1.1 The Multifaceted Challenge for a New Generation of Dosimetry Detectors
 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).
 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.
 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.  find in CRaTER that the dose rate in water is 33% larger than the dose rate in silicon.
 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.
 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.
 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].
 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.
 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.
 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.
 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.