Influence Of Linear Energy Transfer Research Articles

Simulation and analysis of the single event transient characteristics of SiGe HBT at low-temperature environment

This study investigates the temperature dependence of single event transient (SET) effects in silicon germanium heterojunction bipolar transistors (SiGe HBTs). Using Silvaco TCAD simulations, we analyze the influence of linear energy transfer (LET), emitter bias voltage, and striking angle across a temperature range from 100 K to 300 K. The results reveal that temperature significantly affects emitter pulse current and charge collection induced by heavy ions. Higher temperatures increase charge collection, while lower temperatures correspond to higher emitter current and shorter pulse width. The study also observes an increase in bandgap energy (from 1.12 eV to 1.16 eV) and electrostatic potential (from 1.19 V to 1.25 V) with decreasing temperature. The study highlights the crucial role of temperature in SiGe HBT performance under radiation threats and emphasizes drift and diffusion mechanisms as dominant for charge collection.

Radiation-induced DNA damage by proton, helium and carbon ions in human fibroblast cell: Geant4-DNA and MCDS-based study

Background. Radiation-induced DNA damages such as Single Strand Break (SSB), Double Strand Break (DSB) and Complex DSB (cDSB) are critical aspects of radiobiology with implications in radiotherapy and radiation protection applications. Materials and Methods. This study presents a thorough investigation into the effects of protons (0.1–100 MeV/u), helium ions (0.13–100 MeV/u) and carbon ions (0.5–480 MeV/u) on DNA of human fibroblast cells using Geant4-DNA track structure code coupled with DBSCAN algorithm and Monte Carlo Damage Simulations (MCDS) code. Geant4-DNA-based simulations consider 1 μm × 1 μm × 0.5 μm water box as the target to calculate energy deposition on event-by-event basis and the three-dimensional coordinates of the interaction location, and then DBSCAN algorithm is used to calculate yields of SSB, DSB and cDSB in human fibroblast cell. The study investigated the influence of Linear Energy Transfer (LET) of protons, helium ions and carbon ions on the yields of DNA damages. Influence of cellular oxygenation on DNA damage patterns is investigated using MCDS code. Results. The study shows that DSB and SSB yields are influenced by the LET of the particles, with distinct trends observed for different particles. The cellular oxygenation is a key factor, with anoxic cells exhibiting reduced SSB and DSB yields, underscoring the intricate relationship between cellular oxygen levels and DNA damage. The study introduced DSB/SSB ratio as an informative metric for evaluating the severity of radiation-induced DNA damage, particularly in higher LET regions. Conclusions. The study highlights the importance of considering particle type, LET, and cellular oxygenation in assessing the biological effects of ionizing radiation.

Reliability studies on bipolar transistors under different particles radiation

The reliability of silicon bipolar junction transistors (Si BJTs) and silicon germanium heterojunction bipolar transistors (SiGe HBTs) were studied under 80 MeV nitrogen (N6+) ions and 60Co gamma radiation in a total dose from 1 to 100Mrad at 300 K. Different DC electrical parameters namely Gummel Characteristics (IB, IC–VBE), excess base current (ΔIB), current gain (hFE), and output characteristics (IC–VCE) were studied before as well as after being subjected to radiations. Both radiations caused substantial degradation in parameters such as an increase in IB, a decrease in the hFE, and a decrease in ICsat. The parametric degradation is more in Si BJT (99% degradation in hFE after ion irradiation) when compared to SiGe HBT (45% degradation in hFE). The SRIM/TRIM simulations were employed to analyze the energy loss phenomenon and ion range in device structures. Annealing studies were also conducted to study the recovery behavior of irradiated devices. The influence of linear energy transfer (LET) of radiations on damage mechanism is also discussed based on irradiation and annealing results. The results show that SiGe HBTs are more radiation hardened than Si BJTs.

Impact of Hypoxia on Carbon Ion Therapy in Glioblastoma Cells: Modulation by LET and Hypoxia-Dependent Genes.

Tumor hypoxia is known to limit the efficacy of ionizing radiations, a concept called oxygen enhancement ratio (OER). OER depends on physical factors such as pO2 and linear energy transfer (LET). Biological pathways, such as the hypoxia-inducible transcription factors (HIF), might also modulate the influence of LET on OER. Glioblastoma (GB) is resistant to low-LET radiation (X-rays), due in part to the hypoxic environment in this brain tumor. Here, we aim to evaluate in vitro whether high-LET particles, especially carbon ion radiotherapy (CIRT), can overcome the contribution of hypoxia to radioresistance, and whether HIF-dependent genes, such as erythropoietin (EPO), influence GB sensitivity to CIRT. Hypoxia-induced radioresistance was studied in two human GB cells (U251, GL15) exposed to X-rays or to carbon ion beams with various LET (28, 50, 100 keV/µm), and in genetically-modified GB cells with downregulated EPO signaling. Cell survival, radiobiological parameters, cell cycle, and ERK activation were assessed under those conditions. The results demonstrate that, although CIRT is more efficient than X-rays in GB cells, hypoxia can limit CIRT efficacy in a cell-type manner that may involve differences in ERK activation. Using high-LET carbon beams, or targeting hypoxia-dependent genes such as EPO might reduce the effects of hypoxia.

LET dependent response of GafChromic films investigated with MeV ion beams

The change in optical properties of GafChromic films depends not only on the absorbed dose, but also on the linear energy transfer (LET) of the ionizing radiation. The influence of LET on the film dose-response relationship is especially important when films are applied for dosimetry of energetic charged particles. In the present study, we examined the response of the unlaminated EBT3 and MD-V3 films to proton, deuterium and helium beams with energies in the range of several megaelectronvolts (MeV). Films were exposed to doses up to 200 Gy and a model based on the bimolecular chemical reaction was chosen to fit the measured film signals. The LET in the active layers of the films and the dose correction factors were computed with Monte Carlo software TRIM. Signal quenching, observed for all ion beams in comparison to x-rays, was investigated as a function of the LET in the range of 10–100 keV µm−1. The response of the films got weaker with increasing the LET and showed no dependence on the ion species. The LET effect was quantified by introducing a modified expression for the relative effectiveness (RE) by which a unique RE value is assigned to a single LET. The RE defined in that way decreased from about 90% for LET of 10 keV µm−1 to less than 50% for LET of 100 keV µm−1. Similar behavior was observed for EBT3 and MD-V3 film models.

Influence of Linear Energy Transfer on the Nucleo-shuttling of the ATM Protein: A Novel Biological Interpretation Relevant for Particles and Radiation

Linear energy transfer (LET) plays an important role in radiation response. Recently, the radiation-induced nucleo-shuttling of ATM from cytoplasm to the nucleus was shown to be a major event of the radiation response that permits a normal DNA double-strand break (DSB) recognition and repair. Here, we aimed to verify the relevance of the ATM nucleo-shuttling model for high-LET particles and various radiation types. ATM- and H2AX-immunofluorescence was used to assess the number of recognized and unrepaired DSB in quiescent fibroblast cell lines exposed to x-rays, γ-rays, 9- and 12-MeV electrons, 3- and 65-MeV protons and 75-MeV/u carbon ions. The rate of radiation-induced ATM nucleo-shuttling was found to be specific to each radiation type tested. By increasing the permeability of the nuclear membrane with statin and bisphosphonates, 2 fibroblast cell lines exposed to high-LET particles were shown to be protected by an accelerated ATM nucleo-shuttling. Our findings are in agreement with the conclusion that LET and the radiation/particle type influence the formation of ATM monomers in cytoplasm that are required for DSB recognition. A striking analogy was established between the DSB repair kinetics of radioresistant cells exposed to high-LET particles and that of several radiosensitive cells exposed to low-LET radiation. Our data show that the nucleo-shuttling of ATM provides crucial elements to predict radiation response in human quiescent cells, whatever the LET value and their radiosensitivity.

Heavy Ion Radiation Effects on a 130-nm COTS NVSRAM Under Different Measurement Conditions

The 6T SRAM cell with fast access speed and silicon–oxide–nitride–oxide–Silicon (SONOS) cell with nonvolatility in a 130-nm commercial-off-the-shelf (COTS) nonvolatile SRAM (NVSRAM) is exposed to heavy ion irradiation under different test conditions. In 6T SRAM mode, the influence of linear energy transfer (LET), supply voltage, ambient temperature, and periphery circuits on single-event effects (SEE) is studied and analyzed. The anomaly of multiple upsets proportion with increasing the LET value may be ascribed to the difference of ion track structure. The observed nonmonotonic relationship between the upset cross section and supply voltage can be explained by a competing mechanism of critical charge and collected charge in a sensitive node. In SONOS nonvolatile mode, the upset threshold of 60.9 MeV/(mg/cm2) and two types of errors are observed. The recoverable upset error may be caused by ion-induced discharge of nonvolatile cell and the hard error that can be annealed to vanish in room temperature probably results from the ion-induced charge trapping in oxide. The above results show that the 6T SRAM cell in this NVSRAM is vulnerable to SEE, whereas the SONOS nonvolatile cell is quite resistant. Based on the prediction of space upset rate, the potential applications and issues of this COTS NVSRAM and deep sub-micrometer SONOS process in space component are discussed.

Influence of linear energy transfer on the scintillation decay behavior in a lithium glass scintillator

We analyzed the effects of linear energy transfer (LET) on the scintillation properties of a Li glass scintillator, GS20. The temporal profiles of scintillation were measured by using pulsed ion beams with different LETs. The rise in the temporal profiles of scintillation was faster for higher LET, whereas the decay part did not vary significantly for different LETs. The LET effects in the rise were ascribed to excited state interactions during the energy transfer process from the host glass to the luminescent centers, Ce3+ ions. If the light yield decreases with LET, the fast rise at high LET could be explained by the competition between energy transfer and quenching due to the excited state interactions.

Modeling and assessing the influence of linear energy transfer on multiple bit upset susceptibility

The influence of the metric of linear energy transfer (LET) on single event upset (SEU), particularly multiple bit upset (MBU) in a hypothetical 90-nm static random access memory (SRAM) is explored. To explain the odd point of higher LET incident ion but induced lower cross section in the curve of SEU cross section, MBUs induced by incident ions 132Xe and 209Bi with the same LET but different energies at oblique incidence are investigated using multi-functional package for single event effect analysis (MUFPSA). In addition, a comprehensive analytical model of the radial track structure is incorporated into MUFPSA, which is a complementation for assessing and interpreting MBU susceptibility of SRAM. The results show that (i) with the increase of incident angle, MBU multiplicity and probability each present an increasing trend; (ii) due to the higher ion relative velocity and longer range of δ electrons, higher energy ions trigger the MBU with less probability than lower energy ions.

SU-E-T-118: Characterization of EBT3 Films in Photon and Proton Beams.

To measure the calibration curves of EBT3 dosimetry films in photon and proton beams and to quantify the related uncertainties from one beam type to another. EBT3 Gafchromic films have similar properties than EBT2 with a symmetric construction and a matte polyester substrate to prevent Newton's ring artefacts. Films from a same batch were exposed in three different beam qualities, an Elekta SL25 6 MV photon beam, a 100 MeV 5×5cm2 proton beam delivered by pencil-beam scanning dedicated system from IBA and a 60 MeV fixed proton beam (2.5cm in diameter) at Clatterbridge Center for Oncology (CCO), UK. The films were read using an EPSON 10000 XL/PRO scanner. Film calibration curves were acquired for all modalities within a range of 0.05 to 20 Gy. Influence of increasing linear-energy transfer (LET) on film response was investigated by comparing dose measured by EBT3 to a silicon diode detector in depth for a fully-modulated beam using the CCO beam line (homogeneous dose with distal end at 3.1cm in water). A comprehensive uncertainty budget (reproducibility, uniformity'¦) was estimated on films irradiated by Elekta SL25. The main source of uncertainty was the non-uniformity of the scanner response. By placing all the irradiated films at the center of the scanner, the uncertainty could be reduced from 5.8% to 1.9% (1 sigma). For all beams and energies, the calibration curves were matched within uncertainties. Along the fully-modulated depth dose curve, diode and EBT3 measurement were in a 4% agreement point-to-point, indicating films weak dependence with LET. The weak influence of LET, beam type and energy on film response as well as its small uncertainty make EBT3 suitable for relative dosimetry and a promising candidate for measuring correction factors (quality, recombination,'¦) for reference dosimetry with ion chambers of non-standard beams (e.g pencil-beam scanning proton-therapy). “This work is supported by the Walloon Region under the project name InVivoIGT, convention number 1017266.â€.

RBE of α particles and postradiation recovery in a temperature conditional mutant of diploid yeast for various criteria of cell death

The influence of linear energy transfer (LET) on relative biological effectiveness (RBE) and the liquid-holding recovery (LHR) ability was studied for a temperature conditional mutant of diploid yeast. Both the RBE and the rate of LHR were much lower for reproductive killing at 36°C and cell killing without division at both 23 and 36°C. However, the probability of recovery was independent of the culture temperature being identical for both low-and high-LET radiations. It was concluded that the LHR process itself is not damaged after densely ionizing radiation, while the enhanced RBE value and the reduced rate and volume of recovery after high-LET radiation may be entirely caused by the increased yield of the irreversible radiation damages.

Dose and time dependent apoptotic response in a human melanoma cell line exposed to accelerated boron ions at four different LET

The aim was to investigate and compare the influence of linear energy transfer (LET), dose and time on the induction of apoptosis in a human melanoma cell line exposed to accelerated light boron (10B) ions and photons. Cells were exposed in vitro to doses up to 6 Gy accelerated boron ions (40, 80, 125 and 160 eV nm – 1) and up to 12 Gy photons (0.2 eV nm – 1). The induction of apoptosis was measured up to 9 days after irradiation using morphological characterization of apoptotic cells and bodies. In parallel, measurements of cell-cycle distribution, monitored by DNA flow cytometry, and cell survival based on the clonogenic cell survival assay, were performed. In addition, the induction and repair of DNA double-strand breaks (DSB), using pulsed-field gel electrophoresis (PFGE) were studied. Accelerated boron ions induced a significant increase in apoptosis as compared with photons at all time points studied. At 1 – 5 h the percentage of radiation-induced apoptotic cells increased with both dose and LET. At the later time points (24 – 216 h) the apoptotic response was more complex and did not increase in a strictly LET-dependent manner. The early premitotic apoptotic cells disappeared at 24 h following exposure to the highest LET (160 eV nm – 1). A postmitotic apoptotic response was seen after release of the dose-, time- and LET-dependent G2/M accumulations. The loss of clonogenic ability was dose- and LET-dependent and the fraction of unrejoined DSB increased with increasing LET. Despite the LET-dependent clonogenic cell killing, it was not possible to measure quantitatively a LET-dependent apoptotic response. This was due to the different time course of appearance and disappearance of apoptotic cells.

Synergism between gemcitabine and low LET (gamma-rays), but not high-let alpha-particles on multiple myeloma cell lines

Purpose/ObjectiveTo study the influence of Linear Energy Transfer (LET) on the radiosensitizing effects of gemcitabine, by comparing low LET (gamma-rays) and high LET particles (alpha-radioimmunotherapy).Materials/MethodsOn 3 multiple myeloma cell lines (LP1, RPMI 8226 and U266), we measured clonogenic survival after gamma-rays irradiation from Cobalt 60 (0 to 4 Gy) alone or combined with 10 nM gemcitabine after a 24h incubation prior to radiation. We also measured survival after alpha-radioimmunotherapy using B-B4 a monoclonal antibody that targets multiple myeloma cells radiolabeled with bismuth 213, alone or combined with 10 nM gemcitabine. Apoptosis was analyzed by apo 2.7 FACS staining.ResultsGemcitabine acted synergistically with gamma-rays on LP1 and U266 cell lines (Radiation Enhancement Ratio : 1.56 and 1.49 respectively), but not on RPMI 8226. Gemcitabine combined with alpha-radioimmunotherapy did not provoke a synergistic effect on all 3 cell lines. In order to explore the mechanisms of synergism we studied apoptosis 24h after irradiation combined with gemcitabine. Gemcitabine combined with gamma-rays increased apoptosis as compared with gemcitabine or gamma-rays alone on LP1 and U266 cell lines, but not on RPMI 8226. Gemcitabine combined with alpha-radioimmunotherapy increased apoptosis as compared with gemcitabine or alpha-radioimmunotherapy alone on U266 but not LP1 or RPMI 8226 cell lines.ConclusionsGemcitabine radiosensitization depended on the kind of radiation, and did not enhance high LET particles cellular effects. Gemcitabine acted synergistically with gamma-rays, but not with alpha-radioimmunotherapy on multiple myeloma cell lines. Gemcitabine enhanced the apoptotic effects of low LET particles, but this cannot be the only explanation of the observed synergistic effects Purpose/ObjectiveTo study the influence of Linear Energy Transfer (LET) on the radiosensitizing effects of gemcitabine, by comparing low LET (gamma-rays) and high LET particles (alpha-radioimmunotherapy). To study the influence of Linear Energy Transfer (LET) on the radiosensitizing effects of gemcitabine, by comparing low LET (gamma-rays) and high LET particles (alpha-radioimmunotherapy). Materials/MethodsOn 3 multiple myeloma cell lines (LP1, RPMI 8226 and U266), we measured clonogenic survival after gamma-rays irradiation from Cobalt 60 (0 to 4 Gy) alone or combined with 10 nM gemcitabine after a 24h incubation prior to radiation. We also measured survival after alpha-radioimmunotherapy using B-B4 a monoclonal antibody that targets multiple myeloma cells radiolabeled with bismuth 213, alone or combined with 10 nM gemcitabine. Apoptosis was analyzed by apo 2.7 FACS staining. On 3 multiple myeloma cell lines (LP1, RPMI 8226 and U266), we measured clonogenic survival after gamma-rays irradiation from Cobalt 60 (0 to 4 Gy) alone or combined with 10 nM gemcitabine after a 24h incubation prior to radiation. We also measured survival after alpha-radioimmunotherapy using B-B4 a monoclonal antibody that targets multiple myeloma cells radiolabeled with bismuth 213, alone or combined with 10 nM gemcitabine. Apoptosis was analyzed by apo 2.7 FACS staining. ResultsGemcitabine acted synergistically with gamma-rays on LP1 and U266 cell lines (Radiation Enhancement Ratio : 1.56 and 1.49 respectively), but not on RPMI 8226. Gemcitabine combined with alpha-radioimmunotherapy did not provoke a synergistic effect on all 3 cell lines. In order to explore the mechanisms of synergism we studied apoptosis 24h after irradiation combined with gemcitabine. Gemcitabine combined with gamma-rays increased apoptosis as compared with gemcitabine or gamma-rays alone on LP1 and U266 cell lines, but not on RPMI 8226. Gemcitabine combined with alpha-radioimmunotherapy increased apoptosis as compared with gemcitabine or alpha-radioimmunotherapy alone on U266 but not LP1 or RPMI 8226 cell lines. Gemcitabine acted synergistically with gamma-rays on LP1 and U266 cell lines (Radiation Enhancement Ratio : 1.56 and 1.49 respectively), but not on RPMI 8226. Gemcitabine combined with alpha-radioimmunotherapy did not provoke a synergistic effect on all 3 cell lines. In order to explore the mechanisms of synergism we studied apoptosis 24h after irradiation combined with gemcitabine. Gemcitabine combined with gamma-rays increased apoptosis as compared with gemcitabine or gamma-rays alone on LP1 and U266 cell lines, but not on RPMI 8226. Gemcitabine combined with alpha-radioimmunotherapy increased apoptosis as compared with gemcitabine or alpha-radioimmunotherapy alone on U266 but not LP1 or RPMI 8226 cell lines. ConclusionsGemcitabine radiosensitization depended on the kind of radiation, and did not enhance high LET particles cellular effects. Gemcitabine acted synergistically with gamma-rays, but not with alpha-radioimmunotherapy on multiple myeloma cell lines. Gemcitabine enhanced the apoptotic effects of low LET particles, but this cannot be the only explanation of the observed synergistic effects Gemcitabine radiosensitization depended on the kind of radiation, and did not enhance high LET particles cellular effects. Gemcitabine acted synergistically with gamma-rays, but not with alpha-radioimmunotherapy on multiple myeloma cell lines. Gemcitabine enhanced the apoptotic effects of low LET particles, but this cannot be the only explanation of the observed synergistic effects

Treatment planning with heavy ions

The use of heavy charged particles in radiotherapy potentially represents an advance towards better local tumour control and a decrease in morbidity related to radiation injury of healthy tissues surrounding the target volume. This assertion only holds, however, if treatment planning systems give a real representation of the three-dimensional dose distribution, including physical and biological aspects, especially for heavier ions. The influence of linear energy transfer on the biological effects, its variations related to depth, particle, target tissue, position in the Bragg peak, etc. make the possible models for treatment planning extremely complex. A brief review of the problems to be addressed and some solutions is presented from the radiation oncologist's point of view.

The importance of radical density in cosmic chemistry

In various laboratory experiments chemical processes induced by ionizing radiation have been simulated which occur in the outer layers of comets, in interstellar clouds, and in the atmospheres of planets and moons of our Solar System. An obvious problem for the interpretation of the results is that the duration of the experiments is by many orders of magnitude shorter than the time intervals of the simulated systems. Hence in most ice and gas phase experiments much higher dose rates of ionizing radiation are applied in order to obtain a total absorbed dose which is realistic for the simulated system. In this paper the influence of the dose rate on reactions in gaseous systems shall be discussed as well as the influence of linear energy transfer and of scavengers on the reactions and their yields. The central parameter is the density of reactive species. Some consequences for ice systems will be given.

Influence of Linear Energy Transfer on the Radioresistance of Budding Haploid Yeast Cells

The radioresistant component of haploid yeast cells (SC-7) was established by previous investigators to be due to budding cells, and cells with small buds are known to be undergoing DNA synthesis. The response of budding cells of haploid strain SC-7 (Saccharomyces cerevisiae) as a function of LET was studied in this investigation by use of heavy ions (in the ${\rm LET}_{\infty}$ range 420 to 17000 MeV ${\rm g}^{-1}{\rm cm}^{2}$ ) from the Berkeley heavy-ion linear accelerator. The maximum sensitivity for budding cells occurs at approximately the same ${\rm LET}_{\infty}$ (1300-1500 MeV ${\rm g}^{-1}{\rm cm}^{2}$ ) as that for nonbudding cells. The radiosensitivity as a function of LET, of budding haploid yeasts relative to nonbudding cells, is found to be remarkably similar to that of S-phase Chinese hamster cells in culture relative to cells not undergoing DNA synthesis.

Influence of Linear Energy Transfer and Oxygen Tension on the Effectiveness of Ionizing Radiations for Induction of Mutations and Lethality in Saccharomyces cerevisiae

The radiations available from heavy-ion linear accelerators (HILAC) permit an extension of radiobiological studies to regions of linear energy transfer (LET) not previously available. The variation of the relative biological effectiveness (RBE) as a function of LET for the different particle beams has been determined for a variety of effects in different biological systems (1-5). For inactivation of protein molecules or viruses (3), the efficiency decreases continuously with increasing LET. For induction of lethality in yeast (1, 2) or in mammalian cells (5), the RBE rises to a maximum and then decreases as the LET increases. It was of interest to determine if the relation between RBE and LET for the induction of mutations paralleled those for effects on molecules or viruses, or was similar to the relation observed for induction of lethality in cells. Stapleton et al. (6), using Aspergillus, found that a-particles were more effective for killing but less effective for mutation than Xrays. Deering (4) has reported that oxygen ions are less effective for both killing and mutation than are less densely ionizing radiations, although mutability at high LET was decreased relatively more. Hrishi and James (7) have found that thermal neutrons are more effective than X-rays for inducing reversions in histidinerequiring diploid yeast cells. For a given degree of inactivation, however, neutrons were less effective for inducing mutation. The present study is concerned with the determination of the relative effectiveness of a number of particle beams in inducing specific genetic effects and lethality in yeast cells.

Effects of ploidy and linear energy transfer on radiobiological survival curves

The radiobiological influence of linear energy transfer (energy transferred per unit length of the track of an ionizing particle) was investigated with respect to inhibition of cell division in three microorganisms: a diploid strain of the yeast Saccharomyces cerevisiae; a haploid yeast derived form the diploid; and the unicellular green alga Stichococcus. Complete survival curves were obtained for each organism at each of nine different values of linear energy transfer (LET), ranging from 0.73 to 190 k.e.v. per micron of path in tissue. Most of these values of LET were obtained by using different portions of the paths of very speedy deuterons, accelerated helium nuclei, and natural α-particles in such fashion that, in any given exposure, the value of LET was essentially the same for all particles that traversed the cell sample. At all values of LET, the survival curves of the haploid yeast were exponential, while those of the diploid were strongly sigmoid and of a shape essentially independent of LET. The relative biological effectiveness (RBE) (a quantity inversely proportional to the 50 % survival dose) of the nine radiations for both yeasts was practically constant for LET values from 0.73 to 27 k.e.v./μ but increased by about a factor of four or five between 27 and 190 k.e.v./μ. The yeast survival curves are consistent with a theory that, in the haploid, cell division can be inhibited by inactivating any one of some tens of chromosomal sites by means of a single ionizing particle, whereas in the diploid it is necessary to inactivate both members of any one allelic pair of corresponding sites. The variation of RBE with LET is not consistent with simple target theory but is readily explained in terms of chemical intermediates which can diffuse from their places of origin in the ionization tracks to the chromosomal sites. The survival curve of Stichococcus was sigmoid, and the RBE increased by a factor of twelve as LET varied from 1.5 to 190 k.e.v./μ.