CCSD3ZF0000100000001NJPL3IF0PDSX00000001 PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = ULY INSTRUMENT_ID = "COSPIN-HET" OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "COSPIN-HIGH ENERGY TELESCOPE" INSTRUMENT_TYPE = "CHARGED PARTICLE ANALYZER" INSTRUMENT_DESC = " (description excerpted from [SIMPSONETAL1992A]) Instrument Overview =================== The high energy telescope (HET) is a large geometric factor cosmic ray telescope that uses particle trajectory determination together with the dE/dX vs. residual E technique to measure the energy and identify the mass and charge of cosmic rays. The HET with its analogue and digital electronics forms the largest part of SIM2, with the HFT, mounted on top. Detectors D[1]-D[6] provide signals that allow determination of the trajectory of the incident particle, and detectors K[1]- K[6] provide accurate measurement of the energy loss rates and total energy of particles which stop in the K detectors. For particles which stop in the detector stack, the telescope provides charge and mass resolution sufficient for studies of the chemical and isotopic composition of cosmic rays from hydrogen through nickel (1 >= Z >= 28). Scientific Objectives ===================== Through measurements of the intensity and chemical and isotopic composition of nucleonic cosmic rays with energies of order 10 through a few hundred MeV/nucleon, the telescope provides data relevant to a number of important questions in heliospheric and galactic cosmic ray physics, including acceleration of particles in solar flares, the nature of solar modulation, the structure of the heliospheric modulation region, the characteristics of the interstellar medium, conditions in the acceleration region itself, and results of nucleosynthesis in supernovae. In addition, the telescope provides measurements of electrons with energies of a few MeV for investigation of the propagation of solar and Jovian electrons throughout the heliosphere. Detectors ========= Detectors D[1]-D[6] are multi-strip position sensitive Li- drifted silicon solid state detectors that measure both the energy loss in the detector and the location (by strip number) of the cosmic ray trajectory through the detector. The detectors were developed and fabricated at the University of Chicago (Lamport et al., Nucl. Instr. and Meth. 134, 71, 1976), and are operated at a bias voltage of 35 volts. The electrical contacts on the front surface of the detector consist of evaporated gold strips which are individually connected to a resistive divider chain on the detector ring, one end of which is connected to ground. On the back surface, a single evaporated aluminum contact covers the entire sensitive area of the detector. Thus, two signals are derived from each detector. The signal from the back surface (E signal) measures the total energy lost in the detector, while the signal from the front surface (P signal) is proportional to the location along the resistive divider chain of the strip in which the charge was collected. The position resolution achieved by this technique in the individual detectors is approximately 150 microns. With the strips of successive detectors (e.g., D[1], D[2], D[3]) rotated by 60 [deg.], the trajectory of incident cosmic rays can be determined to an accuracy of better than 1 [deg.], as verified by accelerator calibration. Furthermore, use of three detectors in both the front (D[1]-D[3]) and back (D[4]-D[6]) planes provides a viable position-sensing backup in case one of the detectors fails during the mission. Detectors K[1]-K[6] are thick (nominal 5 mm) Li-drift silicon detectors that, in combination, provide approximately 7 g cm^-2 for stopping incident cosmic rays. Detectors K[1]-K[6] were supplied by the Lawrence Berkely Laboratory, and detectors K[5] and K[6] were supplied by the Kevex corporation. The K[1]-K[6] detectors are operated at a bias of 650 volts, which provides better than 99.5% charge collection within the amplifier shaping time constant of 5 microseconds. Anticoincidence protection for the telescope is provided by a scintillator shield (S) surrounding the telescope, and by a solid state detector, A, which identifies particles which completely penetrate the telescope. Electronics =========== Signals from the front and back surfaces of D[1]-D[6] and from K[1]-K[6] (18 signals in all) are fed through charge-sensitive amplifiers and two sets of shaping post-amplifiers. Signals from K[1]-K[6] and the E signals from D[1]-D[6] are fed to a fast (1 microsecond shaping time constant) set of amplifiers to provide inputs for discriminators which, through the digital logic, are used to identify the particle type for counting rates and to set logic flags. In parallel, all 18 detector signals are fed to a slower (5 microsecond rise to maximum) dual-gain set of amplifiers which provide shaped signals for accurate amplitude measurement by the pulse height analysis circuitry. Gains for the dual-gain amplifiers are selected based on signal size as determined by discriminators operated off the differentiated rise of the charge-sensitive amplifier signal. To maintain the accuracy required for isotopic resolution through iron, the amplifiers in the PHA chain have been designed for extreme stability, and tests have demonstrated a drift in gain of < 0.007%/[deg.]C over the temperature range -20[deg.]C to +23[deg.] C. Gains and thresholds are monitored by an in-flight calibration sequence which is normally performed monthly, initiated by ground command. If the combination of fired discriminators satisfies one of the logic conditions required for pulse-height analysis, output from the amplifiers goes to two peak-detector/sample-hold circuits that hold the signal amplitudes for processing by two 4096 channel (12 bit) analogue to digital converters, each of which processes 9 of the signals. Total processing time for a complete event is approximately 2 ms. The pulse height analysis data for an individual event consists of 280 bits containing the 18 pulse height analyses described above and 64 flag bits describing the state of discriminators attached to the individual detectors, the command state of the instrument, and the spin phase, divided into 8 sectors, at the time that the event was detected. Six HET PHA events are recorded during each spacecraft telemetry format period, or 32 seconds at the nominal bit rate. Since the actual event rate in the telescope is much higher than can be returned as PHA data with the available telemetry, the PHA can only sample the events recorded by the telescope. To maximize the scientific return from the PHA sample, three priority levels have been established to govern selection of events for retention in the PHA sample. The levels correspond, roughly, to heavy nuclei which stop in the telescope (P[1]), to any particle which stops in the telescope without triggering the anti-coincidence after penetrating at least to detector D[4] (P[2]), and to any nucleonic particle which triggers detectors D[1] and D[2] but not detectors A and S (P[3a]), or, with a 50% duty cycle, to any particle which triggers D[1], D[2], and D[4] (P[3b]). P[3b] includes both high energy penetrating particles and background events. P[3] is the lowest priority, and any P[3] event can be displaced by a P[2] or P[1] event. Similarly, any P[2] event can be displaced by a P[1] event. In quiet times since turn-on of the HET, about 40% of the events have been of type P[3], and of the order of 1% have been of type P[1]. In addition to pulse height analysis, the HET provides 29 digital counting rates. Of these 13 are derived by logic from the discriminators to correspond to electrons, protons, and heavy nuclei in well defined energy ranges. The counting rates are true spin averages, with accumulation intervals of an integral number of spins, as determined by the software of the data processing unit. Two counting rates provide 8 sectored anisotropy information for protons and electrons, while the remaining 14 counting rates monitor the counting rates of individual detectors as a housekeeping function. The characteristics of the counting rates are more fully described in Table 7 (below). Energy ranges are based on computations from range energy tables for nucleons. All counting rates are telemetered to earth in 27 --> 12 bit compressed format. Because of the duration of the mission, the possibility of a detector failure must be considered. Consequently a number of commands have been implemented to allow reconfiguration of the telescope logic to compensate for failure of one or more detectors. The telescope logic is fully protected to the extent that every term in the logic can be modified in a predetermined way by ground command. In addition, there are commands to turn off high voltage supplies, to initiate (and turn off) the in- flight calibrate sequence, and to control power to three small heaters mounted in the analogue electronics of the HET to help maintain a stable thermal environment throughout the mission. Calibration =========== The HET unit now on the Ulysses spacecraft was tested on several occasions from 1982 - 1989 using beams of heavy nuclei from Ne through Fe accelerated by the Lawrence Berkeley Laboratory Bevalac. The calibration data have been used to verify proper function of the telescope, to characterize the detector response, and to develop algorithms for selection of the data and for determination of mass and charge of incident particles by use of the multiple dE/dx vs. residual E plus trajectory information returned by the telescope. Electron response was investigated over the energy range 3-35 MeV making use of a linear electron accelerator at the University of Chicago Argonne Cancer Hospital. Electron energy ranges were found to be very broad and poorly defined since no effort has been made to optimize the telescope for electron response. Some uncertainty remains in the calibration results also because of the difficulty of using the accelerator at the very low intensities required for our tests. Further experimental work on the electron calibration using the flight spare HET is planned. Response to the RTG radiations was tested using a simulated RTG (sRTG) at JPL in 1982 and, immediately before launch, by exposure to the flight RTG during the RTG mating test at the Kennedy Space Center (KSC). Initial tests with the sRTG showed RTG-induced events in prime HET data channels at intensity levels comparable to those expected from galactic cosmic rays. After adjustments of discriminator thresholds and modifications to the telescope logic the level of interference was shown to be markedly reduced by a second exposure to the sRTG. To further reduce background from the RTG's, a 1.6 kg tungsten shield was placed between the SIM-1 and SIM-2 units. The configuration and placement of the shield was determined by use of Monte-Carlo techniques to provide the maximum shielding effect for the allotted tungsten mass given the known locations and shapes of the RTG radiation source and the detectors to be shielded. Since detectors D[4] - D[6] are key detectors for the HET pulse height analysis priority system and logic and for the electron counting rates (H[6] - H[8]), they were chosen as the detectors in which the maximum possible reduction in the rate of RTG-induced events should be achieved. Tests at KSC and in- flight experience show that the strategy was effective, and that RTG-induced events no longer make significant contributions to HET data channels except for the H[1] and H[6] counting rates, which, during quiet times, respond primarily to RTG-induced events. In flight, an in-flight calibrator (IFC) is commanded on once per month. The IFC provides an exhaustive check of the gains and non-linearities of each amplifier by presenting at the inputs to the charge sensitive amplifiers a series of 2048 pulses, timed to the readout cycle, which cover the entire dynamic ranges of both the high and low gain amplifiers used in the pulse-height analysis. A normal IFC run consists of two passes through the pulse sequence, and requires approximately six hours at the nominal cruise telemetry bit rate. In addition, house-keeping channels monitor the regulated voltage lines that supply the amplifiers and the detector biases, the temperatures of the detectors and electronics, and the zero- offset of the digital to analogue converter (DAC) of the IFC. Measured Parameters =================== Table 7. High energy telescope data channels. Name Prim. Energy Geometric Avg Sectors PHA Part. Range Factor Time Priority Type (MeV(/n)) (cm^2sr) Res(s) ------------------------------------------------------------------- H1 Proton 5.4-14 94 16 -- none H2 Proton 14-19 87 16 -- P3 H3 Proton 24-31 16.2-14.9 16 -- P3 H4 Proton 34-68 8.2-5.5 16 -- P2 H5 Proton 68-92 5.2-3.6 16 -- P2 H6 Elec. ~1-3 87 16 -- P3 H7 Elec. ~5-10 8.2-5.5 16 -- P2 H8 Elec. ~3-5 16.2-14.9 16 -- P3 H9 Proton >92 3.6 16 -- P2 H10 Z>=3 26-36(^12C) 87 128 -- P2 H11 Z>=3 44-127(^12C) 16.2-5.5 128 -- P1 H12 Z>=3 127-173(^12C) 5.2-3.6 128 -- P1 H13 Z>=3 >173(^12C) 3.6 64 -- P1 H45S Proton 34-92 8.2-3.6 128 8 P2 H7S Elec. ~5-10 8.2-5.5 128 8 P2 H14-H19 -- -- -- 128 -- -- H20-H25 -- -- -- 128 -- -- H26,H27 -- -- -- 128 -- -- References Lamport J.E., Mason G.M., Perkins M.A. and Tuzzolino A.J., Nucl. Instr. and Meth. 134, 71, 1976." END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "KUNOWETAL1991" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "MARSDENETAL1991" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "SIMPSONETAL1992A" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END