PDS_VERSION_ID = PDS3
LABEL_REVISION_NOTE = "2007-08-14 Peter Ford Initial version;"
RECORD_TYPE = STREAM
OBJECT = INSTRUMENT
INSTRUMENT_HOST_ID = LRO
INSTRUMENT_ID = CRAT
OBJECT = INSTRUMENT_INFORMATION
INSTRUMENT_NAME = "Cosmic Ray Telescope for the Effects of
Radiation"
INSTRUMENT_TYPE = "ENERGETIC PARTICLE DETECTOR"
INSTRUMENT_DESC = "
Abstract:
=========
The investigation hardware consists of a single, integrated sensor
and electronics box with simple electronic and mechanical interfaces
to the LRO spacecraft. The CRaTER sensor front-end design is based on
standard stacked-detector, cosmic ray telescope systems that have been
flown for decades, using detectors developed for other NASA flight
programs. The analog electronics design is virtually identical to the
robust and flight-proven design of the NASA/POLAR Imaging Proton
Spectrometer that has been operating flawlessly on orbit since 1996.
The digital processing unit is a simple and straightforward design
also based on similar instruments with excellent spaceflight heritage.
No new technology developments or supporting research are required for
the final design, fabrication, and operation of this instrument..
The text of this instrument description has been abstracted from the
instrument paper [CHINETAL2007] and from a preliminary draft of the
CRaTER Instrument Calibration Plan [KASPER2007].
Scientific Objectives:
======================
The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) is
designed to answer key questions to enable future human exploration of
the Solar System, and to address one of the prime objectives of LRO.
Specifically, CRaTER addresses an objective required by NASA's
Exploration Initiative to safely return humans to the Moon; CRaTER is
designed to achieve characterization of the global lunar radiation
environment and its biological impacts and potential mitigation, as
well as investigation of shielding capabilities and validation of other
deep space radiation mitigation strategies involving materials. CRaTER
will fill knowledge gaps regarding radiation effects, provide
fundamental progress in knowledge of the Moon's radiation environment,
and provide specific path-finding benefits for future planned human
exploration. CRaTER's primary measurement goal is to measure directly
the linear energy transfer (LET) spectra caused by space radiation
penetrating shielding material. Such LET spectra are a missing link,
currently derived by models which require experimental measurements to
provide ground truth. CRaTER will provide this essential information
about the lunar radiation environment.
LET is defined as the mean energy absorbed locally, per unit path
length, when an energetic particle traverses material. A LET
spectrometer measures the amount of energy deposited in a detector of
known thickness and material property when an energetic particle passes
through it, usually without stopping. LET measurements behind various
thicknesses and types of material are of great importance to spacecraft
engineers and radiation health specialists. Such measurements are
especially important to modelers who study the impacts of the
penetrating radiation; LET is one of the most important inputs for
predictive models of human health risks and radiation effects in
electronic devices. While LET spectrometers do not necessarily resolve
particle mass, LET measurements do include all the species, with the
possible exception of neutrons, that are relevant to the energy
deposited behind a known amount of spacecraft shielding. A LET
spectrometer essentially provides the key direct measurement needed to
bridge the gap between well measured cosmic ray intensities (that will
be available from other spacecraft) and specific energy deposition
behind shielding materials, exploration-enabling knowledge vital to the
safety of humans working in the harsh space radiation environment.
Accordingly, CRaTER is designed to measure this important quantity and
thereby provide critical closure between measurements, theory, and
modeling.
CRaTER will measure LET spectra produced by incident galactic cosmic
rays (GCRs) and solar energetic protons (SEPs). GCRs and SEPs with
energies ❯10 MeV have sufficient energy to penetrate even moderate
shielding. When they interact with matter, they leave behind energy,
damaging the human tissue and electronic parts they pass through. GCRs
and SEPs possess both short and long timescale variations (see Fig.
10), some of which are predictable and others that are not presently
predictable. GCRs are a slowly-varying and uniform source of cosmic
radiation that bathes the solar system. SEPs are episodic and rare, but
come in extreme bursts associated with intense solar magnetic activity.
Both GCRs and SEPs pose serious risks to humans venturing above the
relative safety of low-Earth orbit and the Earths powerful magnetic
shield; areas including the Moon and the interplanetary space between
Earth and Mars may be dangerous to humans.
In order to achieve the LRO radiation mission requirement, CRaTER is
designed to return the following required data products:
* Measure and characterize that aspect of the deep space radiation
environment, LET spectra of galactic and solar cosmic rays
(particularly above 10 MeV), most critically important to the
engineering and modeling communities to assure safe, long-term, human
presence in space.
* Investigate the effects of shielding by measuring LET spectra behind
different amounts and types of areal density, including
tissue-equivalent plastic.
The CRaTER measurement concept is shown in the see-thru telescope
drawing below (Fig. 11). The investigation hardware consists of a
single, integrated telescope and electronics box with straightforward
electronic and mechanical interfaces to the spacecraft. The
zenith-nadir viewing telescope employs a stack of three pairs of
detectors embedded within aluminum structure and tissue-equivalent
plastic (TEP) to establish the LET spectra of cosmic radiation relevant
for human health and electronics part concerns.
Primary GCRs and SEPs enter the telescope through the zenith,
deep-space entrance, depositing energy in the telescope stack through
ionizing radiation and producing secondary particles through nuclear
interactions. The primary and secondary particles interact with one or
more of the six detectors through the stack: the thin (thick) detectors
are optimized for high (low) LET interactions. Events with sufficient
energy deposition in a detector cross a trigger threshold. Digital
logic then compares multi-detection coincidences with predefined event
masks to identify desirable events. Pulse height analysis is performed
on every detector to measure LET at each point in the stack.
The measurement team will use observations taken during the mission to
construct LET spectra behind the different amounts of material,
including TEP, as a function of particle environment (GCR vs. SEP;
foreshock vs. magnetotail vs. solar wind; etc.). The team will also
test models of radiation effects and shielding by verifying/validating
deterministic models.
Model predictions of energy transport of incident GCR and SEP spectra
(available contemporaneously on other missions) through the CRaTER
instrument will be compared to the measured LET spectra. Thus, CRaTER
will provide not only direct measurements of LET in the lunar
environment, but will also better constrain radiation effects models
that are being used to assess the effects of other radiation
environments, including in interplanetary space and at Mars.
Calibration:
============
The calibration of the CRaTER Flight Models is based on lessons learned
from experiences with the Engineering Model: the relationship between
the value returned by the detectors and the original energy deposited is
essentially linear. Gains and offsets for each detector are determined
with high precision by calibrating the instrument with a beam of high
energy protons produced by the Northeast Proton Therapy Center (NPTC) of
Massachusetts General Hospital (MGH) in Boston. A 300 MeV proton beam at
MGH is degraded in energy using sheets of plastic until a beam is produced
with large energy dispersion and a peak energy tuned to the response of
a pair of thin and thick CRaTER detectors. The dispersed beam produces
a characteristic track in energy deposition in the pair of detectors.
The gains and offsets for each of the detectors is then determined by
iteratively varying the free parameters of the instrument response until
the measurements match the predictions of GEANT numerical simulations
of the energy loss.
Operational Modes:
===========================
The CRaTER instrument has only two modes: powered up and powered down.
There are no operational constraints on these modes. In particular,
CRaTER can be powered up during the cruise phase of the mission, and
will return scientifically useful data.
Sensors:
==========
The investigation hardware consists of a single, integrated sensor and
electronics box with simple electronic and mechanical interfaces to the
LRO spacecraft. The CRaTER sensor front-end design is based on standard
stacked-detector, cosmic ray telescope systems that have been flown for
decades, using detectors developed for other NASA flight programs. The
analog electronics design is virtually identical to the robust and flight-
proven design of the NASA/POLAR Imaging Proton Spectrometer that has been
operating flawlessly on orbit since 1996. The digital processing unit is
a simple and straightforward design also based on similar instruments
with excellent spaceflight heritage. No new technology developments or
supporting research are required for the final design, fabrication, and
operation of this instrument.
The CRaTER telescope consists of six ion-implanted silicon detectors,
mounted on detector boards, and separated by pieces of tissue-
equivalent plastic, hereinafter referred to as TEP. All six of the
silicon detectors are 2 cm in diameter. Detectors 1, 3, and 5 are 140m
thick; the others are 1000m thick. TEP (such as A-150 manufactured by
Standard Imaging) simulates soft body tissue (muscle) and has been used
for both ground-based as well as space-based (i.e., Space Station)
experiments.
Solid-state detectors use semi-conducting crystals (in CRaTERs case,
silicon) with n-type (electron-rich, electron conducting) and p-type
(electron-deficient, hole conducting) regions.
When a reversed bias voltage is applied across the junction, the
un-bonded electrons in the semiconductor are pushed away from the
voltage source, while the holes are pulled towards it. This leaves a
neutral area void of charge and current at the junction of the sectors,
called the depletion region. As incoming radiation (e.g., a solar
proton or cosmic ray particle) collides with the depletion region,
electron-hole pairs are formed in the material (where a once bonded
electron is freed from its atom, leaving a hole). The electron and the
hole respond to the applied voltage, and a small current is created.
This current can be detected and later analyzed.
A cold environment greatly reduces the transmission of thermal
signals. In addition, the solid state of the semiconducting material
makes it easier to detect those signals attributable to freed
electrons.
Tissue equivalent plastic (or TEP) is a plastic recipe designed to
simulate human tissue. It includes hydrogen and nitrogen
percentages-by-composition that are similar to that found in human skin
and muscle. Scientists can use the atomic-level effects that radiation
has on the TEP to deduce what sort of similar effects may occur in
humans.
Electronics:
============
The front-end analog electronics utilize charge amplifiers to collect
signals from the six silicon detectors, amplify, and pulse-shape them
for high-level processing. The backend digital electronics receives the
six signals converts them in parallel to 12-bit digital quantities and,
using a programmable coincidence mask, filters out the events of
interest. Those events are then packed into standard CCSDS data packets
and forwarded to the spacecraft data system for storage and eventual
telemetry to the ground. A field programmable logic array contains all
of the digital circuitry that receives commands from the spacecraft and
handles telemetry packet formatting as well as collecting useful
secondary science and analog housekeeping information. There is no
processor or software within the instrument.
The maximum event rate that the instrument can telemeter is limited to
1200 events/second, enforced separately for every 1 second interval;
excess events are discarded. The instrument has an internal deadtime of
12 microseconds. Events are time-tagged to an accuracy of 1 second.
"
END_OBJECT = INSTRUMENT_INFORMATION
OBJECT = INSTRUMENT_REFERENCE_INFO
REFERENCE_KEY_ID = "CHINETAL2007"
END_OBJECT = INSTRUMENT_REFERENCE_INFO
OBJECT = INSTRUMENT_REFERENCE_INFO
REFERENCE_KEY_ID = "KASPER2007"
END_OBJECT = INSTRUMENT_REFERENCE_INFO
END_OBJECT = INSTRUMENT
END
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