01 / 32-148 / Initial Release / JCKasper / 06/20/2006
Functional Instrument Description
and
Performance Verification Plan
Dwg. No. 32-05002
Revision 01
June 20, 2006
Table of Contents
Preface
1Introduction
1.1The Cosmic Ray Telescope for the Effects of Radiation
1.2Scope of this document
1.3Outline of this document
1.4Related documents
1.4.1GSFC Configuration Controlled Documents
1.4.2CRaTER Configuration Controlled Documents
2CRaTER Overview
2.1LRO Level 1 Measurement Objectives Relevant to CRaTER
2.1.1RLEP-LRO-M10
2.1.2RLEP-LRO-M20
2.2CRaTER Level 2 and Level 3 Requirements
2.3Overall Design
3Telescope Design
3.1Overview
3.2Detectors
3.2.1Detector Description
3.2.2Thin and Thick Detectors
3.2.3Detector Diameter and Maximum Event Rates
3.2.4Leakage Current, Detector Noise, and Operating Temperature
3.3Tissue Equivalent Plastic
3.3.1Description and Composition
3.3.2Use of TEP in CRaTER
3.4Telescope Stack
3.4.1Fields of Regard
3.4.2Optimizing Fields of View and Geometric Factors
3.5Telescope Board
3.6Electronics Box
4Electrical Design
4.1Overview
4.2Telescope Board
4.3Analog Processing Board
4.4Digital Processing Board
4.4.1Test Pulse Generator
4.4.2Low Level Discriminator
4.5Power
4.5.1DC-DC conversion
4.5.2Bias Supplies
5Measurement Process
5.1Science Measurements
5.1.1Overview
5.1.2Description of Pulse Height Analysis
5.1.3Internal Calibration Capability
5.1.4Responding to Solar Energetic Particle Events
5.1.5Primary Science Data Products
5.1.6Secondary Science Data Products
5.2Housekeeping
5.2.1Overview
5.2.2Variables Monitored
5.3Commands
6Instrument Requirements Verification Plan
6.1Description
6.2Level 2 Requirements Verification Matrix
6.3Level 2 Requirements Verification Plan
6.3.1CRaTER-L2-01 Measure the Linear Energy Transfer Spectrum
6.3.2CRaTER-L2-02 Measure Change in LET Spectrum through TEP
6.3.3CRaTER-L2-03 Minimum Pathlength through total TEP
6.3.4CRaTER-L2-04 Two asymmetric TEP components
6.3.5CRaTER-L2-05 Minimum LET measurement
6.3.6CRaTER-L2-06 Maximum LET measurement
6.3.7CRaTER-L2-07 Energy deposition resolution
6.3.8CRaTER-L2-08 Geometrical factor
6.4Level 3 Requirements Verification Matrix
6.5Level 3 Requirements Verification Plan
6.5.1CRaTER-L3-01 Thin and thick detector pairs
6.5.2CRaTER-L3-02 Minimum energy
6.5.3CRaTER-L3-03 Nominal instrument shielding
6.5.4CRaTER-L3-04 Nadir and zenith field of view shielding
6.5.5CRaTER-L3-05 Telescope stack
6.5.6CRaTER-L3-06 Full telescope pathlength constraint
6.5.7CRaTER-L3-07 Zenith field of view
6.5.8CRaTER-L3-08 Nadir field of view
6.5.9CRaTER-L3-09 Calibration system
6.5.10CRaTER-L3-10 Event selection
6.5.11CRaTER-L3-11 Maximum event rate
7Glossary
Preface
Revision 01 of this document is being released for CDR.
1Introduction
1.1The Cosmic Ray Telescope for the Effects of Radiation
The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument is designed to characterize the lunar radiation environment on the Lunar Reconnaissance Orbiter (LRO) spacecraft. CRaTER will investigate the effects of solar and galactic cosmic rays on tissue-equivalent plastics as a constraint on models of biological response to radiation in the lunar environment.
1.2Scope of this document
This Functional Instrument Description (FID) document provides an overview of the CRaTER instrument design as it took shape before the Critical Design Review in June 2006. The design described in this FID was developed to meet the requirements levied by the Level 2 through Level 3 requirements in the Instrument Requirements Document (IRD). The FID has two purposes, namely to describe the design that has been developed to meet the requirements levied in the IRD, and to present the procedures we have developed to verify that CRaTER meets the requirements.
1.3Outline of this document
Section 2 presents an overview of the functioning of the instrument. Section 3 discusses the mechanical design. Section 4 covers the electrical design.
1.4Related documents
1.4.1GSFC Configuration Controlled Documents
- ESMD-RLEP-0010 (Revision A effective November 30 2005)
- LRO Mission Requirements Document (MRD) – 431-RQMT-00004
- LRO Technical Resource Allocation Requirements – 431-RQMT-000112
- LRO Electrical ICD – 431-ICD-00008
- CRaTER Electrical ICD – 431-ICD-000094
- CRaTER Data ICD – 431-ICD-000104
- Mechanical Environments and Verification Requirements – 431-RQMT-00012
- CRaTER Mechanical ICD – 431-ICD-000085
- CRaTER Thermal ICD – 431-ICD-000118
1.4.2CRaTER Configuration Controlled Documents
- 32-01203Contamination Control Plan
- 32-01204Performance Assurance Implementation Plan
- 32-01205Instrument Requirements Document
- 32-01206Performance and Environmental Verification Plan
- 32-01207Calibration Plan
- 32-02003.02Mechanical Interface Document
- 32-02052Analog to Digital Subsystem Electrical Interface Control Document
- 32-03001.01Electrical Grounding Diagram
- 32-03010Digital Subsystem Functional Description Document
- 32-04003.01Reliability Assessment Document Drawing
- 32-04011.01Analog Electronics Worst Case Analysis Drawing
- 32-05001Detector Specification Document
- 32-05203Electronics Subsystem Mechanical Interface Control Document
2CRaTER Overview
NASA has established investigation measurement requirements for LRO based on RLEP Requirements and the LRO AO and refined further from the mission instrument selections and Project trade studies. In this section, the LRO Level 1 measurement requirements and rationales relevant to CRaTER are reproduced from Section 3.1.1 of ESMD-RLEP-0010, along with the associated product listed in Section 6.2 the instrument will produce in response to the LRO measurement requirements.
2.1LRO Level 1 Measurement Objectives Relevant to CRaTER
2.1.1RLEP-LRO-M10
2.1.1.1Requirement
The LRO shall characterize the deep space radiation environment at energies in excess of 10 MeV in lunar orbit, including neutron albedo.
2.1.1.2Rationale
LRO should characterize the global lunar radiation environment in order to assess the biological impacts on people exploring the moon and to develop mitigation strategies.
2.1.1.3Data Product
Provide Linear Energy Transfer (LET) spectra of cosmic rays (particularly above 10 MeV), most critically important to the engineering and modeling communities to assure safe, long-term, human presence in space.
2.1.2RLEP-LRO-M20
2.1.2.1Requirement
The LRO shall measure the deposition of deep space radiation on human equivalent tissue while in the lunar orbit environment.
2.1.2.2Rationale
The radiation environment needs to be characterized in order to assess its biological impacts and potential mitigation approaches, including shielding capabilities of materials and validation of other deep space radiation mitigation strategies.
2.1.2.3Data Product
Provide LET spectra behind different amounts and types of areal density, including tissue equivalent plastic.
2.2CRaTER Level 2 and Level 3 Requirements
Tabulated copies of the CRaTER Level 2 and Level 3 requirements listed in the IRD are repeated in this section for reference.
Item / Sec / Requirement / Quantity / ParentCRaTER-L2-01 / 4.1 / Measure the Linear Energy Transfer (LET) spectrum / LET / RLEP-LRO-M10
CRaTER-L2-02 / 4.2 / Measure change in LET spectrum through Tissue Equivalent Plastic (TEP) / TEP / RLEP-LRO-M20
CRaTER-L2-03 / 4.3 / Minimum pathlength through total TEP / > 60 mm / RLEP-LRO-M10, RLEP-LRO-M20
CRaTER-L2-04 / 4.4 / Two asymmetric TEP components / 1/3 and 2/3 total length / RLEP-LRO-M20
CRaTER-L2-05 / 4.5 / Minimum LET measurement / 0.2 keV per micron / RLEP-LRO-M10, RLEP-LRO-M20
CRaTER-L2-06 / 4.6 / Maximum LET measurement / 7 MeV per micron / RLEP-LRO-M10, RLEP-LRO-M20
CRaTER-L2-07 / 4.7 / Energy deposition resolution / < 0.5% max energy / RLEP-LRO-M10, RLEP-LRO-M20
CRaTER-L2-08 / 4.8 / Minimum full telescope geometrical factor / 0.1 cm2 sr / RLEP-LRO-M10
Table 2.2.1: CRaTER Level 2 instrument requirements and LRO parent Level 1 requirements.
Item / Ref / Requirement / Quantity / ParentCRaTER-L3-01 / 6.1 / Thin and thick detector pairs / 140 and 1000 microns / CRaTER-L2-01, CRaTER-L2-05, CRaTER-L2-06, CRaTER-L2-07
CRaTER-L3-02 / 6.2 / Minimum energy / < 250 keV / CRaTER-L2-01
CRaTER-L3-03 / 6.3 / Nominal instrument shielding / > 1524 micron Al / CRaTER-L2-01
CRaTER-L3-04 / 6.4 / Nadir and zenith field of view shielding / <= 762 micron Al / CRaTER-L2-01
CRaTER-L3-05 / 6.5 / Telescope stack / Shield, D1D2, A1, D3D4, A2, D5D6, shield / CRaTER-L2-01, CRaTER-L2-02, CRaTER-L2-04
CRaTER-L3-06 / 6.6 / Pathlength constraint / < 10% for D1D6 / CRaTER-L2-01, CRaTER-L2-02, CRaTER-L2-03
CRaTER-L3-07 / 6.7 / Zenith field of view / <= 34 degrees D2D5 / CRaTER-L2-01, CRaTER-L2-02
CRaTER-L3-08 / 6.8 / Nadir field of view / <= 70 degrees D4D5 / CRaTER-L2-01
CRaTER-L3-09 / 6.9 / Calibration system / Variable rate and amplitude / CRaTER-L2-07
CRaTER-L3-10 / 6.10 / Event selection / 64-bit mask / CRaTER-L2-01
CRaTER-L3-11 / 6.11 / Maximum event transmission rate / >= 1000 events/sec / CRaTER-L2-01
CRaTER-L3-12 / 6.12 / Telemetry interface / 32-02001
CRaTER-L3-13 / 6.13 / Power interface / 32-02002
CRaTER-L3-14 / 6.14 / Thermal interface / 32-02004
CRaTER-L3-15 / 6.15 / Mechanical interface / 32-02003
Table 2.2.2: CRaTER Level 3 instrument requirements and parent Level 2 requirements.
2.3Overall Design
The two drawings in Figure 2.3.1 illustrate the overall mechanical design of CRaTER. The drawing on the left is of the entire assembled instrument. CRaTER consists of a rectangular electronics box with a tilted top cover (visible on the left) and a telescope assembly (visible on the right side of the first drawing and rendered in cross section in the drawing on the right).
Figure 3.2.1: Drawings of the CRaTER instrument. The image on the left is from CRaTER Mechanical Interface Document 32-02003.02 and shows the entire assembled instrument. The four electrical connectors all visible on the left side of the instrument, consists of the two redundant 1553 communications interfaces, temperature monitors to the spacecraft, and power from the spacecraft. The telescope assembly and the electronics box are assembled separately. The figure on the right is a cross-section of the telescope assembly, showing the stack of detectors and TEP, the connections between the detectors and their preamplifiers, and the single connector to the electronics box.
The electronics box houses a digital processing board (DPB) and an analog processing board (APB). The DPB interfaces with the spacecraft through the four connecters seen on the side of the instrument in the drawing on the left in Figure 3.2.1. From the left, there are redundant 1553 command and telemetry interfaces, feedthroughs for thermometers used for survival heaters and monitoring the instrument health by the spacecraft, and a connector for the spacecraft supplied 28V DC power.
The telescope assembly holds the telescope stack and the telescope electronics board (TEB). The TEB connects the telescope to the electronics box, delivers bias voltages to the detectors, and sends detector signals and calibration pulses through preamplifiers back to the APB. The preamplifiers are very sensitive, therefore it is desirable to limit electrical interference near the detectors. Therefore the telescope assembly is electrically isolated from the electronics box, and grounded on the same path as the signals from the preamplifiers to the APB. The telescope stack, visible in the cross section in the drawing on the right in Figure 3.2.1, consists of aluminum shields on the nadir and zenith sides to block low energy particles, followed by pairs of thin and thick silicon detectors surrounding sections of A-150 Tissue Equivalent Plastic (TEP).
Figure 3.2.2 is a functional block diagram of the entire instrument. It is based on the CRaTER reliability assessment and shows the critical components necessary for the functioning of the instrument. As described above, the telescope assembly houses the telescope stack and the telescope board. The electronics box assembly houses the APB and the DPB. The APB is described in detail in Section. It acts to shape the pulses from each of the detector preamplifiers, to further amplify the signals, and to generate calibration pulses for testing the response of each signal path. The DPB identifies and processes particle events and generates scientific measurements, controls power distribution within the instrument, records housekeeping data, and receives commands and sends telemetry to the spacecraft.
Figure 3.2.3: Block diagram of CRaTER showing the critical components of the instrument (Similar to reliability assessment drawing 32-04003.01). The telescope assembly consists of the telescope stack of detectors and TEP, and a telescope board with preamplifiers. The electronics box assembly consists of an analog processing board (APB) on which the pulses from the detectors are shaped and the digital processing board (DPB) which monitors the detectors for particle events, conducts the pulse height analysis, and tracks housekeeping data. The DPB also interfaces with the spacecraft through 1553 for telemetry, and receives a 1 Hz timing signal and 28V for power.
3Telescope Design
3.1Overview
In this section, we outline the physical construction of CRaTER, with a focus on the sensing portion of the instrument, or the telescope assembly. As described in the instrument overview in Section 2.3, CRaTER consists of two physical parts, the telescope assembly and the electronics box. The telescope is mechanically mounted to the electronics box, but the two structures are electrically isolated from one another. The entire telescope assembly is instead grounded to the digital signal ground. This is done to reduce noise on the spacecraft chassis ground reaching the detectors and the preamplifiers.
As specified in CRaTER L2 requirement CRaTER-L3-05 the telescope stack consists of components three pairs of thin and thick detectors surrounding two pieces of TEP. From the zenith side of the stack the components are the zenith shield (S1), the first pair of thin (D1) and thick (D2) detectors, the first TEP absorber (A1), the second pair of thin (D3) and thick (D4) detectors, the second TEP absorber (A2), the third pair of thin (D5) and thick (D6) detectors, and the final nadir shield (S2).
Figure 3.1: Cross section of the telescope assembly, showing the pairs of thin and thick detectors and the TEP in the telescope stack, and the associated telescope electronics board. A pigtail with signal and ground runs from each of the detectors to one of six preamplifiers on the telescope electronics board.
These components may be seen in Figure 3.1.1 below. Pairs of thin and thick silicon detectors are used to measure the LET spectrum. Section 3.2 reviews the detectors selected for CRaTER and discusses the need for the pairs of thin and thick detectors to cover the full range of LET possible in silicon. The three pairs are needed to cover the range in LET expected by SEP and GCR ions in silicon and after evolving through the TEP. Section 3.3 describes the A-150 Tissue Equivalent Plastic. Section 3.4 describes the optimization of the location of components in the telescope stack.
3.2Detectors
3.2.1Detector Description
The rate (dE/dx) at which ions will loose energy (E) in the detectors is given by the stopping power of the material,
,(1)
which is a combination of electronic and nuclear interactions. For the energy range we are interested in, and the dominant process for energy loss by ions in the detectors is through electromagnetic interactions. Note that nuclear interactions such as fragmentation will occur, especially for the higher energy galactic cosmic rays we will be measuring with CRaTER. Nonetheless, the signal produced by a shower of fragments will still be produced by electromagnetic processes. Within the detector the most of the ionizing energy is used to free electron-hole pairs in the detector. The signal collected from the detector is the current pulse produced by these electron-hole pairs. A small fraction, several percent, of the energy loss will also go into generating phonons in the detectors. Since we do not measure the energy deposited into phonons, it is important to calibrate the relationship between energy deposited into the detectors and the final signal.
Figure 3.2 depicts a simplified detector cross-section illustrating the key components of the CRaTER detectors. Silicon semiconductor detectors are useful for characterizing the energy loss of ionizing radiation as it traversed the detector. The silicon is doped to make it a semiconductor, with electron-hole pairs in valance levels with energy levels much smaller than the first ionizing potential of an atom, making the detector much more sensitive than a proportional counter. The silicon is composed of both an N-type and a P-type substrate, making it a large diode that can be biased at a relatively high voltage with little leakage current. When an energetic charged particle passes through the silicon, it liberates electron-hole pairs, which rapidly drift to either end of the detector in the electric field established by the bias voltage. The resulting signal from the drifting electron-hole pairs is linearly proportional to the total energy deposited. The purpose of the instrument is to amplify these signals when they occur in multiple detectors, and to stably and accurately determine the height of each pulse.
Two types of detectors, thin and thick circular disks, have been procured from Micron Semiconductor Ltd for the engineering and flight models of CRaTER. The thin detectors are nominally 140μm thick and the thick detectors are 1000μm thick. Both detectors are ion implanted totally depleted structures formed from an N-type substrate. The Phosphorous-implanted N-type substrate is the ohmic side of the detector and the Boron-implanted P-side is the junction. These implants require lower energy and result in low implant depths of ~0.3μm. Both detectors are circular, have thin junction and ohmic windows, and have fast timing capability (i.e., although fast timing is not critical for CRaTER, it is desired to have the metallization is made in such a fashion as to reduce surface resistivity). There is a guard ring ( indicated by the green region labeled Guard) around the active junction to improve edge uniformity and a neighboring field plate (FP) ring to aid discharge of oxide stray charge. Each thin and thick detector is mounted to its own small passive PCB and connected to the telescope electronics board by a wire pigtail. Details of the expected detector performance and the detector validation procedures are described in the Detector Specification document 32-05001.
Figure 3.2: Schematic cross section of the silicon detectors ordered from Micron Semiconductor Ltd for the CRaTER instrument (Figure 1 of Detector Specification 32-05001).
3.2.2Thin and Thick Detectors
Thin and thick detector pairs are used to span the entire range of LET and to provide a system that is less sensitive to low energy protons in the event of intense Solar Energetic Particles (SEPs) from solar flares or coronal mass ejections.
The need for pairs of thin and thick detectors is reflected in CRaTER-L3-01, namely that he telescope stack shall contain adjacent pairs of thin and thick Silicon detectors. The thickness of the thin detectors will be approximately 140 microns and the thick detectors will be approximately 1000 microns. The need for pairs of detectors arises from the large range in LET that ions can produce in silicon through ionizing radiation and from the different amplitudes of energy loss expected from light and heavy ions.
To understand the range of energy deposits expected in the detectors, consider the Bethe-Bloch equation for the energy loss of a particle passing through matter and generating ionization. The relativistically invariant version is,
,(2)