Radiation Assessment Detector (RAD)

Instrument Overview

The Radiation Assessment Detector (RAD) investigation is an investigation to detect and analyze the most biologically-significant energetic particle radiation on the Martian surface as a key element of the Mars Science Laboratory (MSL) mission. Fully characterizing and understanding the radiation environment on Mars is fundamental to quantitatively assessing the habitability of the planet, and is essential for future crewed Mars missions. RAD also addresses significant aspects of the MSL investigation, including the radiation effects on biological potential and past habitability, as well as keys to understanding the chemical alteration of the regolith due to impinging space radiation.

Scientific Objectives

Characterize the Energetic Particle Spectrum on Mars

Galactic cosmic rays (GCRs) are high energy (100 MeV/nuc to 10 GeV/nuc and above) particles thought to be produced by supernovae shocks outside the heliosphere and are composed of roughly 89% protons, 10% alpha particles (He), and 1% heavier nuclei (Reedy and Howe, 1999). GCRs are modulated by the solar wind and the 11-year solar cycle, with roughly 30% higher flux at solar minimum, and show variability with respect to elemental composition and energy. Specifically, near solar minimum, substantially higher fluxes of lower-energy particles can access the inner heliosphere compared to times near solar maximum. Because of their high energies and continuous nature, GCRs are the dominant source of background radiation at the Martian surface, and are responsible for the production of secondary particles via complex interactions in the atmosphere and regolith. The radiation dose from these secondary particles is comparable to that from the primary GCR. The Earth's magnetic field (magnetosphere) and deep atmosphere (~ 1000 gcm-2) effectively shield us from most of the hostile interplanetary radiation environment. No primary cosmic rays reach the surface of the Earth. However, this is not the case for Mars. Mars has no significant magnetosphere and only ~1-2% of the atmospheric mass of Earth. In the thick terrestrial atmosphere, most of the GCR energy deposition, and secondary particle production, occurs in the top 20 km of the atmosphere, while in the thin Martian atmosphere this occurs at ground level.

The secondary particles generated within the atmosphere include neutrons and gamma rays that, due to their lack of electric charge, penetrate the remaining column of the Martian atmosphere rather freely. The gamma rays do not contribute significantly to the radiation dose at the surface, but the neutrons do. Also, some secondary neutrons generated within the regolith backscatter to the surface, where they contribute to the dose. GCR heavy ions also collide with carbon and oxygen in the atmosphere and regolith to produce a flux of energetic charged nuclear fragments at the surface. Some GCR heavy ions, such as C, N, and O, are relatively abundant and have a significant probability to survive traversal of the atmosphere intact. Despite their relatively limited range in matter, these particles have high quality factors (a measure of biological effectiveness) and therefore need to be considered in radiation risk assessments.

Solar energetic particles (SEPs) are produced by the Sun as a result of shocks from coronal mass ejections (CMEs) associated with large solar storms and flares; they are dominantly protons. Although most SEPs have energies lower than 100 MeV/nuc, the flux of SEPs is highly variable and can vary by more than 5 orders of magnitude on time scales of hours to days (Posner and Kunow, 2003), reaching energies as high as several GeV. About 140 MeV/nuc of kinetic energy is needed for protons and helium ions to penetrate the average column depth of the Martian atmosphere. Typical SEP events produce a flux composed of 98% protons, 1% alpha particles and 1% heavier nuclei (Reedy et al., 2001). Because most SEPs have energies below 100 MeV, much of their flux does not reach the Martian surface, although even non-penetrating SEPs can create secondary neutrons that do reach the surface. Also, some events produce a significant SEP flux at energies above 100 MeV/nuc. Thus, although sporadic, SEP events may overwhelm the background GCR radiation at the Martian surface.

Determine the Radiation Dose Rate for Humans on Mars

Presently, no radiation exposure limits are established for surface Mars missions, but limits for low Earth orbit (LEO) provide a reasonable baseline from which to compare astronaut safety and risk. The LEO limits are classified into short (30-day), annual, and career durations, and are also a function of the exposed organs. Astronauts conducting Martian surface operations would be exposed to continuous GCR radiation, and potentially large bursts of SEP radiation. Although the GCR flux is less at solar maximum, the probability of large SEP events is greater, and the combined dose equivalent can easily approach annual exposure limits for blood forming organs, particularly at high elevations where the atmospheric column above is minimal. Thus, it is critical to quantify through direct measurement the total radiation environment, including the baseline GCR flux and the secondaries it produces, as well as the range of episodic SEP radiation at the surface of Mars well in advance of any future manned missions in order to properly assess the safety risks and to develop potential mitigation strategies. RAD will provide the precursor measurements necessary to fully characterize GCR and SEP radiation, assess potential risks, and enable mitigation strategies to be adequately designed in preparation for future manned Mars missions.

Transport of HZE Particles through the Martian Atmosphere

The lack of direct observations necessitates the use of radiation transport models of the Mars atmosphere (e.g., HZETRN, SIREST). However, radiation transport models that provide input to dosimetry models are static, driven by radiation inputs at the top of the atmosphere and MOLA topographic data only. Unfortunately, the true nature of the surface radiation environment is still highly uncertain. The model outputs need to be tested (Wilson et al., 1999), and compared to observational ground truth in order to be validated and considered complete to the point of ensuring astronaut safety on future manned missions. Measurements from RAD will be compared to output from existing models of these interactions. Disagreement between observations and model results elucidate weaknesses in the model physics, or the understanding of the modeled interactions, and will be used as feedback for improvement.

Characterize the Radiation Hazard for Extant Life on Mars

The radiation hazards for indigenous Martian life forms are unknown, but most current studies assume that life elsewhere will be based on polymeric organic molecules (Pace, 2001), and will in an overall sense, share with terrestrial life the vulnerability to energetic radiation. Thus the risks to extant organisms are assumed to be analogous to the risks to future human explorers. Energetic charged particles ionize molecules along their tracks. This ionization creates OH and other damaging free radicals which can in turn break DNA strands within cells. Double strand breaks are most significant, as they may be mis-repaired, leading to mutagenesis. Surviving cells may become cancerous. While Martian life may not be based on DNA, most astrobiologists assume that it will require some system of heredity based on large polymeric organic molecules. Thus it will likely have similar vulnerability to energetic radiation.

RAD will quantify the flux of biologically hazardous radiation at the surface of Mars today, and measure how these fluxes vary on diurnal, seasonal, solar cycle and episodic (flare, storm) timescales. Through such measurements, we can learn how deep life would have to be today for natural shielding to be sufficient. This depth can be compared to the calculated diffusion depth of strong oxidants which will destroy organic molecules in the near surface environment of Mars today (Bullock et al., 1994), and thus learn whether radiation or oxidizing chemistry will determine the minimum depth needed to drill to look for extant life on Mars today.

Much attention has been given to the possibility of life in subsurface voids (caves) that will be protected from the surface radiation environment (Boston et al., 1992). It has been noted that the cave environments likely to exist on Mars could possibly facilitate the evolution of macroscopic life in the subsurface, as opposed to merely microbial life (Boston et al., 2001). The shielding required to make such an environment suitable for life will depend on the surface radiation. Measurements of the surface radiation will allow us to determine how deeply buried such voids must be to be safe from the high-energy radiation environment at the surface. This, in turn, will directly impact future strategies involving drilling and digging to search for subsurface life.

While the idea that life exists today on Mars is controversial, the idea of life on Mars in the past is much less so. The recent discoveries by the Mars Exploration Rovers (MER) and Mars Express of evidence for abundant surface liquid water in the past reinforce the widespread view that Mars, in the past, may have been a habitable planet. In seeking to understand the limits of surface habitability in the past on Mars, it is important to be able to characterize the radiation environment during past epochs when surface water existed, the climate was more moderate, and presumably the atmosphere was substantially thicker than at present. Radiation is an important source of biological mutations, and as such may have been the dominant source of genetic diversity in the past on Earth and presumably on any planet (perhaps including Mars) where life is based on a genetic code (which is part of most definitions of life). How would the thicker past atmosphere, required for a warm, wet early Mars, modify the radiation environment? According to accepted models of atmospheric evolution, how have the dose rates of radiation capable of doing tissue damage, and radiation-induced mutation, varied throughout Martian history? How would extreme radiation events (solar flares, gamma ray bursts) have affected evolution of past organic life on Mars? For any effort to understand this past radiation environment of Mars, the starting point must be a more thorough understanding of the role that the current atmosphere plays in modulating and altering the radiation from space. Understanding how radiation interacts with the contemporary atmosphere permits the extrapolation of this interaction with the ancient, thicker atmospheres.

Chemical and Isotopic Effects of Radiation on the Martian Surface and Atmosphere

Space 'weathering' is a well-known but fairly poorly understood phenomenon that alters the chemistry and appearance of the surfaces of airless bodies (Hapke 2001, Chapman 2004). It usually consists of two components, that due to micrometeorite bombardment, and that caused by the impingement of charged particles on the surface of asteroids and airless satellites. An enormous fluence of high-energy charged primary and secondary particles has interacted with the Martian regolith throughout its history. The annual dose rate at the surface, and in the first several tens of cm of regolith, is expected to be on the order of 0.1 Gy/year. Extremely large doses accumulate over the eons. There is thus reason to believe that radiation contributes significantly to the unique chemistry of the Martian surface. The unique space weathering on Mars can only be understood and quantified with direct observations of energetic particles at the surface.

One of the primary objectives of the MSL mission is to emplace mobile analytical chemistry instruments at the surface of Mars, including those that can quantify light elements. A detailed analysis of the makeup of both bulk rocks and their surfaces will pave the way for a far greater understanding of the weathering and alteration processes active on Mars. RAD will supply the basic input to chemistry models that up to now has been lacking - the space radiation environment of the surface of Mars. Together with the analytic chemistry experiments on MSL, RAD will provide real constraints on how primary rocks weather to their current, highly altered state.

Calibration

Because RAD has limited storage and telemetry, much of the data is stored in the form of histograms. These depend on real-time calibration. The parameters that control this calibration are loaded into RAD via a configuration table. In addition, the RAD electronics, and some of the detectors, are temperature sensitive. Prior to a RAD observation, the temperature of the detectors is taken and recorded. The initial temperature is used to identify an additional onboard table of correction parameters. Therefore, observational data is received on the ground having gone through onboard calibration.

Operational Considerations

The RAD experiment requires long time integration to capture the statistics of GCR radiation and frequent observations to capture the random and nearly unpredictable SEP events. A minimum of 15 minutes of observation per hour every hour is sufficient to achieve RAD science objectives.

Detectors

RAD's particle detection capabilities are achieved with a solid-state detector (SSD) stack (A, B, C), a CsI(Tl) scintillator (D), and a plastic scintillator (E) for neutron detection, as shown below. The D and E detectors are surrounded by an anticoincidence shield (F), also made of plastic scintillator. All scintillators are optically coupled to silicon diodes which convert scintillation light to electrons.

                    
       |------------| <--Si A Detector
       |            | 
       |            | 
       |            | 
       |            | 
       |------------| <--Si B Detector
       |------------| <--Si C Detector
     //  / /     \ \ \\
    //  / /	      \ \ \\ 
   //  / /    D    \ \ \\
  //  / /    CsI    \ \ \\
 //  / /             \ \ \\ <-- anti-coincidence wrapper F1
||  --------------------  ||
||    ----------------    || 
||   |   Bicron 432   |   ||
||   |   E Detector   |   ||
||    ----------------    || 
|----------------------------|
|----------------------------|<-- anti-coincidence wrapper F2
      //  /|-----|\ \\
       |------------|  

The D calorimeter stops protons with energies up to about 95 MeV and Fe ions up to 270 MeV/n. At high energies, charged particles produce ionization electrons in the detector material and undergo no significant change in direction. Thus, the path of energetic ions through RAD is, to a good approximation, along a straight line. The opening angle of 65 degrees, defined by trajectories that hit both A and B (i.e., the A*B coincidence), provides for 1 cm^2 sr geometric factor of the instrument. This gives a fairly narrow distribution of possible path lengths through the detectors. The shape of the CsI scintillator conforms to the field of view as defined by the A*B coincidence. Identification of ion species is achieved with the dE/dx vs. E method (McDonald and Ludwig, 1964).

The high-energy threshold for ions is determined by the trigger settings for detectors A and B. Particles that penetrate the chain of detectors ABCD (and possibly E) and leave the system through F2 are accepted and counted in the 'penetrating particle' histograms. It has been determined that the A*B trigger can and will be operated at a low threshold so that RAD efficiently detects minimum-ionizing, singly-charged particles (e.g., GCR protons). Limited particle identification at energies beyond ~100 MeV/n is possible for these events. The penetrating particle histograms extend the energy range of the instrument to accommodate up to ~500 MeV/n ion spectra with elemental resolution based on multiple dE/dx analysis.

The A detector has outer and inner segments, referred to as A1 and A2 respectively. During high fluence events such as SEPs, the outer segment (A1) can be disabled so as not to overwhelm the electronics with a high event rate.

The use of a scintillator anti-coincidence shield F1 and F2 is a requirement for neutron detection. Since neutral particles do not interact with electrons, which are the source for light emission in scintillator materials, the detection requires other means of energy transfer. In the D/E scintillator system, neutrons of 2 to about 100 MeV are detected indirectly by elastic scattering with protons in the plastic detector E. Recoil protons carry on average half the kinetic energy of the incoming neutron. The detector has a 4-pi field of view. High energy neutrons may cause D/E coincidences. Neutrons that produce very high-energy recoils can go undetected if the recoil leaves E or D and strikes the F anti-coincidence shield. This process effectively sets the upper limit of reasonable neutron detection efficiency at about 100 MeV.

The lower energy range of neutrons detectable by RAD will be dominated by emission from the RTG that powers MSL. RAD thresholds will be raised accordingly. It is not known exactly where these thresholds will be, but a preliminary test with the RTG at the Idaho National Laboratory suggests that thresholds will have to correspond to about 4-5 MeV of energy deposited in both D and E to keep the event rate manageable. Additionally, when active, the DAN Pulse Neutron Generator (PNG) will provide a source of neutrons.

Gamma-rays are a by-product of high-energy nuclear interactions and can penetrate the anti-coincidence shield without interacting, in which case signals can be produced in E and (more likely) D via the photoelectric effect, Compton scattering, and pair production. The high-Z CsI material of D has a much higher sensitivity for the detection of gamma-rays than the plastic of detector E. D can be used as a gamma-ray spectrometer, albeit with poor resolution compared to the Odyssey GRS in orbit around Mars, above the detector noise level of several hundreds of keV. However the RTG will produce a flux of gamma-rays in D at energies up to a few MeV.

Solar flares and coronal mass ejections can accelerate electrons to energies of several MeV. Observations of relativistic solar electrons are vital for event onset timing studies. Electrons are low linear energy transfer (LET) particles with modest ranges in detector materials. HETn will measure electrons in the range from 150 keV up to 15 MeV. The distinction from ion scan be performed with dE/dx vs. E analysis. A distinct signature in the detectors comes from positrons, which are by-products of flare processes absinthe corona and also produced by interactions of high-energy particles in the atmosphere. The annihilation of the positron with a detector B electron generates characteristic 511 keV X-rays. A*B coincidences with electron-type energy loss signals in coincidence with a light pulse in D equivalent to the characteristic X-ray energy deposit define a positron detection.

Quantitative assessment of energy spectra depends on detailed response functions derived from calibration data and from Monte Carlo simulations. All particle detectors, and some of the analog electronics, are contained in the RAD Sensor Head (RSH). Additional analog electronics, and all of the digital electronics, are contained in the RAD Electronics Box (REB).

Electronics

The silicon diodes used both for direct detection of charged particles (A, B, and C) and for detection of scintillation light (D, E, F) are in all cases connected to charge-sensitive preamplifiers and shaping amplifiers in the sensor head. These are of a standard design, optimized for low noise and wide dynamic range. There are seventeen analog signals at the output of the RSH; these are split into 34 redundant signal pairs at the input of a mixed signal ASIC known as the VIRENA (Voltage-Input Readout for Nuclear Applications). The VIRENA provides, for each channel, an additional amplification stage, two adjustable threshold comparators, and a peak-hold circuit. The VIRENA is a 36-channel device, of which 34 channels are used as described above to read out the 17 RSH signals. The firmware requires that 32 of the 34 channels be selected to be used in the onboard analysis, i.e., 2 of the 34 channels are not used. The choice of which 32 channels are used is configurable. Furthermore, depending on which Level 2 (L2) trigger fired to initiate the event readout(a process explained in more detail below), different sets of channels may be read out, ranging from a few to the full 32.

The VIRENA output signals are multiplexed into a single 14-bit analog-to-digital converter. For events with a valid L2 trigger, the appropriate set of pulse heights is read out and kept in local memory for analysis.

Triggers

RAD has two trigger levels, Level 1 (L1) and Level 2 (L2). Level 1 triggers are initiated by the VIRENA 'fast' discriminators. These are enabled for one channel each for the A1, A2, B, C, D, and E detectors. When the firmware recognizes an L1 trigger, the 'slow' discriminator outputs are examined to see if any L2 trigger patterns (which are for the most part coincidence conditions) are matched. If they are, the pulse height readout commences according to the readout mask for the specific L2 trigger that was matched.

Real-time Analysis

Valid events are analyzed in RAD's Level 3 (L3) firmware. Events are sorted as to whether they are caused by penetrating particles (those that go all the way through the RAD detector stack), stopping particles (those that hit at least A and B and possibly others, but not reaching F2), or neutral particles (those hitting only D and/or E). All events that meet selection criteria are entered into the appropriate histogram. A subset of these are stored in the form of full pulse height records (sometimes referred to as 'list data' format), but storage space for the latter is quite limited.

Location

The instrument sits within the body of the rover. The telescope extends upward and sits nearly flush with the rover deck. The telescope is covered by a kapton window. The field of view is unobstructed subject to the variable position of the robotic arm. RAD is sensitive to the neutron emissions from the RTG.

Operational Modes

RAD uses a simple operational strategy. Nominally, the instrument wakes once per hour, takes a fifteen minute observation, and then returns to sleep mode. In the absence of specific changes to this sequence, RAD will operate in this wake-sleep cycle without commanding.

Upon waking, RAD takes a 'pre-observation' to check if event counts are above a threshold indicative of a solar event. In the event of solar event, the outer ring of the A detector is disabled.