RAD-TDM: Instrument Capability Development in the Vision for Space Exploration: Radiation Assessment Detector (RAD) Calibration Campaigns, 15-9531

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Principal Investigator
Arik Posner (Don Hassler)

Inclusive Dates:  02/01/05 – 08/02/05

Background - To date, SwRI has not fabricated an instrument that has been calibrated and tested with particles at energies equivalent to galactic cosmic rays or solar energetic particles. With the selection of the Radiation Assessment Detector (RAD) for the Mars Science Laboratory (MSL) rover mission, NASA offered SwRI scientists the unique opportunity to test a laboratory model at the NASA Space Radiation Laboratory (NSRL), a facility that provides access to calibrations with high-energy heavy ions abundant in space. The calibration results we have obtained with galactic cosmic ray energy ions will have multiple benefits for the Institute including: Expertise in designing, fabricating, testing, and operating energetic particle instrumentation that is critical for the Space Exploration Initiative; Feedback of instrument performance will improve design of instrument hardware, layout, and electronics that we plan to propose for future missions. Most direct benefits from these calibration results flow toward the MSL/RAD project.

Approach - The overlying philosophy for the RAD Technology Demonstration Model (TDM) was to design a calibration instrument as close as possible to the MSL/RAD baseline. Matching detector geometries guarantee that calibration results obtained in this project can be applied directly to the MSL/RAD program. For this purpose, collaborative RAD detector design efforts between the University of Kiel and SwRI RAD teams and direct hardware contributions from our Kiel partners were incorporated in the RAD-TDM. Our calibration results cover the responses of all RAD main sensor components, photo-diodes (PDs), and solid-state detectors (SSDs), in combination with their pre-amplifiers to high-energy particles. The signals generated by the relativistic heavy ion beams are up to 30,000 times stronger than can be provided by laboratory radioactive sources.

Accomplishments - Comparison of derived energy losses in our detectors with the actual measurements confirms that over most part of the spectrum the SSD and photo-diode detector response is approximately linear. Only beyond kinetic energy deposits of 15 GeV, we find that signals generated deviate significantly from a linear scale, therefore slightly reducing energy resolution but also reducing required dynamic range by approximately 15 percent. We have shown that the faintest (800 keV) and strongest (20 GeV) signals can be measured with the CsI calorimeter read out with photo-diodes. With a design goal for the dynamic range of 20,000, the minimum dynamic range already achieved with three photo-diode gain stages is, according to our results, a factor of 5,800, i.e., a factor of 17.9 per gain stage. We found that the energy resolution of the sensor components for ions at these energy deposits is sufficient for identifying groups of main elements of galactic cosmic rays on the surface of Mars, as proposed. The energy resolution values we achieved with the RAD CsI scintillator read-out with photo-diodes is dE/E = 4.8 percent at 12 GeV. This result is consistent with results, achieved in an earlier IR&D project (15. 9469), in which we found an energy resolution of 11 percent at approximately 1 MeV (energy losses differ by a factor of 12,000) with a slightly larger CsI scintillator. The solid-state detector energy resolution was found to be dE/E = 6.1 percent at 52 MeV. The combined calibration results are sufficient to generate an updated energy loss matrix for MSL/RAD. We used the relativistic particle beams provided to us in our first calibration campaign for a total of 23 tests. These tests included probing the dynamic range of the detector system, a critical aspect that we claimed in the MSL/RAD proposal. The tests also include angle and shielding tests to modify the energy deposits in our detectors. Specifically, the shielding tests provided us with extra information on the elemental resolution of the instrument. The interaction of the beam particles with shielding generated secondary ions from spallation, i.e., breaking up of the atomic nucleus. Hence, ending with a mixed beam, we were able to detect and identify number of elements with which we established an elemental charge (Z) scale. From these data, we deduced that the elemental resolution of our photo-diode/scintillator system is sufficient, i.e., Z/dZ, is, at least up to iron, always larger than Z. This means that in principle, we will be able to detect all light elements up to Fe as proposed for MSL/RAD.

Figure 1. Don Hassler (right), the MSL/RAD principal investigator, and Arik Posner, leader of this calibration campaign, check connectors and positioning of the RAD-TDM instrument system in the target room before beam activation. The trajectory of high-energy ions is along the guardrails, originating from the rear left. A laser pointing system helps with the accurate placement of equipment in the beam line.

Figure 2. In measurements with a CsI scintillator read out with photo-diodes, the RAD-TDM detected the relative energy losses of spallation products of a 1 GeV/n iron beam from the interaction with tungsten. In this graph, we displayed Fe from the beam and all spallation-produced elements over their respective elemental charge number. This Brookhaven calibration result demonstrates a key capability for RAD: resolving cosmic-ray energy ions by element with a RAD technology demonstration model.

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