device types at a cyclotron. The experimental results and model predictions are described for a wide variety of device types, including NMOS, PMOS, CMOS/bulk, CMOS/SOS, and ANOS.

Status report: Direct track detector dosimetry in negative pion beams

Cellulose nitrate and Lexan polycarbonate track detectors have been exposed in a water phantom to 350 Rad in a stopping negative pion beam. The dose is measured at the “peak” or star region. The vertical beam from the Biomedical Channel at the Los Alamos Medium-Energy Physics Facility (LAMPF) was used. Heavy ions from negative pion interactions in the “peak” region and “plateau”, or entrance, region were determined by the step etch technique. The ions, and thus the linear energy transfer spectrum from pion interactions with oxygen, have been tentatively identified. The cellulose nitrate track detectors have also been used to measure superficial dose contours of the high LET portion of the peak pion beam in the first human biology trails. These results will be discussed.

Microdosimetry of negative poins at LAMPF.

Lineal energy distributions and linear energy transfer (LET) spectra are being measured at the biomedical facility of the Los Alamos Scientific Laboratory for a negative pion beam having an initial momentum of 168 MeV/c. The results show that the fraction of the dose for lineal energies greater than 50 keV/mum is approximately 2% in the plateau and approximately 12% at the Bragg peak. Preliminary data indicate that the high lineal energy (and high LET) component increases significantly in the region slightly downstream from the peak.

Qualitative alteration in radiation injury under hypoxic conditions.

The nature of the absorption of x and gamma rays is such that a significant portion of the energy is dissipated in dense ion clusters with a high linear energy transfer (LET) (1). Such high LET radiations have been called the α component by Widerö (2). Oxygen is known to enhance the effects of low LET radiations (3, 4); hence, in the absence of oxygen the relative importance of the high LET or α component of x and gamma rays would increase. Since high LET radiations produce injury which apparently differs from that of low LET radiations in the degree of repair demonstrated by split-dose studies (5), it might be expected that repair of injury caused by x or gamma rays under hypoxic conditions would be decreased. Any decrease in repair of injury would be important in radiotherapy, since it would increase the killing of hypoxic tumor cells relative to the euoxic when fractionated treatment is used. Previous investigations in our laboratory with the mouse LD50/30 dose (6) and the endogenous spleen colony technic (7) demonstrated a decrease in early recovery between doses when hypoxic cells were irradiated with 250 kVp x rays. The earlier studies did not establish complete dose response curves and did not prove that severe hypoxia was obtained in vivo by exposure of mice to 5 per cent oxygen. Since the LET spectra of 60Co gamma rays and 200 kV x rays differ (8), it also seemed important to investigate the effect of radiation quality on repair of sublethal injury. The present experiments demonstrate that a qualitative change in injury under hypoxic conditions does occur with 60Co radiations and that the in vivo method does produce severe hypoxia with true alterations in the results of fractionated radiation doses. Material and Methods Female LAF1 mice, aged twelve to sixteen weeks, from the Jackson Laboratories, Bar Harbor, Maine, were used in all experiments. They were housed 9 per cage and fed ordinary laboratory chow. The spleen colony technic of Till and McCulloch (9) was employed to measure cell survival and recovery with split doses. In the endogenous technic the mouse is exposed to radiation doses in the sublethal ranges that allow survival of hematopoietic colony-forming units (CFU) which can be recognized in the spleen. Radiation dose-response curves were determined from spleen colony counts made ten days after the exposure (Diagram A). The exogenous CFU technic involved injection of bone marrow from donor mice to irradiated recipients by the intravenous tail vein route. The recipients received a sufficient total-body dose (1,000 rads) to prevent endogenous colony formation and to permit the growth of transplanted colony-forming cells in the spleen.

Radiation dosimetry aboard manned space vehicles.

Measurements necessary to characterize radiation exposure in space besides total dose are dose rate, distribution of dose in tissue, and local density of energy deposition, the latter generally characterized by the quality linear energy transfer (LET). In routine low-level exposures to known qualities of radiation on earth, measurement of total dose alone usually suffices to define an exposure; moreover, the known relation of dose in tissue for most radiations permits the measurement to be made in air or at the surface of the body and the tissue dose to be inferred from this. In contrast, the highly variable, high-energy paniculate radiation of solar or cosmic origin entering the small metal-enclosed volume of air containing the astronaut can be expected to produce a buildup of secondaries, which depend on the local geometry. Hence, either direct tissue dose and LET measurement or a means of relating instrumental data to these is necessary in each case of space radiation exposure. Dose information required for this purpose can be acquired by relatively unsophisticated and highly reliable tissue equivalent instrumentation. Such instrumentation should give reliable values of the absorbed dose and some coarse breakdown of the LET spectra of the incident radiation in at least a sufficient number of ranges to allow weighting for changes of radiobiological effectiveness of the various LET groups.

Radiobiological research with fast neutrons and the implications for radiotherapy.

There has recently been an upsurge of interest in the use of high linear energy transfer (LET) radiations in radiotherapy (21). At the present time a beam of fast neutrons is the only type of radiation available with a LET substantially higher than that of x rays. The physical and radiobiological properties of the fast neutron beam generated by the Medical Research Council cyclotron at Hammersmith Hospital are being investigated with radiotherapeutic applications in mind, and a number of results up to the beginning of 1962 have already been published as a symposium (16). The work has now reached a stage where the treatment of patients can be considered, and the purpose of the present paper is to summarize the results to date and to relate them to the problem of radiotherapy with high LET radiation. A beam of fast neutrons of several MeV is sufficiently penetrating for therapy of deep-seated cancer. A monoenergetic beam of 14 MeV neutrons has depth-dose characteristics similar to those of megavoltage x rays (19). The neutron beam at Hammersmith is produced by directing a beam of 16 MeV deuterons onto a thick beryllium target. The spectrum extends up to 19 MeV with a modal value at about 6 MeV. The depth-dose distributions are similar to those of 250 kV x rays, making therapy possible at many sites. The LET spectrum in tissue is complex, extending from low values characteristic of gamma radiation up to nearly 1,000 keV/μ. It is not meaningful to describe the distribution in terms of a single effective LET since different parts assume varying degrees of importance depending on circumstances, e.g., test material, size of dose, presence of oxygen or chemical protectors, etc. Since the publication of the symposium in 1963 (16), the neutron target has been removed to a new position, allowing experiments to be performed in a separate room next to the cyclotron chamber (10). This change has been of great value and makes the treatment of patients possible. Basic physical information about the beam is given elsewhere (8, 10). Oxygen Ratios The rationale for using fast neutrons or other high LET radiations in radiotherapy lies in their reduced oxygen enhancement ratio. The maximum dose of radiation which can be delivered depends on the response of normal tissues which are mostly well oxygenated; the oxygen enhancement ratio indicates the reduction in effective dose received by anoxic regions in a tumour. Consequently the ratio of the two oxygen enhancement ratios, that of x rays to that of neutrons, gives the increase in the effective dose received by these regions when neutrons rather than x rays are used. This ratio has been called the Gain Factor by Alper (1). Table I gives all the results of measurements of this type, which have been made using the fast neutron beam. The bacterial experiments given at the bottom of the table were designed to assess the effects of different culture conditions during and after irradiation.

A FAST PROTON SOURCE FOR RADIOBIOLOGY.

THE majority of radiobiological reactions are markedly influenced by the linear energy transfer (LET) as well as the dose of ionizing radiation, and comparative investigations at different LET values are important in the methodology of radiobiological research. Irradiation at high values of LET has been achieved most commonly by means of fast neutrons produced in accelerators or reactors. The choice of nuclear reactions for production of mono-energetic neutrons is rather restricted, and even then the LET spectrum in an irradiated material is quite broad because: (a) the initial energies of the ionizing secondary particles range from a maximum value down to zero; (b) a given cell may be irradiated by a secondary particle with any energy from the initial value down to nearly zero. For irradiation of most cells and micro-organisms it is far preferable to use mono-energetic charged particles directly, provided the dimensions of the cells, etc., are small compared with the range of the particles. In this ‘track segment’1 method the dose can be concentrated in a narrow range of LET, apart from the inevitable contribution from δ-rays, and different values of LET can be selected by slowing the particles in foils. α-Particles from radioisotopes have been used, but the low penetrating power, the restricted range of LET available and the hazard from contamination limit the usefulness of this method. Direct acceleration of charged particles, for example heavy ions in the Hilac2, to the energies required is a major undertaking of nuclear engineering and correspondingly expensive.

Limitations of the Concept of Linear Energy Transfer (LET)

Variations in the relative biological effectiveness of ionizing radiations are commonly attributed to differences in specific ionization or linear energy transfer (LET). The implication has been that for each radiation field there exists one definite distribution of dose in LET (LET spectrum) that may be computed from an analysis of the existent particle energies and corresponding values of specific energy loss. Thus Boag (1) has furnished LET spectra in water for neutrons of various energies. We have designed a proportional counter instrument (2) for the experimental determination of LET spectra and performed measurements (3) that show a certain discrepancy with Boag's computations. Recent experiments indicate, moreover, that the LET spectrum recorded by the instrument depends on the internal gas pressure, which implies a change of LET spectrum with change of the length of track that is sampled. We believe that this effect is real and is caused by the existence of statistical fluctuations that must affect biological systems as much as the physical measuring device. Computations exist that provide a theoretical value of — (dE/dx), the specific energy loss of a particle of given charge and speed. This representation, however, is an idealization, since in reality the charged particle loses energy discontinuously and hence the energy ΔE lost in traversing a thickness Δx will fluctuate because of variations in both the number of collisions and the energy expended per collision. Thus, the real quantity — (ΔE/Δx) has a range of values for identical particles of the same energy, and the spread becomes wider as Δx is decreased, since statistical variations are larger in smaller samples. On the other hand, if Δx is increased, the spread becomes less, but since a greater number of collisions is involved, δE is larger and the energy of the particle may change sufficiently so that the average value of —(ΔE/Δx) obtained in the sample may differ. For example, the theoretical value of —(dE/dx) for a proton at the peak of the Bragg curve is about 91 Kev/μ. When a small section of proton track is examined, a distribution of values will be observed which has the above figure as a mean value. As the test interval is increased, the distribution curve will become more peaked but the mean value will drop below 91 Kev/μ. Our experimental data indicate that, in the case of LET spectra produced by neutrons, these effects are sufficiently marked that appreciable deviations from the theoretical distribution must occur, no matter what sample is chosen. The further conclusion must be drawn that the LET spectrum is a function of the size of the biological test object and is, in fact, rather different in biological structures of different dimensions. Because of these complications, the analysis we perform to obtain the LET spectra is itself affected by second-order errors, but these should be less than the actual discrepancy between theoretical and practical distributions.