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A horizontal multi-purpose microbeam system with a single electrostatic quadruplet focusing lens has been developed at the Columbia University Radiological Research Accelerator Facility (RARAF). It is coupled with the RARAF 5.5 MV Singleton accelerator (High Voltage Engineering Europa, the Netherlands) and provides micrometer-size beam for single cell irradiation experiments. It is also used as the primary beam for a neutron microbeam and microPIXE (particle induced x-ray emission) experiment because of its high particle fluence. The optimization of this microbeam has been investigated with ray tracing simulations and the beam spot size has been verified by different measurements.

Radiation-induced bystander effects have been observed in vitro and in cell and tissue culture models, however, there are few reported studies showing these effects in vivo. To our knowledge, this is the first reported study on bystander effects induced by microbeam irradiation in an intact living mammal. The mouse ear was used to investigate radiation-induced bystander effects in keratinocytes, utilizing a 3 MeV proton microbeam (LET 13.1 keV/μm) with a range in skin of about 135 μm. Using a custom-designed holder, the ear of an anesthetized C57BL/6J mouse was flattened by gentle suction and placed over the microbeam port to irradiate cells along a 35 μm wide, 6 mm long path. Immunohistochemical analysis of γ-H2AX foci formation in tissue sections revealed, compared to control tissue, proton-induced γ-H2AX foci formation in one of the two epidermal layers of the mouse ear. Strikingly, a higher number of cells than expected showed foci from direct irradiation effects. Although the proton-irradiated line was ~35 μm wide, the average width spanned by γ-H2AX-positive cells exceeded 150 μm. Cells adjacent to or in the epidermal layer opposite the γ-H2AX-positive region did not exhibit foci. These findings validate this mammalian model as a viable system for investigating radiation-induced bystander effects in an intact living organism.

A laser ion source that will be installed on the new High Voltage Engineering (HVE) 5 MV Singletron accelerator at the Columbia University Radiological Research Accelerator Facility (RARAF) will expand the linear energy transfer (LET) range available for irradiation experiments with mammalian cells. Through laser ablation the laser ion source can produce heavy ions with high charge states from a solid target; after acceleration, these ions will have sufficient energy to irradiate cells on a thin surface at atmospheric pressure. A high-power 100 Hz pulsed Nd:YAG laser used with the laser ion source has produced aluminum ions with charge states greater than nine. Proper power management issues are important in obtaining the high charge states while protecting sensitive laser optics. We expect that the laser ion source will enable us to use ions from hydrogen to iron, providing an LET range of about 10–4500 keV/lm for cell targets.

The stand-alone microbeam, under development at RARAF, presents a novel approach to biological microbeam irradiation studies. Foregoing a conventional accelerator as a source of energetic ions, we propose to use a small, high-specific-activity, alpha emitter. Alpha particles emitted from this source are focused using a compound magnetic lens consisting of 24 permanent magnets arranged in two quadrupole triplets. Using a ‘‘home made’’ 6.5 mCi Polonium source, a 1 alpha particle/s, 10 lm diameter microbeam can be realized. As the alpha source energy is constant, once the microbeam has been set up, no further adjustments are necessary apart from a periodic replacement of the source. The use of permanent magnets eliminates the need for bulky power supplies and cooling systems required by other types of ion lenses and greatly simplifies operation. It also makes the microbeam simple and cheap enough to be realized in any large lab.

A central tenet in understanding the biological effects of ionizing radiation has been that the initially affected cells were directly damaged by the radiation. By contrast, evidence has emerged concerning “bystander” responses involving damage to nearby cells that were not themselves directly traversed by the radiation. These long-range effects are of interest both mechanistically and for assessing risks from low-dose exposures, where only a small proportion of cells are directly hit. Bystander effects have been observed largely by using single-cell in vitro systems that do not have realistic multicellular morphology; no studies have as yet been reported in three-dimensional, normal human tissue. Given that the bystander phenomenon must involve cell-to-cell interactions, the relevance of such single-cell in vitro studies is questionable, and thus the significance of bystander responses for human health has remained unclear. Here, we describe bystander responses in a three-dimensional, normal human-tissue system. Endpoints were induction of micronucleated and apoptotic cells. A charged-particle microbeam was used, allowing irradiation of cells in defined locations in the tissue yet guaranteeing that no cells located more than a few micrometers away receive any radiation exposure. Unirradiated cells up to 1 mm distant from irradiated cells showed a significant enhancement in effect over background, with an average increase in effect of 1.7-fold for micronuclei and 2.8-fold for apoptosis. The surprisingly long range of bystander signals in human tissue suggests that bystander responses may be important in extrapolating radiation risk estimates from epidemiologically accessible doses down to very low doses where nonhit bystander cells will predominate.

A single-cell/single-particle microbeam has been designed in which the radiation source is a high specific-activity alpha-particle emitting radioisotope, rather than a particle accelerator. The source is imaged at the focal point of a compound magnetic lens array. A novel precision microscope stage with an extremely compact design is included with the endstations locating, targeting and imaging system. This arrangement allows a 4 m long vertical beamline without any bending magnets. The permanent magnet lens design is derived from an electrostatic lens used in our laboratory in an accelerator-based microbeam. The stand-alone microbeam is expected to produce a 10 lm diameter beam, and will serve as a high-end instrument in a biology laboratory, to precisely deposit ionizing radiation damage within single cells.

The Columbia University Microbeam II has been built to provide a focused ion beam for irradiating designated mammalian cells with single particles. With the interest in irradiating non-stained cells and cells in three-dimensional tissue samples, the endstation was designed to accommodate a variety of imaging techniques, in addition to fluorescent microscopy. Non-stained cells are imaged either by quantitative phase microscopy (QPm) [IATIA, Box Hill North, Victoria, 3129, Australia [1]] or by Mirau interferometry. In conjunction with the focused microbeam, a proposed multiphoton microscope will offer the opportunity to probe live cells at selected depths in tissue samples; dynamic events in such tissue samples are more representative of biological system response than those in cell cultures.

We describe a novel secondary-electron ion microscope (SEIM), designed for diagnostics of the upcoming submicron Columbia University charged-particle microbeam. This secondary-electron ion microscope allows much higher resolutions, at higher single particle detection efficiencies, than previously available, for rapid and accurate diagnostics of sub-micron charged-particle beams. Based on ion electron-emission microscopy (IEEM) and photo-electron microscopy (PEM), the SEIM involves conversion of the incident projectiles on a secondary-electron emitting film. The ejected electrons are focused using a unipolar electrostatic lens and conical electrostatic mirror to form a magnified image on a microchannel plate (MCP). The flight path of the electrons includes two 45� bends; this ‘‘folded’’ geometry results in lower aberrations than a ‘‘straight’’ design, and enables efficient beam imaging down to 100 nm resolution with >50% single electron transfer efficiency. � 2005 Elsevier B.V. All rights reserved.

Normal human fibroblasts in plateau phase ( congruent with 95% G(1) phase) were stained with the vital nuclear dye Hoechst 33342 (blue fluorescence) or the vital cytoplasmic dye Cell Tracker Orange (orange fluorescence) and plated at a ratio of 1:1. Only the blue-fluorescing nuclei were microbeam-irradiated with a defined number of 90 keV/microm alpha particles. The orange-fluorescing cells were then "bystanders", i.e. not themselves hit but adjacent to cells that were. Hit cells showed a fluence-dependent induction of micronuclei as well as delays in progression from G(1) to S phase. Known bystander cells also showed enhanced frequencies of micronuclei (intermediate between those seen in irradiated and control cells) and transient cell cycle delays. However, the induction of micronuclei in bystander cells did not appear to be dependent on the fluence of the particles delivered to the neighboring hit cells. These are the first studies in which the bystander effect has been visualized directly rather than inferred. They indicate that the phenomenon has a quantitative basis and imply that the target for radiation effects cannot be considered to be the individual cell.

Mitchell, S. A., Randers-Pehrson, G., Brenner, D. J. and Hall, E. J. The Bystander Response in C3H 10T‰ Cells: The Influence of Cell-to-Cell Contact. Radiat. Res. 161, 397‐401 (2004). Although radiation-induced heritable damage in mammalian cells was thought to result from the direct interaction of radiation with DNA, it is now accepted that biological effects may occur in cells that were not themselves traversed by ionizing radiation but are close to those that were. However, little is known about the mechanism underlying such a bystander effect, although cell-to-cell communication is thought to be of importance. Previous work using the Columbia microbeam demonstrated a significant bystander effect for clonogenic survival and oncogenic transformation in C3H 10T‰ cells. The present study was undertaken to assess the importance of the degree of cell-to-cell contact at the time of irradiation on the magnitude of this bystander effect by varying the cell density. When 10% of cells were exposed to a range of 2‐12 a particles, a significantly greater number of cells (P , 0.0001) were inactivated when cells were irradiated at high density (.90% in contact with neighbors) than at low density (,10% in contact). In addition, the oncogenic transformation frequency was significantly higher in high-density cultures (P , 0.0004). These results suggest that when a cell is hit by radiation, the transmission of the bystander signal through cell-to-cell contact is an important mediator of the effect, implicating the involvement of intracellular communication through gap junctions. q 2004 by Radiation Research Society

Two conflicting phenomena, bystander effect and adaptive response, are important in determining the biological responses at low doses of radiation and have the potential to impact the shape of the dose-response relationship. Using the Columbia University charged-particle microbeam and the highly sensitive AL cell mutagenic assay, we show here that non-irradiated cells acquire mutagenesis through direct contact with cells whose nuclei have been traversed with a single alpha particle each. Pretreatment of cells with a low dose of X-rays four hours before alpha particle irradiation significantly decreased this bystander mutagenic response. Results from the present study address some of the fundamental issues regarding both the actual target and radiation dose effect and can contribute to our current understanding in radiation risk assessment. c2004 COSPAR. Published by Elsevier Ltd. All rights reserved.

A laser ion source is scheduled as an upgrade for the 4 MV Van de Graaff accelerator at the Columbia Universitys Radiological Research Accelerator Facility (RARAF). The source has been designed for use with the RARAF singleparticle single-cell microbeam, though it will be used also for broad-beam irradiations. The operating principle, laser ablation, can produce heavy-ions with high charge states so that their energies will be high enough to provide sufficient range – at least 20 lm – for irradiating cells on a thin surface at atmospheric pressure. The laser ion source being implemented at RARAF consists of three main components: a high-power 100 Hz Nd:YAG laser, a source vacuum chamber and a 24 spherical electrostatic analyzer and a double focusing element with point-to-point focusing in both the horizontal and vertical planes. We expect that the laser ion source will enable us to use ions of sufficient range from hydrogen to iron, providing a range of linear energy transfer from about 10 to 4500 keV/lm. 2003 Elsevier B.V. All rights reserved.

Two conflicting phenomena, bystander effect and adaptive response, are important in determining the biological responses at low doses of radiation and have the potential to impact on the shape of the dose–response relationship. Using the Columbia University charged-particle microbeam and the highly sensitive A L cell mutagenic assay, we show here that non-irradiated cells acquire mutagenesis through direct contact with cells whose nuclei had been traversed with two alpha particles each. Pre-treatment of cells with a 0.1-Gy dose X-rays 4 h before alpha particle irradiation significantly decreased this bystander mutagenic response. Results from the present study address some of the fundamental issues regarding both the actual target and radiation dose effect and can impart on our current understanding in radiation risk assessment.

A noninvasive, self-referencing biosensor/probe system has been integrated into the Columbia University Radiological Research Accelerator Facility Microbeam II end station. A single-cell oxygen consumption measurement has been conducted with this type of oxygen probe in 37° C Krebs–Ringer Bicarbonate buffer immediately before and after a single-cell microbeam irradiation. It is the first such measurement made for a microbeam irradiation, and a six fold increment of oxygen flux induced during a 15-s period of time has been observed following radiation exposure. The experimental procedure and the results are discussed.

Sawant, S. G., Randers-Pehrson, G., Metting, N. and Hall, E. J. Adaptive Response and the Bystander Effect Induced by Radiation in C3H 10T½ Cells in Culture. Radiat. Res. 156, 177–180 (2001). This paper discusses two phenomena of importance at low doses that have an impact on the shape of the dose–response relationship. First, there is the bystander effect, the term used to describe the biological effects observed in cells that are not themselves traversed by a charged particle, but are neighbors of cells that are; this exaggerates the effect of small doses of radiation. Second, there is the adaptive response, whereby exposure to a low level of DNA stress renders cells resistant to a subsequent exposure; this reduces the effect of low doses of radiation. The present work was undertaken to assess the relative importance of the adaptive response and the bystander effect induced by radiation in C3H 10T½ cells in culture. When the single-cell microbeam delivered from 1 to 12 α particles through the n...

Sawant, S. G., Randers-Pehrson, G., Geard, C. R., Brenner, D. J. and Hall, E. J. The Bystander Effect in Radiation Oncogenesis: I. Transformation in C3H 10T½ Cells In Vitro can be Initiated in the Unirradiated Neighbors of Irradiated Cells. It has long been accepted that radiation-induced genetic effects require that DNA be hit and damaged directly by the radiation. Recently, evidence has accumulated that in cell populations exposed to low doses of α particles, biological effects occur in a larger proportion of cells than are estimated to have been traversed by α particles. The end points observed include chromosome aberrations, mutations and gene expression. The development of a fast single-cell microbeam now makes it possible to expose a precisely known proportion of cells in a population to exactly defined numbers of α particles, and to assay for oncogenic transformation. The single-cell microbeam delivered no, one, two, four or eight α particles through the nuclei of all or just 10% of C3H 10...

A design is given of a laser ion source for the 4.2 MV Van de Graaff accelerator at the Columbia University Radiological Research Accelerator Facility (RARAF). The source has been designed with application in mind for the RARAF single-particle single-cell microbeam, though it will also be used for broad-beam irradiations. The operating principle, laser ablation, can produce heavy ions with high charge states so that their energies will be high enough to provide sufficient range—at least 20 μm—for irradiating cells on a thin surface at atmospheric pressure. The laser ion source being implemented at RARAF is based on the laser operated ion source used by Hughes at the University of Arkansas and consists of three main components: laser generator, source vacuum chamber, and spherical electrostatic analyzer.

A soft x-ray microbeam using proton-induced x-ray emission (PIXE) of characteristic titanium (K(α) 4.5 keV) as the x-ray source has been developed at the Radiological Research Accelerator Facility (RARAF) at Columbia University. The proton beam is focused to a 120 μm × 50 μm spot on the titanium target using an electrostatic quadrupole quadruplet previously used for the charged particle microbeam studies at RARAF. The proton induced x-rays from this spot project a 50 μm round x-ray generation spot into the vertical direction. The x-rays are focused to a spot size of 5 μm in diameter using a Fresnel zone plate. The x-rays have an attenuation length of (1/e length of ~145 μm) allowing more consistent dose delivery across the depth of a single cell layer and penetration into tissue samples than previous ultra soft x-ray systems. The irradiation end station is based on our previous design to allow quick comparison to charged particle experiments and for mixed irradiation experiments.

Presented here is a novel microbeam technology--the Flow-And-ShooT (FAST) microbeam--under development at RARAF. In this system, cells undergo controlled fluidic transport along a microfluidic channel intersecting the microbeam path. They are imaged and tracked in real-time, using a high-speed camera and dynamically targeted, using a magnetic Point and Shoot system. With the proposed FAST system, RARAF expects to reach a throughput of 100,000 cells per hour, which will allow increasing the throughput of experiments by at least one order of magnitude. The implementation of FAST will also allow the irradiation of non-adherent cells (e.g. lymphocytes), which is of great interest to many of the RARAF users. This study presents the design of a FAST microbeam and results of first tests of imaging and tracking as well as a discussion of the achievable throughput.

A novel neutron microbeam is being developed at the Radiological Research Accelerator Facility (RARAF) of Columbia University. The RARAF microbeam facility has been used for studies of radiation bystander effects in mammalian cells for many years. Now a prototype neutron microbeam is being developed that can be used for bystander effect studies. The neutron microbeam design here is based on the existing charged particle microbeam technology at the RARAF. The principle of the neutron microbeam is to use the proton beam with a micrometre-sized diameter impinging on a very thin lithium fluoride target system. From the kinematics of the 7Li(p,n)7Be reaction near the threshold of 1.881 MeV, the neutron beam is confined within a narrow, forward solid angle. Calculations show that the neutron spot using a target with a 17-µm thick gold backing foil will be

A lens system consisting of two electrostatic quadrupole triplets has been designed and constructed at the Radiological Research Accelerator Facility (RARAF) of Columbia University. The lens system has been used to focus 6 MeV 4 He ions to a beam spot in air with a diameter of 0.8 μm. The quadrupole electrodes can withstand voltages high enough to focus 4 He ions up to 10 MeV and protons up to 5 MeV. The quadrupole triplet design is novel in that alignment is made through precise construction and the relative strengths of the quadrupoles are accomplished by the lengths of the elements, so that the magnitudes of the voltages required for focusing are nearly identical. The insulating sections between electrodes have had ion implantation to improve the voltage stability of the lens. The lens design employs Russian symmetry for the quadrupole elements.

Charged-particle microbeams, developed to provide targeted irradiation of individual cells, and then of sub-cellular components, and then of 3-D tissues and now organisms, have been instrumental in challenging and changing long accepted paradigms of radiation action. However the potential of these valuable tools can be enhanced by integrating additional components with the direct ability to measure biological responses in real time, or to manipulate the cell, tissue or organism of interest under conditions where information gained can be optimized. The RARAF microbeam has recently undergone an accelerator upgrade, and been modified to allow for multiple microbeam irradiation laboratories. Researchers with divergent interests have expressed desires for particular modalities to be made available and ongoing developments reflect these desires. The focus of this review is on the design, incorporation and use of multiphoton and other imaging, micro-manipulation and single cell biosensor capabilities at RARAF. Additionally, an update on the status of the other biology oriented microbeams in the Americas is provided.

The understanding of complex radiation responses in biological systems, such as non-targeted effects as represented by the bystander response, can be enhanced by the use of genetically amenable model organisms. Almost all bystander studies to date have been carried out by using conventional single-cell in vitro systems, which are useful tools to characterize basic cellular and molecular responses. A few studies have been reported in monolayer explants and bystander responses have been also investigated in a three-dimensional normal human tissue system. However, despite the well-know usefulness of in vitro models, they cannot capture the complexity of radiation responses of living systems such as animal models. To carry out in vivo studies on the bystander effect we have developed a new technique to expose living organisms using proton microbeams. We report the use of a nematode C. elegans strain with a Green Fluorescent Protein (GFP) reporter for the hsp-4 heat-shock gene as an in vivo model for radiation studies. Exposing animals to heat and chemicals stressors leads to whole body increases in the hsp-4 protein reflected by enhanced fluorescence. We report here that gamma-rays also can induce stress response in a dose dependent manner. However, whole body exposure to stress agents does not allow for evaluation of distance dependent response in non targeted tissues: the so-called bystander effect. We used the RARAF microbeam to site specifically deliver 3 MeV protons to a site in the tail of young worms. GFP expression was enhanced after 24 hours in a number dependent manner at distances100 microm from the site of irradiation.

Multiphoton microscopy has been added to the array of imaging techniques at the endstation for the Microbeam II cell irradiator at Columbia University’s Radiological Research Accelerator Facility (RARAF). This three-dimensional (3D), laser-scanning microscope functions through multiphoton excitation, providing an enhanced imaging routine during radiation experiments with tissuelike samples, such as small living animals and organisms. Studies at RARAF focus on radiation effects; hence, this multiphoton microscope was designed to observe postirradiation cellular dynamics. This multiphoton microscope was custom designed into an existing Nikon Eclipse E600-FN research fluorescence microscope on the irradiation platform. Design details and biology applications using this enhanced 3D-imaging technique at RARAF are reviewed.

Ionizing radiations have been shown to be able to induce the death of mammalian cells and initiate mutagenic or carcinogenic change. While all three end points are related through chromosomal changes, the latter in particular is of profound concern to human populations. We have undertaken a series of studies wherein mammalian cells were irradiated with low fluences of charged particles (protons, deuterons, helium ions) of defined LET from 10 to about 200 keV per micrometer. Frequencies of induced chromosomal changes were related to fluence at each LET, such that the induction of chromosomal changes per cell per charged particle could be estimated. However, for human exposures from densely ionizing radiation, such as the alpha particles from radon daughters, effects are dominated by the traversal of cells by single particles. Conventional experiments inevitably result in cells being exposed to a distribution (Poisson) of particle traversals. As the effect is unlikely to be a linear function of the number of traversals, a preferred approach would be to irradiate cells with exactly one (or any known number) of particles. To this end we are developing a dedicated beam line (microbeam) on a 4.2 MV Van de Graaff accelerator such that individual particles will vertically traverse individual living mammalian cells positioned by a microscope-based imaging system under computer control. “Conventional” low-fluence and “single-particle” studies will be compared, allowing critical evaluations of the potential of individual high LET charged particles to initiate change. This will have particular relevance both to consideration of the human health risks of radon daughter alpha particles and of basic mechanisms of chromosome aberration formation.

The stand-alone microbeam at Columbia University presents a novel approach to biological microbeam irradiation studies. Foregoing a conventional accelerator as a source of energetic ions, a small, high-specific-activity, alpha emitter is used. Alpha particles emitted from this source are focused using a compound magnetic lens consisting of 24 permanent magnets arranged in two quadrupole triplets. Using a 'home made' 6.5 mCi polonium source, a 1 alpha particle s(-1), 10 microm diameter microbeam can, in principle, be realised. As the alpha source energy is constant, once the microbeam has been set up, no further adjustments are necessary apart from a periodic replacement of the source. The use of permanent magnets eliminates the need for bulky power supplies and cooling systems required by other types of ion lenses and greatly simplifies operation. It also makes the microbeam simple and cheap enough to be realised in any large lab. The Microbeam design as well as first tests of its performance, using an accelerator-based beam are presented here.

Based principally on the cancer incidence found in survivors of the atomic bombs dropped in Hiroshima and Nagasaki, the International Commission on Radiation Protection (ICRP) and the United States National Council on Radiation Protection and Measurements (NCRP) have recommended that estimates of cancer risk for low dose exposure be extrapolated from higher doses by using a linear, no-threshold model. This recommendation is based on the dogma that the DNA of the nucleus is the main target for radiation-induced genotoxicity and, as fewer cells are directly damaged, the deleterious effects of radiation proportionally decline. In this paper, we used a precision microbeam to target an exact fraction (either 100% or ≤20%) of the cells in a confluent population and irradiated their nuclei with exactly one α particle each. We found that the frequencies of induced mutations and chromosomal changes in populations where some known fractions of nuclei were hit are consistent with non-hit cells contributing significantly to the response. In fact, irradiation of 10% of a confluent mammalian cell population with a single α particle per cell results in a mutant yield similar to that observed when all of the cells in the population are irradiated. This effect was significantly eliminated in cells pretreated with a 1 mM dose of octanol, which inhibits gap junction-mediated intercellular communication, or in cells carrying a dominant negative connexin 43 vector. The data imply that the relevant target for radiation mutagenesis is larger than an individual cell and suggest a need to reconsider the validity of the linear extrapolation in making risk estimates for low dose, high linear-energy-transfer (LET) radiation exposure.

Randers-Pehrson, G., Geard, C. R., Johnson, G., Elliston, C. D. and Brenner, D. J. The Columbia University Single-Ion Microbeam. Radiat. Res. 156, 210–214 (2001). A single-ion microbeam facility has been constructed at the Columbia University Radiological Research Accelerator Facility. The system was designed to deliver defined numbers of helium or hydrogen ions produced by a van de Graaff accelerator, covering a range of LET from 30 to 220 keV/μm, into an area smaller than the nuclei of human cells growing in culture on thin plastic films. The beam is collimated by a pair of laser-drilled apertures that form the beam-line exit. An integrated computer control program locates the cells and positions them for irradiation. We present details of the microbeam facility including descriptions of the collimators, hardware, control program, and the various protocols available. Various contributions to targeting and positioning precision are discussed along with our plans for future developments. Beam tim...

Domestic, low-level exposure to radon gas is considered a major environmental lung-cancer hazard involving DNA damage to bronchial cells by alpha particles from radon progeny. At domestic exposure levels, the relevant bronchial cells are very rarely traversed by more than one alpha particle, whereas at higher radon levels-at which epidemiological studies in uranium miners allow lung-cancer risks to be quantified with reasonable precision-these bronchial cells are frequently exposed to multiple alpha-particle traversals. Measuring the oncogenic transforming effects of exactly one alpha particle without the confounding effects of multiple traversals has hitherto been unfeasible, resulting in uncertainty in extrapolations of risk from high to domestic radon levels. A technique to assess the effects of single alpha particles uses a charged-particle microbeam, which irradiates individual cells or cell nuclei with predefined exact numbers of particles. Although previously too slow to assess the relevant small oncogenic risks, recent improvements in throughput now permit microbeam irradiation of large cell numbers, allowing the first oncogenic risk measurements for the traversal of exactly one alpha particle through a cell nucleus. Given positive controls to ensure that the dosimetry and biological controls were comparable, the measured oncogenicity from exactly one alpha particle was significantly lower than for a Poisson-distributed mean of one alpha particle, implying that cells traversed by multiple alpha particles contribute most of the risk. If this result applies generally, extrapolation from high-level radon risks (involving cellular traversal by multiple alpha particles) may overestimate low-level (involving only single alpha particles) radon risks.

We fabricated ultrathin metal-semiconductor Schottky diodes for use as transmission particle detectors in the biological microbeam at Columbia University’s Radiological Research Accelerator Facility (RARAF). The RARAF microbeam can deliver a precise dose of ionizing radiation in cell nuclei with sub-micron precision. To ensure an accurate delivery of charged particles, the facility currently uses a commercial charged-particle detector placed after the sample. We present here a transmission detector that will be placed between the particle accelerator and the biological specimen, allowing the irradiation of samples that would otherwise block radiation from reaching a detector behind the sample. Four detectors were fabricated with co-planar gold and aluminum electrodes thermally evaporated onto etched n-type crystalline silicon substrates, with device thicknesses ranging from 8.5 μm – 13.5 μm. We show coincident detections and pulse-height distributions of charged particles in both the transmission detector and the commercial detector above it. Detections are demonstrated at a range of operating conditions, including incoming particle type, count rate, and beam location on the detectors. The 13.5 μm detector is shown to work best to detect 2.7 MeV protons (H+), and the 8.5 μm detector is shown to work best to detect 5.4 MeV alpha particles (4He++). The development of a transmission detector enables a range of new experiments to take place at RARAF on radiation-stopping samples such as thick tissues, targets that need immersion microscopy, and integrated microfluidic devices for handling larger quantities of cells and small organisms.