Dose spectra from energetic particles and neutrons



[1] Dose spectra from energetic particles and neutrons (DoSEN) are an early-stage space technology research project that combines two advanced complementary radiation detection concepts with fundamental advantages over traditional dosimetry. DoSEN measures not only the energy but also the charge distribution (including neutrons) of energetic particles that affect human (and robotic) health in a way not presently possible with current dosimeters. For heavy ions and protons, DoSEN provides a direct measurement of the lineal energy transfer (LET) spectra behind shielding material. For LET measurements, DoSEN contains stacks of thin-thick Si detectors similar in design to those used for the Cosmic Ray Telescope for the Effects of Radiation. With LET spectra, we can now directly break down the observed spectrum of radiation into its constituent heavy-ion components and through biologically based quality factors that provide not only doses and dose rates but also dose equivalents, associated rates, and even organ doses. DoSEN also measures neutrons from 10 to 100 MeV, which requires enough sensitive mass to fully absorb recoil particles that the neutrons produce. DoSEN develops the new concept of combining these independent measurements and using the coincidence of LET measurements and neutron detection to significantly reduce backgrounds in each measurement. The background suppression through the use of coincidence allows for significant reductions in size, mass, and power needed to provide measurements of dose, neutron dose, dose equivalents, LET spectra, and organ doses. Thus, we introduce the DoSEN concept: a promising low-mass instrument that detects the full spectrum of energetic particles, heavy ions, and neutrons to determine biological impact of radiation in space.

1 Introduction

1.1 The Multifaceted Challenge for a New Generation of Dosimetry Detectors

[2] We are preparing for human exploration beyond low Earth orbit (LEO). The radiation hazard is potentially severe but not sufficiently well characterized to determine if long missions outside LEO can be accomplished with acceptable risk [Cucinotta et al., 2001; Schwadron et al., 2010; Cucinotta et al., 2010]. Radiation hazards may be overstated or understated through incomplete characterization in terms of net quantities such as accumulated dose. Time-dependent characterization often changes acute risk estimates [National Council on Radiation Protection and Measurements (NCRP), 1989; Cucinotta, 1999; Cucinotta et al., 2000; George et al., 2002]. Protons, heavy ions, and neutrons all contribute significantly to the radiation hazard. However, each form of radiation presents different biological effectiveness. As a result, quality factors and radiation-specific weighting factors are needed to assess biological effectiveness of different forms of radiation [e.g., International Commission on Radiological Protection (ICRP), 1991; NCRP, 1993]. More complete characterization must account for time-dependent radiation effects according to organ type, primary and secondary radiation composition, and acute effects (vomiting, sickness, and, at high exposures, death) versus chronic effects (such as cancer).

[3] For heavy ions and protons, there are considerable advantages of providing direct measurements of the energy deposition spectra both behind shielding material and with no shielding. The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) [Spence et al., 2010] is designed for this purpose. Lineal energy transfer (LET) is the basis of risk assessment in a mixed radiation field; it is the mean energy absorbed (ΔE) locally per unit path length (Δx) when a charged particle traverses material. LET is expressed in units of MeV/g/cm2 or, in biological systems, keV/µm. A LET spectrometer measures the amount of energy deposited in a detector of known thickness as a high-energy particle passes through it without stopping. Figure 1 shows the direct relationship between LET and quality factor defined by the International Commission on Radiological Protection (ICRP) [ICRP, 1991], which is needed to quantify biological impact in terms of factors such as dose equivalents and organ doses. There has been discussion within NASA about replacing the ICRP quality factor with a somewhat different definition that does not directly depend on LET [Cucinotta et al., 2012]. However, there remain major questions about how a new definition can be implemented with instrumentation. In this work, we continue to use LET while recognizing the possibility that future changes in characterization of biological impact may necessitate the use of new measurement quantities.

Figure 1.

Quality factors are directly linked to lineal energy transfer (LET). By directly measuring LET, DoSEN is able to provide a direct connection between measurements and biological impact via the quality factor.

[4] Measurements of space radiation continue to be made largely using silicon detectors. Measurements of energy deposited in silicon must be converted into tissue dose. A single scaling factor [e.g., Beaujean et al., 2002; Schwadron et al., 2012] is often used to perform this conversion. For example, Schwadron et al. [2012] find in CRaTER that the dose rate in water is 33% larger than the dose rate in silicon.

[5] Figure 2 shows the relationship between measured particle fluences versus modeled LET for a solar energetic particle (SEP) event in October 1989 (J. Barth and M. Xapsos, SEP measurements required for environment models and system design and testing, paper presented at GOES R+ EP Workshop, Boulder, CO, 28–29 October 2002) While LET spectrometers do not (necessarily) resolve mass, LET measurements should include all the contributing charged particle species (see Figures 3 and 4). The dose and dose equivalent from neutrons are an additional important part of the radiation exposure behind shielding and must also be accounted for. Dosimeters such as tissue equivalent proportional counters [Perez-Nunez and Braby, 2010] and plastic nuclear track detectors [Benton and Richmond, 1986] measure all energy depositions, including those induced by neutrons, with tissue-like response, but do not provide separate neutron and charged particle spectra.

Figure 2.

The relationship between measured particle fluences versus modeled LET for a SEP event in October 1989 (discussed by Barth and Xapsos (presented paper, 2002)).

Figure 3.

Galactic cosmic ray LET spectrum observed by the CRaTER instrument. The labeled peaks correspond to the minimum ionizing LET for each of the indicated species [from Case et al., 2013]. Here D1/D2, D3/D4, and D5/D6 refer to the three thin (~150 µm)/thick (~1000 µm) silicon detector pairs with each detector pair separated by tissue equivalent plastic [Spence et al., 2010]. The LET shown here is separated between “thick” detector LET measurements (< 50 keV/µm) and “thin” detector LET measurements (>50 keV/µm). The LET spectrum is shown for galactic cosmic rays incident on CRaTER (on the Lunar Reconnaissance Observatory orbiting the Moon) from the zenith direction with D1 facing zenith and D6 facing nadir. Therefore, galactic cosmic rays typically first penetrate the D1/D2 detector pair and then pass through the other silicon detectors and tissue equivalent plastic (TEP). The fact that the fluxes are highest in the D1/D2 detector pair and lower in D3/D4 and D5/D6 reflects the energy losses incurred in the TEP separating the D1/D2 from D3/D4 detector pairs and in the TEP separating D3/D4 from D5/D6.

Figure 4.

DoSEN's LET coincidence in multiple solid-state detectors (SSDs) allows species and quality factors, providing a direct relationship between LET measurements and biological impact (e.g., dose equivalents and organ doses). Shown here are “crossplots” from CRaTER: the LET from the CRaTER D1 and D2 detectors versus LET from D3 and D4 detectors. We clearly resolve the different species that contribute to the radiation dose and dose equivalent. The labels for H, He, C, and O show where these particles are identified in the crossplot. Note that there are tracks that curve up from the diagonal coincidence region. Each track is associated with an individual cosmic ray element. The black diagonal solid lines extending from the C and O coincidence peaks indicate the associated element tracks, whereas the neon, magnesium, silicon, and iron (Fe) tracks are identified off the diagonal coincidence region.

[6] The CRaTER instrument, currently operating in lunar orbit, provides ground truth measurements of LET spectra with and without shielding (Figure 3). These data provide a direct link between the unshielded deep-space radiation environment and the environment after modification by shielding. With LET spectra (Figure 3), we can directly break down the observed spectrum of radiation into its constituent heavy-ion components (Figure 4) and through biologically based quality factors (Figure 1) that provide not only doses and dose rates but also dose equivalents, associated rates, and even organ doses in shielded and unshielded environments.

[7] A second important element of dosimetry detection is the ability to detect neutrons with an active, low-mass device. Neutrons represent a serious radiation hazard encountered in low Earth orbit (LEO) or in deep space. This is because neutrons are copiously produced in cosmic ray reactions with all material (e.g., in the form of a space vehicle, cargo, or a nearby planetary body) and because they are remarkably penetrating. They cannot be suppressed by shielding unless specific measures (e.g., use of hydrogenous shielding material) are taken. For passengers or crew, this means that they interact throughout one's entire body. For avionics, all sensitive electrical components, regardless of how deeply embedded in the mechanical structure, are exposed. The radiation assessment detector (RAD) [Posner et al., 2005; Hassler et al., 2012] on the Mars Science Laboratory and the International Space Station provides excellent examples of instruments that characterize the effects of radiation from neutrons in addition to protons and heavy ions [e.g., Zeitlin et al., 2013].

[8] The neutron energy range of greatest concern for both avionics and crew health and safety starts around 10 MeV, but lower energy neutrons (<1 MeV) are also of concern. The typical spallation spectrum above 10 MeV is hard but falls off above 100 MeV, thus the danger from many MeV neutrons and their great numbers make them a serious problem. This energy range for avionics is a problem because these neutrons exceed the threshold for inducing reactions with silicon—the key constituent of microelectronics. These neutron reactions release alpha particles inside the electronics that produce enough ionization to upset the digital logic of circuits, which are known as soft errors. This source of microelectronic errors is only alleviated with shielding, increasingly extensive error correction firmware or hard resets.

[9] For crew health and safety, 10 MeV neutrons and above have a mean free path in biological tissue of the same order as human dimensions. Once they deposit their energy by collision in the tissue, the thermalized neutrons often capture hydrogen in the organic matter, in turn releasing a gamma ray deep inside the body that induces further damage. The properties of a neutron that make it so penetrating also make it difficult to detect and measure, especially those in the problematic range of 10 to 100 MeV. That is the main reason why monitoring this range is typically neglected, i.e., the instrumentation necessary for measuring the 10 to 100 MeV neutron intensity is relatively massive and complex—not ideal attributes for space-based equipment. The other irony is that for the case of deep-space missions, either manned or unmanned, that any attempt to shield crew or payload from cosmic rays produces increasing levels of neutron intensity.

[10] Measuring neutrons from 10 to 100 MeV with high efficiency requires considerable mass and volume due to the relatively small interaction cross sections in this energy range. For spaceborne instruments, mass is at a premium, so efficiency must usually be sacrificed. Furthermore, distinguishing energy depositions caused by neutrons (typically a fraction of the incident neutron energy) from those caused by gamma rays or charged particles requires additional information, typically obtained either by pulse shape discrimination (PSD) or by use of an active anticoincidence system (see section 2). Converting the partial energy depositions from neutrons into spectra of incident energies requires an additional unfolding step in the data analysis. The low efficiency of neutron detectors may be—depending on the details of the environment—mitigated by relatively high neutron fluxes. Exposure times required to obtain statistically significant neutron spectra may therefore be long compared to times required to obtain charged particle spectra. Hydrogenous scintillator is known to be a good material for neutron detection, chiefly because neutron interactions with the hydrogen nuclei cause measurable recoils. Because it is prohibitive to make a detector large enough to absorb the full energy of each neutron, the response of the instrument is broad, but still the task of measuring the spectrum and intensity in the featureless neutron spectrum is feasible [Matzke and Weise, 1985]. Such technology has been in use for decades, but adapting it to the smallest, most efficient, and lowest-mass envelope is challenging.

[11] In this paper, we provide a new concept for complete characterization of radiation biological effectiveness in a small and lightweight device. Such a device must be capable of measurement of LET spectra and neutrons. The DoSEN concept developed here combines these independent measurements and uses the coincidence of LET measurements and neutron detection to significantly reduce backgrounds in each measurement. The background suppression through use of coincidence allows for significant reductions in size, mass, and power needed to provide measurements of dose, neutron dose, dose equivalents, LET spectra, and organ doses. The use of coincidence techniques has a long history in space physics. Often, the use of such techniques results in transformational shifts in research. For example, the use of triple coincidence in spectrometry led to measurements of ion composition within plasmas [e.g., Gloeckler et al., 1992], and on the Interstellar Boundary Explorer Mission [McComas et al., 2004], triple coincidence techniques are used to pick out a very weak signal of neutral atoms from many competing backgrounds [e.g., Wurz et al., 2009]. Without such coincidence measurements, many of the in situ discoveries over the last two decades in space science would not have been possible. The CRaTER instrument itself combines a stack of six solid-state detectors (SSDs) with three sets of thin and thick SSDs separated by tissue equivalent plastic (TEP) [Spence et al., 2010]. Coincidence provides not only suppression of backgrounds but also separation between energetic particle sources from beyond the Moon and albedo sources from the Moon itself [Wilson et al., 2012]. Section 2 introduces the DoSEN instrument, section 3 provides results from a laboratory DoSEN prototype, and concluding remarks are provided in section 4.

2 DoSEN Instrument Concept

[12] DoSEN provides an innovation for LET and neutron coincidence to provide complete characterization of radiation biological effectiveness in a small and lightweight device. The DoSEN concept combines a CRaTER-like LET measurement via a stack of four SSDs with neutron measurements using an organic stilbene scintillator with pulse shape discrimination (PSD) coupled to silicon photomultipliers (SiPMs). The SiPM (also known as the solid-state photomultiplier or the multipixel photon counter) is a compact light sensor that provides performance similar to that of a photomultiplier tube, but with at least an order of magnitude less mass and volume. Because SiPMs are compact and low mass, they will eventually allow the SSDs to go on all six sides of the detector for full 3-D detection of sources.

[13] The SiPM was originally developed in Russia for high-energy physics applications [Buzhan et al., 2001; Buzhan et al., 2003; Balagura et al., 2006], and the technology is currently undergoing rapid advancement for use in nuclear medicine detectors [e.g., Schaart et al., 2009, 2010]. It consists of a two-dimensional array of small cells in a lattice with each cell spaced 50 µm on center with little dead space in between. Each cell acts as an independent avalanche photodiode. These cells are reversed biased slightly above their breakdown voltage so that they operate in “limited Geiger mode”: when a photon is absorbed, an avalanche is quickly generated which produces a large signal independent of the number of photons that were absorbed. A resistor in series with the cell quenches the avalanche after several tens of nanoseconds. The outputs of all the cells are summed together into an analog sum so that the intensity of the incident light is proportional to the number of cells that absorb photons.

[14] The advantages of the SiPM include high gain (~106) at low operating voltages (typically 20–70 V), compactness, insensitivity to magnetic fields, fast timing response, and the potential for low cost through mass production runs. SiPMs have by now been shown by many groups to perform well as readout devices for inorganic scintillators used for gamma ray measurement [e.g., Bloser et al., 2008, 2010a, 2012; Christian et al., 2010]. Relatively few groups [e.g., Stapels et al., 2011] have attempted to use SiPMs to measure neutrons using organic scintillators as we are doing here.

[15] In the DoSEN instrument concept (Figure 5), two stacks of SSDs provide CRaTER-like LET measurements. These Si detectors are mounted on the sides of the organic scintillator with PSD and SiPMs for neutron measurement. DoSEN measurements (Figure 6) combine complementary advanced techniques: (a) CRaTER-like LET measurements and (b) neutron measurements enabled by the combination of SiPMs, organic stilbene scintillator with pulse shape discrimination (PSD), and LET detection of recoil protons:

  1. [16] LET measurements are enabled by the stack of four SSDs with two sets of thin/thick SSDs separated by TEP (Figure 5). These LET measurements are composed of total dose (ΔE) recorded in each of the eight SSDs (D1–D8; odd = thin detector, even = thick detector). Varying column densities between the detector pairs (and all differential combinations) probe biologically important ionizing charged particles (TEP provides link to astronaut safety) and provide a range of doses from shallow (skin) to deep (beat frequency oscillator) total dose with real-time monitoring and active readout capability. The LET measurements enable the total dose rate (ΔE/dt) to be also recorded in each of the eight SSDs. Coincident LET spectra (ΔE/dx) and directionality are computed between coincident SSD detector pairs, which provide real-time capability for separation of ion species in LET (e.g., Figure 4). Ion-separated LET information allows for radiobiological dose equivalent calculation using biologically based weighting and quality factors (e.g., Figure 1). When the LET dosimetry is combined with fast coincidence logic (see DoSEN functional diagram, Figure 7), DoSEN is capable of forming rapid, active readout measurements of dose and dose rates, dose equivalent (DE) and DE rates, and organ dose rates associated with incident ion species and neutrons (via SiPMs). One of the unique and enabling features of the DoSEN design is the ability to determine directionality of ions using detector coincidence. For example, on the CRaTER instrument, this technique involving detector coincidence has been critical in distinguishing between incident cosmic rays measured largely in the zenith-facing D1–D2 detectors and the cosmic ray albedo of energetic particles measured in the nadir-facing D5–D6 detectors [Wilson et al., 2012]. This cosmic ray albedo is generated via nuclear interactions from the lunar regolith due to incident cosmic rays. Figure 8 exemplifies this feature based on CRaTER measurements. In this case, the left-facing detectors, D1–D2, register tracks of right-going protons and right-going alpha particles (labeled in Figure 8). It is important to recognize that recoil protons from the scintillator leave a unique LET track along either D4 or D8 SSDs (depending on their direction out of the scintillator). This enables segregation between primary ions that enter from outside the detector versus recoil protons and ions generated within the scintillator. Another unique benefit is the ability to determine directionality of sources from outside the detector. In principle, a design similar to the DoSEN prototype with SSD detectors on all sides of the organic scintillator provides full 3-D information concerning the sources of ionizing radiation. While larger than DoSEN, such a detector would still be quite small and there is no design limitation that would prevent mounting six sets of silicon detectors in all outward facing directions of the scintillator.

  2. [17] Neutron measurements (Figure 9) are enabled by the interaction of neutrons in the organic stilbene scintillator. Recoil protons from elastic neutron scatters excite atoms in the scintillator via ionization, which in turn produces optical scintillation photons that are detected by the SiPMs. Neutron interactions may be distinguished from gamma ray interactions (recoil electrons from Compton scatters) via PSD: In many organic scintillators, including stilbene, the decay time of the scintillation light depends on the energy loss (dE/dx) of the ionizing particle [e.g., Harihar et al., 1993]. The time required for the detector's electronic pulse to decay a given amount (e.g., from its peak to ~20% of peak) is one measure of this decay time and permits proton signals to be identified. In addition, recoil protons that exit the scintillator may be detected by the SSDs and identified via D4–D8 LET tracks (see Figure 8), permitting discrimination between recoil particles and primary sources of radiation from outside the detector.

Figure 5.

The DoSEN sensor configuration includes a combination of solid-state detectors (SSDs), organic scintillator with PSD, and Si photomultipliers (SiPMs) allowing coincident detection of energetic particle LET and neutrons. The unique coincidence offered by LET and neutron detection promises a significant advance for a new generation of dosimetry measurements.

Figure 6.

LET measurements by DoSEN provide fast, active readout of dose, dose rate, dose equivalent rate, and organ dose rate from galactic cosmic rays, solar energetic particles, and secondary charged particles with sufficient energy to penetrate the housing (>10 MeV p+).

Figure 7.

The DoSEN functional design allows fast coincidence logic applied for the SSD sensor stack and SiPMs. This unique combination allows for rapid active readout of dose, dose rate, dose equivalent rates, organ dose rates, and secondary neutron dose rates.

Figure 8.

DoSEN provides critical directional information using LET coincidence. For example, right-going protons are measured first in D2 where they deposit most of their energy (lower D4 LET). Left-going protons from the scintillator show a track along D4 with low D2 LET.

Figure 9.

DoSEN provides neutron discrimination via SiPM measurements of secondary photon distributions from an organic scintillator with PSD. Recoil protons from the scintillator are detected by the SSDs and distinguished from primary protons via directional LET tracking (Figure 8) and from gamma ray interactions via PSD. The coincidence between SiPMs and SSD measurements provides significant background suppression.

3 Lab Prototype Results

[18] The DoSEN lab prototype has been designed (Figure 10) and implemented (Figure 11) to demonstrate the basic measurement principles. The prototype consists of four Si SSDs, TEP, a 1 inch cylindrical stilbene crystal wrapped in white Teflon tape, and a 2 × 2 array of multipixel photon counter SiPMs.

Figure 10.

The final lab prototype design for the DoSEN detector.

Figure 11.

Photograph of the final lab prototype for DoSEN.

[19] The four SiPMs were mounted on a custom array board to form a total light collection area of 26 mm × 26 mm (Figure 12). Due to the large capacitance of the SiPM array, a very low input impedance readout is critical for achieving a fast time response and avoiding nonlinearities in the output pulse height [Huizenga et al., 2012].

Figure 12.

Photograph of our SiPMs mounted on a custom array board to form a total light collection area of 26 mm × 26 mm.

[20] The 1 inch right cylinder stilbene crystal was wrapped in white reflective Teflon tape and coupled to the SiPM array using optical grease (right side of Figure 11). A custom low-noise power supply provides a bias voltage that automatically scales to compensate for temperature-induced gain variations, as measured by a thermistor bead mounted on the array board.

[21] The first step in validating the DoSEN measurement concept was to demonstrate experimentally that pulse shape discrimination (PSD) could successfully distinguish between γ ray and neutron interactions in the stilbene using a SiPM readout. This was accomplished by exposing the stilbene to a 252Cf fission source and feeding the amplified SiPM output pulses into a pulse shape analyzer (PSA) module. The PSA module outputs timing signals that represent the time required for the pulse to decay from 80% to 20% of its peak; this time (on the order of 100 ns) is converted to a voltage by a time-to-amplitude converter and digitized, and we refer to it as the “PSD parameter.” Figure 13 shows the PSD parameter versus pulse height recorded for the 252Cf source (P. F. Bloser et al., Scintillator gamma-ray detectors with silicon photomultiplier readouts for high-energy astronomy, submitted to Proceedings of SPIE, 2013). The upper band is produced by fast neutron interactions and the low band by gamma rays. The bands are clearly separated. These results demonstrate the advances necessary to integrate the scintillator into DoSEN for measurement and to differentiate between neutrons and γ rays.

Figure 13.

Peak-to-tail pulse decay time (PSD parameter) versus pulse height from a SiPM array coupled to a stilbene crystal exposed to fast neutrons (upper track) and gamma rays (lower track) from a 252Cf fission source. The two tracks are well separated, permitting identification of neutron interactions.

[22] The pulse height response of the stilbene/SiPM detector was calibrated using radioactive gamma ray sources. Because organic scintillators are inefficient at absorbing gamma rays, Compton edges were recorded for 137Cs, 22Na, and 60Co. Shown in Figure 14 are the Compton edges shown in counts versus analog-to-digital units (ADU) of the stilbene/SiPM. These data were analyzed with the assistance of Monte Carlo computer simulations, which allowed the exact electron equivalent energy deposit (~keVee; electron equivalent) associated with the half maximum of the Compton edge to be identified (this technique was previously applied successfully by Bloser et al. [2010b]). The resulting calibration is shown in Figure 15. The dynamic range for the prototype is 55–3300 keVee, corresponding approximately to 500–8200 keVpe (proton equivalent, as required for neutron scatter measurements). The correspondence between proton (Ep in MeV) and electron equivalent (L in MeV) light output is [Hansen and Richter, 2002, equation 1]

display math(1)

where a = 0.693, b = 3.0, c = 0.2, and d = 0.965.

Figure 14.

Gamma ray spectra recorded in the stilbene detector from 137Cs, 22Na, and 60Co. These data were used to convert ADU to energy (keVee) for the stilbene detector.

Figure 15.

Calibration of the D5 detector using the Compton edge fit results from 137Cs, 22Na, and 60Co.

[23] Calibration of the Si detectors was performed using a several radioactive isotopic sources along with pulser sweeps. For the thick detectors (D2, D4, with thickness of 1000 µm), a 137Cs source with peaks with β peaks at 624 keV and 656 keV was used to calibrate the detectors at relatively low energy. Pulser sweeps were then used to extend the responses to higher energies. In addition, measurement of a 241Am α source with a line at ~5.45 MeV was used for each detector to provide a separate calibration point (“X” in Figure 16). In the case of the thin detectors (150 µm), 241Am α particles provided low-energy calibration and, as with the thick detectors, the response is extended to high energies using pulser sweeps. The conversion between analog digital units (ADU) and energy is

display math(2)

where G is the gain (units of keV/ADU) and Z (units of keV) is the zero offset. Figure 16 shows the response curves used for these calibration runs.

Figure 16.

Calibration runs for each of the detectors using radioisotopes 241Am for thin detectors (D1 and D3) and 137Cs for thick detectors (D2 and D4) followed by extensions to higher energy using pulser sweeps. The “X” on the D2 and D4 curves shows the measurement of the 241Am emission line (5.45 MeV) in the thick detectors.

[24] Finally, coincidence data were taken between the stilbene/SiPM detector and a thick SSD (D4) using coincident gamma rays from 22Na and 60Co sources. In this case, we need to demonstrate that the DoSEN detector operates as a system with the needed background-suppressing coincidence measurements. The 22Na source produces simultaneous 511 keV gamma rays at 180° orientation. The source is placed in the telescope axis between the D4 and D5 detectors and data are collected. The collected coincident data show the ability of DoSEN to record events on a timescale with a coincidence window of 100 ns. In order for the 511 keV Compton data to be above the threshold in the D4 detector, we raised the D4 gain. This constitutes a separate gamma detection mode in the D4′ detector (column 5) in Table 1. The gamma detection mode was calibrated using the 133Ba gamma line at 356 keV line and the 137Cs six β peaks at 624 keV. Figure 17 shows D4′-D5 coincidence demonstrating that the detector has the required capability of coincidence detection.

Table 1. Gains and Zero Offsets Deduced From Calibration Using Radioactive Isotopesa
  1. aProton equivalent (pe) to electron equivalent (ee) conversion using equation ((1)) [Hansen and Richter, 2002].
G (keV/ADU)133.524.618926.60.830.83 (ee)
Z (keV)−8,010−1,180−10,200−1,310−5553 (ee)
Range (MeV)1–5000.3–921–7000.3–1000.1–30.09–3.2 (ee)
0.5–8.2 (pe)
Figure 17.

This example shows a crossplot (D4-D5) with γ mode for D4 (higher gain). The almost rectangular shape in the lower left is the coincident 511 keV gamma rays from 22Na hitting both D4 and D5 (Stilbene), demonstrating good coincidence between the two detectors. The source was placed inline of the telescope axis between D4 and D5 with a coincidence rate of 8.5 Hz. When we move the source off axis, the coincidence rate drops precipitously. There is also a faint vertical line around D4 ADU = 700 that shows the full energy deposit (511 keV) in the D4 Si detector.

4 Summary

[25] DoSEN lays the foundation for a new generation of dosimeters. The use of LET and SiPM coincidence suppresses background while allowing smaller, active readout detectors that provide doses, dose equivalents, and organ doses from neutrons, electrons, protons, and heavy ions. LET coincidence techniques represent a novel method for determining quality factors directly from LET measurements, thereby constituting a new technique for determining dose equivalents and organ doses. A coincident LET measurement directly connects energy measurements and the full spectrum of biological effects. The sensor design provides detailed directional information. A full instrument prototype would have SSDs on all sides of the organic material (outside SiPMs) and yield detailed 3-D information about the direction of radiation sources.

[26] The use of detectors with simultaneous neutron and LET measurements provides a promising approach for obtaining broad information concerning radiation hazards and the generation of secondary radiation. The sensor would be useful for measuring radiation in free space, behind spacecraft shielding, on planetary surfaces, etc. The fact that the detector is small, relatively low mass, and low power and utilizes fast coincidence techniques makes it a sensor that can be used as an active readout device carried by astronauts and may be used in a wide variety of NASA agency-wide scientific missions. DoSEN's fundamental advances in low-power coincidence radiation detection techniques provide the needed capability for quantifying biological damage absent from current dosimetry, thus laying the foundation for a new generation of dosimetry detectors.


[27] This work was supported by the DoSEN project, NASA grant NNX13AC89G.