# Presentation: 078074

## A Charged Particle Spectrometer for Crewed Space Missions 

**D. H. Kaplan****1 ****and J. H. Adams, Jr.****2 **
*1**Department of Physics, Southern Illinois University Edwardsville, Edwardsville, Illinois 62026*
- 2 *NASA Marshall Space Flight Center, Huntsville, Alabama 35805 *

**Abstract:**
-    It is necessary to measure the radiation environment within crewed space systems to check the accuracy of computer models used to determine the radiation exposure of the crew. All radiologically significant components of the radiation field need to be measured.These include neutrons, protons and the nuclei of the nuclei of all the abundant elements from H to Fe over the energy range where their fluxes are high enough to be radiologically significant.The instrumentation to make these measurements includes various types of dosimeters, a Tissue Equivalent Proportional Counter (TEPC), a directional charged particle spectrometer for both internal and external use and an external electron detector. This poster will present a concept for a directional charged particle spectrometer to monitor the nuclei of the elements C though Fe from 20 MeV to 10 GeV within the crew quarters and externally.
**Introduction:**
-     While computer codes aimed at predicting the radiation environment inside spacecraft on deep space missions exist, their predictions need to be validated or corrected. It is therefore important that all radiologically significant components of the radiation field be measured in crew quarters on at least one deep space mission. Subsequently, radiation exposures can be determined by using calculations normalized to dosimetry measurements.
-     Among the significant components will be neutrons, protons, and nuclei of elements up to and including Fe. Due to fragmentation reactions in the spacecraft walls, the abundances of different elements will be enhanced or diminished relative to the progenitor galactic cosmic ray (GCR) background. The enhancement will vary with location within the spacecraft according to the wall thickness distribution and other factors. Necessary corrections to computer code predictions will most likely be due to errors in knowledge of shielding thickness distribution and/or in relevant cross sections used in the modeling. The effects of both of these types of error will be most pronounced at relatively low energies (roughly 100 MeV per nucleon (MeV/n) to 500 MeV/n.) For this reason, measurement in this low energy range is particularly important.
-    While current and planned future instrumentation includes various passive dosimeters and a Tissue Equivalent Proportional Counter, it is necessary to separately measure and monitor the abundance and energy of each elemental component at various locations inside the craft over a range from about 100 MeV per nucleon (MeV/n) to about 10 GeV per nucleon (GeV/n). Existing instruments that have flown on the International Space Station (ISS) and/or on the Space Shuttle (SS) are inadequate to this task, due to intrinsic limitations of capability, insufficient geometrical acceptance, or both. For these and other reasons, a recent NASA report has criticized existing internal active area dosimetry, calling for substantial updating.
-    A suitable, active charged particle spectrometer should should be capable, with daily measurements, of providing statistical accuracy of about 20% on the abundance of each element over the 100MeV/n – 10GeV/n energy range, with binning emphasizing the 100 – 500 MeV/n region. [The daily sampling frequency of updating is minimum frequency for a lunar mission] For deployment within a spacecraft, the instrument volume should be limited to about 0.05 m3, it should be operable on standard spacecraft voltages, should draw no more than 15-20 W power, and should weigh no more than about 10 kg.  Due to the wide range of nuclear charges that must be identified and measured, it is necessary to deploy two instruments. One of these will measure H and He, and a second, more challenging to design, is to identify and measure nuclei the range . This paper is concerned with the design of this second instrument.
** ****Adaptation to Specific Requirements: **
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**    **The broad energy range to be covered suggests a vertically segmented detector with stacked, separated threshold cherenkov counters. The minimum necessary sizes and the optimal ordering of the components within the device are dependent on the rates to be encountered. For this purpose, a computer code, CRÈME96, was run to estimate flux rates through an aluminum spacecraft wall of thickness 10g/cm2, assuming galactic cosmic ray background under Solar minimum (cosmic ray maximum) conditions. It was found that about 2/3 of the flux behind the shielding is above 500 MeV/n in energy. Further, as transmitted particles with energies above about 800 MeV/n are much more susceptible to fragmentation than to stopping in tissue, and because the fragmentation cross sections are less dependent on energy, finer energy binning is necessary in the range below 500 MeV/n than above it. Thus, the identities and energies of the lower energy particles should be determined in the upper layers of a multi-layer detector, as this maximizes the geometrical acceptance factors for such particles.
-    Within this design paradigm, the primary *dE/dx* signal can be provided by planes of relatively coarsely pixellated instrumented Silicon. However, in order to minimize power consumption and allow for dual use as a trigger, instead of Silicon microstrip or mm-scale pixellated Silicon, a scintillating fiber hodoscope with all fibers read out at both ends by multianode PMT and ASIC IC signal processing is proposed. With this scheme, a fast dynode signal can then be used to replace the traditional trigger scintillator layer, thus making a thinner front end. (In addition to its higher power draw, due to its relatively slow readout time, a Silicon-based plane is inappropriate for a trigger.)

** **For measurement of energies above 2.5 GeV/n, an aerogel-silica  (*n* = 1.04) cherenkov radiator is proposed. For the range 300 – 2500 MeV/n, an acrylic plastic (*n* = 1.49) cherenkov radiator can be used. Simple calculations show that, for energies in the range 100 MeV/n – 300 MeV/n, the rangeout technique is not appropriate, because of the additional depth, weight and grammage that would be required in the upper detector layers. Therefore, we propose the use of a layer of CVD-diamond (*n*= 2.4) or of Moissanite (*n* = 2.64) as a separate cherenkov radiator for this purpose. Due to the high index of refraction, such a layer can be quite thin (see below) or even in the form of a solution or paste of small crystals in solid or liquid solution. In this report, we assume a 1mm thick solid layer (15 cm x 15 cm x 0.1 cm).
- In order to homogenize the cherenkov signals against variations in yield with lateral position of the track and such factors as chance proximity to a readout phototube, in this design each radiator will deposit photons into a separate light collection box positioned underneath, the internal walls of which are coated with diffuse reflector, on which cherenkov photons will undergo repetitive bounces. For the lowest energy segment, the 1 mm CVD Diamond or Moissanite layer would be adhered directly to the upper inside surface of the light box.  While the ideal box shape would be cubical, the success of the TIGER mission has demonstrated that an aspect ratio (depth/length or width) of 20% provides an adequate photoelectron yield for ~15%  photocathode efficiency. Thus, for 15 cm x 15 cm planar area, the depth of each light box need only be 3 cm. Light boxes can thus be read out with 1” diameter photomultiplier tubes placed side-by side around the box end faces. The PMT’s are to be followed by ASIC IC’s including discrimination and pulse height analysis.  The expected photoelectron yields of a 1mm thick layer of CVD diamond for incident Carbon and Oxygen nuclei are shown; these  assume a normalization factor similar to that found for cherenkov detectors on the TIGER mission with similar light box aspect ratio.
** **In line with the principle of early low-energy identification, the Diamond or Moissanite detector should be placed as close to the top of the detector stack as possible, above the acrylic and aerogel detectors. The thinness of the radiator layer and light box (< 4cm) is consistent with this. The overall fractional energy resolution of the 3-layer composite cherenkov detector for *Z*=6 (the worst case) is better than adequate for the bin widths of the design.
-     For assessing rate capability requirements for a dosimeter, Iron provides a useful benchmark due both to its high quality factor and the fact that Fe rates behind shielding are somewhat lower than in the background GCRs. Using, for example, the CREME96 result of 0.15 Fe per cm2-sr-day for the range 100 – 200 MeV/n behind 10 g/cm2 of Al shielding, requires a geometrical factor (*S*) of at least 167 cm2-sr to achieve 25 Fe (i.e., ) per day in this range. For a detector surface area of 15 cm x 15 cm (a reasonable transverse area choice for spacecraft deployment), for this *S*, a detector segment thickness of less than 13 cm is required, the less, the better. In our design conception the thickness of the portion of the device measuring energies and charge in the 100-300 Mev/n range is about 6 cm, which implies S 340 cm2 – sr or about 50 Fe/day in this range. The flux of Fe for all other 100 MeV/n wide bins at higher mean energy than this is greater.
**    **The success of the NASA TIGER mission has demonstrated that a ratio of hodoscope pitch to plane separation of 1-2% is adequate for the required 0.25 cu resolution. Using a hodoscope plane separation of 6 cm for the low energy portion of the detector and 1mm square cross section scintillating fiber elements (1 mm pitch) would then provide an adequate pitch to separation ratio of 1.7%. Assuming that the fibers are read out at both ends, the number of readout channels per hodoscope plane is then 300. For redundancy of position measurement, the proposed device incorporates 6 such hodoscope planes instead of four; thus the total number of scintillating fiber channels is 1800. The fibers would be read out with Hamamatsu 64-anode compact multianode photomultipliers (MAPMT’s); 6 per plane are then required, bringing the total to 36 MAPMT’s.
-  Conclusions
-    We have presented a concept for an active on board heavy element monitor capable of identifying and measuring the energies of charged nuclei from Carbon through Iron in the range 100 MeV/n – 10 GeV/n within a crewed spacecraft. The device is compact, light weight and draws less than 15 W.
-    Further work needs to be done in verifying and completing the design, including Monte Carlo simulations and especially beam tests of the proposed Cerenkov system.
- Also, the design presented above does not address the need for monitoring of nuclei with Z<6. For these nuclei, a special attachment needs to be designed  as the photon yields will be low in the relatively thin Cherenkov radiators mentioned above. Due to the higher rates expected at low *Z*, however, such an attachment, with thicker Cherenkov radiators, can have a smaller transverse surface area, and so the total volume would still be manageable.

-  CRÈME 96 results for flux of elements 14 (Si) – 26 (Fe) behind 10
-  g/cm2 of Al shielding.

- (Left): Schematic of a scintillating fiber. Fiber has additives which emit photons upon traversal by nuclei. Photons are transmitted through fiber via repeated total internal reflection. Fibers are fabricated at Washington University.
-  (Right):  A prototype plane of parallel running optical scintillating fibers constructed in the Washington University Laboratory for Experimental Astrophysics (J.T. Link et al., (2003), 28th International Cosmic Ray Conference **4**, 1781).

- Expected Cherenkov photoelectron yields for 1mm thick CVD Diamond layer with incident (a) Carbon and (b) Oxygen nuclei.