HEP is a High Energy Proton telescope, a compact particle telescope which measures highly penetrating protons in space. HEP measures the differential energy spectrum of protons between 25 and 440 MeV and the integral flux above 440 MeV. This instrument combines new detector materials, an innovative sensor geometry, and a combination of active and passive shielding to obtain accurate measurements of highly penetrating protons in an instrument compact and light weight enough for space flight. A description of the instrument is in publication, having been accepted by Nucl. Instrum. Meth. A. It is entitled “Design, Development, and Calibration of a High Energy Proton Telescope for Space Radiation Studies,” by R.H. Redus, B.K. Dichter, M.R. Oberhardt, J.O. McGarity, J. Dalcolmo, S. Woolf, A.C. Huber, J.A. Pantazis.
The high energy proton population in the near-Earth space environment poses one of the most significant hazards to the operation of satellites. These particles penetrate deeply into spacecraft and cause ionizing radiation damage to electronic and optical components as well as single event effects, such as latch-up, in digital circuitry. Significant efforts have been made to measure and model the high energy protons in the Earth’s radiation belts so that their effects on space systems can be understood and predicted. Although significant progress has been made in this area, one population, inner belt protons with energies above 100 MeV, is still relatively poorly understood. In particular, the standard NASA proton model, AP8, had to be extrapolated to energies above 100 MeV because almost no data were available at these high energies. HEP is designed to measure the high energy proton population so that accurate, data based, proton models can be developed for use by spacecraft designers and operators.
The on-orbit measurement of energy spectra of high energy protons in the inner zone of the earth’s magnetosphere is a significant technical challenge. The primary difficulty is due to the fact that the quantity of interest, a differential energy proton spectrum in a small solid angle about some direction, must be measured in the presence of a very large, omni-directional flux of penetrating protons. Clearly, a key challenge is the ability to efficiently discriminate the signal due to in-aperture protons from the far more numerous out-of-aperture ones. A second measurement challenge is posed by the need to make the instrument small and light-weight so that it can be flown aboard a spacecraft. A brute force approach, designing an instrument with sufficient detector material to stop very high energy protons, leads to designs that have dimensions of many tens of centimeters and masses of well over 10 kg. This is both too large for use on today’s small research satellites and the large size of the scintillator makes it extremely sensitive to out-of-aperture particles.
The High Energy Proton telescope (HEP) described here addresses these difficulties through the use of (1) a unique combination of thin semiconductor detectors and a segmented thick scintillation detector, made from the relatively new scintillator Gd2SiO5, to measure the energy of incident protons, (2) a combination of active coincidence requirements and passive shielding to reduce the response to the large omnidirectional flux, and (3) a flexible on-board data processing system to permit an accurate measurement of inevitable complicating effects such as inelastic nuclear scattering. HEP measures the differential energy spectrum of protons from 20 to 440 MeV in twenty-two logarithmically spaced energy channels and the integral flux of protons above 440 MeV. It has an angular resolution of 12° full cone and a geometric factor for high energy protons of 1.8×10-2cm2-ster. HEP also includes twenty-six data channels for measuring background events. The instrument consists of two boxes, a sensor head with a size of 10x10x7 cm3 and an electronics box of 10x10x8 cm3. The total weight is 2.3 kg, total power consumption is 2.5 watts, and the total data rate can be less than 100 bytes per second.
The basic configuration of the sensors is sketched in Figure 1. The sensor includes four thin semiconductor detectors (SDs), denoted D1, D2, D3, and D4 (front to rear), two primary scintillators S1 and S2, and a veto scintillator S3. PIN photodiodes are used to measure the energy deposited in each of the scintillators. The sensor also includes passive shielding and a collimator to reduce the flux from outside the nominal aperture, and a degrader to reduce the flux of in-aperture low energy electrons and protons.
The operation of the instrument can be understood from Figure 2, a plot showing the energy deposited in each on-axis detector as a function of incident proton energy. This plot shows computed values for mean energy deposition. The calculations were implemented using custom software to compute energy depositions in a target with an arbitrary number of layers, using the Bragg-Kleeman rule to estimate the stopping power of compounds and elements not in the Janni tables.
The SDs are used to establish the approximate energy range of the incident protons. For example, a coincident signal from D1, D2, and D3, in anti-coincidence with a signal from D4, implies that the incident proton stopped in S2, the rear scintillator. The minimum proton energy required to reach S2, passing through the degrader, D1, D2, S1, and D3 is 135 MeV, a value computed from the Bethe-Bloch model and verified with calibration measurements. The minimum energy required to reach D4, passing through S2 in addition to those above, is 185 MeV. Thus the pattern of detector triggering given above, (D1)&(D2)&(D3)&(not D4), implies that the incident proton energy was between 135 and 185 MeV. The amount of energy deposited in both primary scintillators, S1 and S2, establishes the exact energy within each range.
Figure 3 shows the energy deposited in S1 versus that deposited in S2, based on the same calculations as Figure 2. The solid line shows the energy deposited in the front scintillator, S1, versus the energy deposited in the rear scintillator, S2, for protons entering from the front. There are three segments to this curve, termed A, B, and C, as noted. Each segment corresponds to a different coincidence pattern from the SDs. HEP uses the combination of the pattern of SDs which are triggered, along with the amount of energy deposited in the scintillators, to uniquely determine the energy of the incident protons.
Protons may also enter from the rear, and these will both trigger the SDs and deposit energy in the scintillators. The dashed line in Figure 3 shows the pattern of energy deposition and detector triggering for these rear entry protons, which can be distinguished from the front entry protons for incident energies below about 300 MeV.
The size of the solid state detectors determines the angular acceptance cone, thus permitting the angular distribution of protons to be measured if the instrument is swept over pitch angles. For type C protons, which must pass through all four detectors, the radii and separation of D1 and D4 define an acceptance cone with a half-angle of 12°. A summary of the HEP geometric properties is shown in Table 1.
|Particle||Energy (MeV)||Number of Channels||Angular Response (half cone)||Geometric factor (cm2-ster)|
|Protons||25 to 135||10||8°||5.7 x 10-2|
|Protons||135 to 185||4||6°||2.1 x 10-2|
|Protons||185 to 440||6||6°||1.8 x 10-2|
|Protons||>440||1||6°||1.8 x 10-2|
Table 1. Summary of HEP in-aperture channels and geometric properties.
The requirement for a simultaneous signal between the SDs ensures that the particles are coming through the desired acceptance cone. The veto scintillator, S3, is used to reject particles which come outside the desired acceptance cone but might otherwise trigger the coincidence logic. A passive shield is also used, consisting on the sides of 5 mm of Cu (proton range of 60 MeV), in the rear of 13 mm of Cu (proton range of 100 MeV), and in the front of 13 mm Cu with 2.5 mm of W (proton range of 130 MeV). This passive shield reduces the omnidirectional flux to minimize accidental coincidences and pulse pile-up. Even with this shield, the flux of protons incident on S3, integrated over the 4p solid angle, is computed from the AP8 model to be 140,000 sec-1. The flux of particles into S1 and S2 is expected to be over 100,000 sec-1. In contrast, the average flux of in-aperture protons above 20 MeV is expected to be only 300 sec-1.
A summary of the important properties of the HEP detectors, their energy depositions and the count rates expected from the AP8 model, is shown in Table 2. The first row shows the minimum incident energy required by an on-axis proton to reach the detector. The S3 value is for an off-axis proton. The second row shows the maximum energy deposited in each detector, while the third row shows the maximum count rate expected from the isotropic AP8 model.
|Minimum Incident Energy (MeV)||26||30||33||139||141||186||60|
|Maximum Deposited Energy (MeV)||8||8||131||8||104||8|
|Maximum Count Rate (sec-1)||2.0E+03||2.0E+03||9.8E+04||6.6E+03||1.1E+05||2.2E+04||1.4E+05|
Table 2. Summary of important properties of HEP detector response.
Ideally, all in-aperture protons would deposit energy as shown in Figures 2 and 3, and no out-of-aperture protons could trigger the coincidence logic. However, there will inevitably be in-aperture protons which deviate from the pattern shown in Figures 2 and 3, due to statistical fluctuations in the interaction processes and to inelastic nuclear scattering events, which are common at these energies. There are also some trajectories along which out-of-aperture protons, incident at the correct energy, will penetrate the HEP collimator and be accepted by the coincidence logic. And there is some probability that random, accidental, simultaneous triggering of two or more detectors will generate false signals. These classes of events form a “background” which must be quantified to obtain an accurate knowledge of the incident spectrum. HEP is designed to minimize these background events but cannot eliminate them. The response of HEP to these event classes has been modeled and measured in ground calibrations. The HEP data processing system is designed to make on-orbit measurements of these “background” event types, allowing correction factors to be determined for these events.
There are three primary means of measuring these background events. First, many of these events produce energy depositions in the S1 and S2 scintillators which do not lie on the curves shown in Figure 3. Up to 20% of the protons passing through HEP will undergo a nuclear interaction, producing an energy deposition pattern deviating from that computed using the Bethe-Bloch formula. For example, a 160 MeV proton would normally stop in S2. If it undergoes a nuclear interaction in S2, then it will deposit the expected energy in S1 but less in S2. There is a region in the S1S2 plane below segment “B” which corresponds to such nuclear interactions. HEP includes 26 “background” data channels, each of which can correspond to an arbitrary portion of the S1S2 plane, to measure particles penetrating from the rear, nuclear interactions, and other effects. Second, the same population of off-axis penetrating particles which produce a false signal, with both logic pattern and S1S2 deposition matching the in-aperture particles, also produce signals with distinct logic patterns. HEP can therefore return the number of counts which do not match the expected logic, for example returning the counts which have energy deposition consistent with segment “C” above but a logic pattern consistent with “A”. Third, to quantify the fraction of pulses lost due to false vetoes or affected by pulse pile-up, the raw count rates in each detector and for each logic pattern are measured.
Although HEP is designed primarily to measure energetic protons, it can also be used to measure energetic electrons. Electrons with an energy exceeding about 1 MeV have a range sufficient to penetrate the copper degrader, D1, and D2. Electrons of 1-10 MeV are nearly minimum ionizing particles, so will generally deposit relatively small amounts of energy in D1 and D2, and will deposit little energy in the S1 scintillator. Protons within a narrow energy range around 25 MeV will also trigger D1 and D2 while not depositing much energy in S1, but these are protons near peak of the Bragg curve so deposit large amounts of energy in D1 and D2. Protons which are energetic enough to have low energy depositions in D1 and D2, will deposit a significant amount of energy in S1. HEP therefore combines the S1 pulse height analysis (requiring a deposition under 6.6 MeV), with pulse height analysis in D1 and D2 (requiring a simultaneous deposition between 0.2 and 2 MeV) to measure the flux of energetic electrons.
The response of HEP to the on-orbit proton environment is not simple, but has been modeled and the model results agree very well with all calibration and experimental results. Some of the response complications, such as those due to inelastic nuclear scattering or to penetrations through the edge of the collimator, are an inevitable consequence of the interactions of energetic protons. Some of the other complications are due to the practical necessities of space flight: limited shielding mass, limited signal bandwidth, and so on. HEP combines new detector materials, an innovative sensor geometry, a combination of active and passive shielding, and a flexible data processing scheme to obtain the most accurate measurements yet of highly penetrating protons in an instrument compact and light weight enough for space flight.
If the HEP raw count rates are multiplied by a factor accounting for the geometric factor and the losses due to inelastic nuclear scattering, then the results should be very accurate for roughly isotopic proton populations above 80 MeV. There will be greater uncertainty at lower energies and in the loss cone. Additional correction for dead times, pile-up, and off-axis penetrations can further reduce the uncertainty. Most importantly, HEP permits one to measure correction factors due to nuclear scattering, pulse pile-up, out-of-aperture penetration, and so on. This will provide the most accurate measurement yet of the energetic protons in the inner belts.
This research was sponsored by the US Air Force Material Command, AFRL/VSBS, under contract F19628-95-C-0227. We wish to acknowledge the support of Brookhaven National Laboratory, especially the staff of the B2 Test Beam at the Alternating Gradient Synchrotron, particularly Alan Carroll and Craig Woody, in providing the high energy calibrations and in loaning important equipment. And we wish to acknowledge the support of the Harvard Cyclotron Laboratory, especially Ethan Cascio, in obtaining the low energy calibration data.
CEASE is a small, low power instrument that provides operators with fully processed, real time, in situ measurements and autonomously generated warnings of the space radiation environment threats. CEASE reports these threats to the host spacecraft:
The S/C operator can use this information to take appropriate action to avoid endangering the mission. The instrument will also provide, if requested, detailed data on particle fluxes incident on the spacecraft over the 72 hours prior to the request. This feature will allow the spacecraft operator, once an anomaly has occurred, to have sufficient data to analyze and understand the cause of the anomaly.
CEASE is designed to be a standard, operational instrument for general use on all spacecraft. The mechanical and electronic design of the instrument was focused on small size, low power requirement, high-reliability and radiation hardness, to allow the incorporation of CEASE into virtually any spacecraft and mission. The instrument’s on-board intelligence permits long-term, unattended operation of the instrument, giving the S/C operator as much, or as little, information as needed.
The first flight of CEASE was on June 7, 2000 on board the TSX-5 spacecraft and was followed by the second launch aboard the Space Technology Research Vehicle 2 on November 15, 2000. The next launch is scheduled for the first quarter of 2001 aboard the Defense Support Program (DSP) operational spacecraft.
Amptek acknowledges the support of the U.S. Air Force, Space Physics Division, Geophysics Directorate, Phillips Laboratory and the contributions of SAIC in the CEASE project.
Digital Ion Drift Meter (DIDM) instruments are designed to measure the velocity vectors of ambient ions at a spacecraft’s location. Measurements of ion density and temperature are also provided. The local electrical field strength can be obtained via the relationship between electrical field, ion drift velocity and the measured magnetic field provided by on-board magnetometer.
The DIDM-1000 series is comprised of two sensors mounted directly onto the electronics housing. The sensors can be pointed to obtain optimal orientation for instrument location and ion species of interest. Easy accommodation of different types of spacecraft interfaces is accomplished by having a dedicated interface card within the instrument.
Incident ions are focused onto a Micro-Channel Plate (MCP) detector, in a manner similar to the operation of a pinhole camera. Trajectories are determined from the impact location of the MCP output onto a wedge and strip anode. The anode provides the azimuth and elevation components of the incident ion’s trajectory, which correspond to the velocity components as well. The normal velocity component is determined separately, by the Retarding Potential Analyzer feature of the instrument.
The instrument is made of state-of-the-art components, detector technology, data processing capability and measurement resolution. It is designed to achieve at least two orders of magnitude improvement over analog instruments of the same type.
Amptek builds the SSJ/4 and SSJ/5 instruments for the Defense Meteorological Satellite Program (DMSP) satellites. They are Electrostatic Analyzer Detectors (ESA) designed for satellite and space systems use that detect and analyze electrons and ions, and provide information on angular distribution of the incoming particles. Small size, low power dissipation, radiation hardening, and high reliability make the ESA systems ideal for measuring the charged particle environment in the Magnetosphere and monitoring Spacecraft Charging events.
The SSJ4 (ESA-200) is a modular instrument of the cylindrical plate type, which can be configured to detect electrons, ions, or both. The energy of the particles to be analyzed can vary from a few eV to 60 keV. The ESA-200 includes the ESA plates, channel electron multiplier (CEM) detectors, precision high voltage generators to drive the analyzer plates and provide bias to the detectors, and output logic to interface with the spacecraft.
The SSJ5 (ESA-500) is also modular, but uses a nested spherical deflection plate system to simultaneously analyze electrons and ions over a 90° field of view. It utilizes a space qualified microprocessor that permits customizing data rates, measurement ranges, on board storage, and specific analysis algorithms, such as Auroral boundary detection or real time charging measurements.
Amptek has now delivered a combination of 18 SSJ4 and SSJ5 instruments for the DMSP program.
J.O. McGarity, D.J. Sperry, A.W. Everest III, A. Huber, J. Pantazis
Amptek, Inc., 6 De Angelo Drive, Bedford, MA 01730
M.R. Oberhardt, D. A. Hardy, and W.E. Slutter
Phillips Laboratory, Geophysics Directorate, Hanscom AFB, MA 01730-5000
M. P. Gough
School of Engineering and Applied Sciences, University of Sussex, Brighton, BN1 9QT, U K
The Shuttle Potential and Return Electron Experiment (SPREE) is a plasma diagnostic system that was flown in the Space Shuttle payload bay as part of the joint NASA/Italy/U.S. Air Force Tethered Satellite System 1 (TSS-1). The SPREE measures energy and angular characteristics of ion and electron particle flux and processes that data to identify wave-particle interactions (WPI). The data SPREE collects are integral in quantifying the electrodynamic behavior of TSS-1. The system consists of a multiple microprocessor based Data Processing Unit (DPU), a pair of nested triquadraspherical electrostatic analyzers mounted on rotary platforms, a space particle correlator system, and two space qualified high density (> 2 Gigabytes) data recorders. The electrostatic analyzers measure both electrons and ions simultaneously over an angular fan of 100, segmented into ten 10 wide zones. A programmable sweep high voltage power supply provides 32 logarithmically spaced deflection voltages to cover a 10 eV to 10 keV energy range. The rotary tables allow the analyzers to view all angles out of the Shuttle bay. The analyzers operate over a dynamic range from 10E3 to 10E12 particles /cm² in particle flux and 0 to 10 MHz in WPI. The SPREE is capable of operating in an environment that may consist of high pressure surges (> 2 x 10E-4 Torr), active electron beams, and RF bursts. Under these conditions, the SPREE DPU uses an on- board algorithm to assess and report the charge level of the Shuttle with respect to the local plasma.
The Shuttle Potential and Return Electron Experiment (SPREE) was developed as part of the instrumentation for the Tethered Satellite System (TSS-1). It was flown on the Space Shuttle (STS-46) in August 1992. The TSS-1 system consisted of a deployable satellite and a suite of instrumentation mounted in the Shuttle bay and on the satellite. Using an electrically conductive tether, the satellite was intended to be deployed to a distance of approximately 20 kilometers from the Shuttle. As the conductive tether passed through the earth’s magnetic field, an emf was induced that tends to drive a current between the satellite and the Shuttle. This current, when permitted to flow, passes through either load resistors to Shuttle ground or to electron guns that emit the collected current1,2,3. Any charge imbalance resulting from either of these configurations will result in charging of the Shuttle, which SPREE measured and reported.
The electron beam system on TSS-1 can emit currents up to 500 mA at voltages up to several kilovolts when the satellite is fully deployed. Even the partial deployment at 256 meters achieved about 50 Volts and 15 mA current. A multi-kilowatt beam may interact with the ambient plasma through wave particle interactions (WPI), producing heating of the plasma as well as other perturbations. The orientation of the local magnetic field or the development of a sheath potential around the Shuttle may cause a significant portion of the electron beam to be returned to the Shuttle. We observed occasional electron fluxes that were many orders of magnitude larger than the flux provided by the local plasma. These return current drove the Shuttle ground negatively with respect to the ambient plasma.
The SPREE system builds on detector and DPU technology developed for the Ampte, Giotto and CRRES satellites. The SPREE system consists of four principal subsystems:
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