CCSD3ZF0000100000001NJPL3IF0PDSX00000001 PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = ULY INSTRUMENT_ID = "COSPIN-LET" OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "COSPIN-LOW ENERGY TELESCOPE" INSTRUMENT_TYPE = "CHARGED PARTICLE ANALYZER" INSTRUMENT_DESC = " (descriptions excerpted from [SIMPSONETAL1992A]) Instrument Overview =================== The COSPIN Low Energy Telescope (LET) measures the flux, energy spectra and elemental composition of solar energetic particles and low energy cosmic ray nuclei from hydrogen up to iron. The instrument covers an energy range from ~ 1 to ~ 75 MeV/n, using a double dE/dX vs. E telescope. Comprehensive on-board particle identifier electronics and an event priority system enable rare nuclei to be analyzed in preference to the more common species. Isotope separation for light nuclei such as He is also achieved. The LET sensor and associated analogue and digital electronics are mounted in the central portion of the SIM-1 package, between the Anisotropy Telescopes and the Digital Processing Unit. The instrument aperture is protected by a hinged cover that was opened by telecommand after launch. A gas purge system was also included to provide additional protection against detector contamination during ground testing and the ascent phase of the launch. Detectors ========= The LET sensor consists of a four-element solid-state detector telescope surrounded by a cylindrical plastic scintillator anticoincidence shield, together with associated analogue electronics. Detectors D[1] and D[2] are large-area (6 cm^2) surface barrier devices having nominal thicknesses of 30 microns (D[1]) and 100 microns (D[2]), while D[3] and D[4] are 2000 micron-thick Li-drifted devices of 10.0 and 12.5 cm^2 active area, respectively. D[4] forms part of the anticoincidence shield. The aperture of the telescope is covered by two thin foils, an inner Ti foil (2 microns) and an outer Kapton foil (8 microns), included for electrical screening and thermal control purposes, respectively. The telescope geometrical factor, defined by two circular collimators mounted in front of D[1] and D[2] in order to reduce unwanted edge effects, has a value of 0.58 cm^2 sr for the coincidence channels. Low resolution single-detector only) measurements of protons and alpha particles are also made. In this case, the geometrical factor is ~ 9.1 cm^2sr. The signals from detectors D[1], D[2] and D[3] are fed into individual amplifier chains consisting of a charge-sensitive preamplifier followed by a parallel combination of three pulse- shaping voltage amplifiers (PSA). To accommodate the large dynamic range required, separate low- and high-gain PSA have been used, selected via an analogue switch controlled by the third (fast) PSA in conjunction with a discriminator. The selected outputs are fed into a common 10 bit (1024 channel) ADC that provides pulse height information. The fast PSA are followed by a number of threshold discriminators, the outputs of which are used in the coincidence logic to define a series of counting rate channels. In addition, the outputs of the slower PSA are fed into a Particle Identifier (PI) circuit that provides both counting rate and event priority information. The LET PI circuit makes it possible to obtain the counting rates corresponding to groups of nuclear species, and comprises a set of analogue function generators and discriminators that divide the instrument response into regions of different nuclear charge. The actual analogue functions used are of two types, namely E[1]+ b[i]E[2] = a[i] (1) and E[2](b[i] + E[i]) = a[i] (2) where E[1], E[2] and E[3] are the energies deposited in detectors D[1], D[2] and D[3], respectively, and a[i], b[i] are constants. For particles stopping in D[2], the loci of points satisfying equation (1) for given pairs of (a[i],b[i]) values define boundaries on the [DELTA]E ([is defined as]E[1]) vs. residual E ( E[2]) diagram that separate the tracks corresponding to different elements or groups of elements into so-called 'charge groups'. The charge groups defined for the D[1] - D[2] range are: protons; (He^-3, He^-4); (Li, Be, B); (C, N, O); Z >= 10 nuclei. In a similar way, charge group boundaries for particles stopping in D[3] are defined by equation (2) with suitable pairs of (a[i], b[i]) values. A different functional form is required in this case because of the pronounced curvature of the [DELTA]E vs. residual E tracks resulting from the large thickness of D[3] relative to D[2]. The same charge groups as before are defined for the D[1] - D[2] - D[3] range, with the addition of a high-Z group corresponding to Z >= 20. Each of the charge groups has an associated counting rate register which is incremented each time a valid event within the group is observed. In the spacecraft prime (real time telemetry) mode, accumulation intervals are 32 s for the proton and alpha particle rate channels and 128 s for the heavy ion rate channels. In addition to providing counting rate information, the PI output is used to generate a 4-bit event code that controls the Event Priority System. The latter is included in order to maximize the LET PHA data-collection efficiency for the rarer nuclear species in the cosmic ray flux. Such a system is needed because the LET telemetry allocation (52 bits per s at the highest bitrate) limits the number of PHA events that can be transmitted to a maximum of 1 per second. Each pulse-height analyzed event is assigned a 4- bit priority value on the basis of its event code via predefined sequences stored in a ROM. Only the pulse heights corresponding to the highest-priority event occurring within every 1 s sampling period are transferred to the telemetry stream. The sequence of priority assignments for all event codes is changed periodically in order to prevent biasing effects. In addition to pulse height data and proton, alpha and heavy ion rates, the LET data frame contains digital status information and analogue housekeeping values, as well as counting rate data for the individual detectors. Housekeeping information includes instrument voltages, the detector leakage currents and temperature values for the detector telescope and electronics. A summary of the LET digital data channels is given in Table 4. Also included in the instrument is an In-Flight Calibrator (IFC) circuit that checks, on command, the electrical characteristics of the instrument by delivering a sequence of pulses having well-defined amplitudes to all amplifier inputs. In-flight Performance ===================== The LET aperture cover was opened by means of the pyrotechnic actuators on October 19. On COSPIN turn-on four days later, all housekeeping data indicated that the instrument status was nominal, and the scientific data showed that a moderate solar flare event was in progress at the time of switch-on. Another example of LET data acquired in flight soon after turn- on . . . covers a 4-day period starting at 00:00 hrs UT on 7 November 1990, at which time a series of impulsive solar flare events occurred on the Sun. Such events are known to produce an enrichment in the isotope ^(3)He relative to the usually more abundant 4He. Both isotopes were present throughout the period and were well resolved by the LET. In addition to the initial flight data from the Ulysses instrument presented above, essentially identical LET sensors that were flown on the Phobos 1 and 2 space probes have provided a wealth of data on the solar and interplanetary energetic particle populations during the rising phase of solar cycle 22 [Marsden et al., Nucl. Instr. and Meth. 290, 211, 1990; Marsden et al., Planet. Space Sci., 39, 57, 1991]. Finally, a substantial database on the performance of the LET has been accumulated during ground testing; the instrument has been extensively calibrated at particle accelerator facilities, with exposures to both heavy ion and proton beams [Kamermans et al., Nucl. Instr. and Meth. 171, 87, 1980; LeBorgne et al., Report ESA STM-224, 1981; Marsden et al., Nucl. Instr. and Meth. 221, 619, 1984]. The in-flight performance of the COSPIN LET instrument has matched that expected from ground calibration and test. While the majority of single detector counting rates are dominated by counts produced by the RTG neutron and gamma ray background, all coincidence channels show good immunity to RTG induced backgrounds. An exception is the low energy 0.3 - 1.5 MeV electron channel, which is dominated by the RTG contribution except during large solar particle events and, we expect, in Jupiter's magnetosphere. The performance in flight so far demonstrates that the experiment is capable of making important contributions to our knowledge of the particle populations in the heliosphere during the exploratory out-of-ecliptic Ulysses mission. Measured Parameters =================== Table 4. Low energy telescope data channels. Name Primary Energy Geom. Avg Sectors PHA Particle Range Factor Time Event Type (MeV(/n)) (cm^2sr) Res(s) Code L1/P1 proton 0.9-1.2 9.1 32* -- -- L2/P2 proton 1.2-3.0 9.1 32* -- 1 L3/P3 proton+ 1.8-3.8 0.58 32* -- 3 L4-L11/P3S proton+ 1.8-3.8 0.58 32 8 -- L12/P4 proton+ 3.8-8.0 0.58 32* -- 4 L13-L20/P4S proton+ 3.8-8.0 0.58 32 8 -- L21/P5 proton 8.0-19.0 0.58 32* -- 5 L22/A1 He 1.0-5.0 9.1 32* -- 2 L23/A2 He+ 1.9-3.7 0.58 32* -- 6 L24/A3 He+ 3.7-8.4 0.58 32* -- 7 L25/A4 He+ 8.4-19 0.58 32* -- 8 L26/H1 Li,Be,B+ 1.9-4.9 0.58 128* -- 9 L27/H2 Li,Be,B+ 4.9-26 0.58 128* -- 10 L28/H3 C,N,O+ 2.6-7.1 0.58 128* -- 11 L29/H4 C,N,O+ 7.1-39 0.58 128* -- 12 L30/H5 Z>=10+ 3.0-9.0 0.58 128* -- 13 L31/H6 10<=Z<=20+ 9.0-50 0.58 128* -- 14 L32/H7 Z>20+ 12-75 0.58 128* -- 15 L33/E1 electron 0.3-1** -- 128* -- -- L34-L38/S1-S5 -- -- -- 128* -- -- * Rigid interval synchronized to telemetry format + Particle identification by hyperbolic discriminator thersholds (I1-I9) ** Dominated by RTG-induced background in quiet times" 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