CCSD3ZF0000100000001NJPL3IF0PDSX00000001 PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = ULY INSTRUMENT_ID = "COSPIN-AT" OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "COSPIN-ANISOTROPY TELESCOPE" INSTRUMENT_TYPE = "CHARGED PARTICLE ANALYZER" INSTRUMENT_DESC = " (the following descriptions are excerpted from [SIMPSONETAL1992A]) Instrument Overview =================== The ATs sensor unit consists of two, identical charged- particle telescopes, each with a geometrical factor of 0.75 cm^2 sr, whose role is to measure the three-dimensional charged-particle distribution in the energy ranges 0.7 to 2.2 MeV for Z >= 1, 2.2 to 6.5 MeV for protons, and 3.1 to 23.0 MeV for Z >= 2. The three dimensional distribution measurements are achieved by inclining the two telescopes at independent angles (AT1 at 145[deg.] and AT2 at 60[deg.]) to the spin axis of the spacecraft and sectoring (8 sectors) the data outputs of the telescopes during each spin. Detectors ========= The AT unit, which is situated at the top of the SIM-1 package, comprises two telescopes, each with independent electronics systems. [Each telescope] consists of a stack of three semiconductor silicon surface-barrier detectors surrounded by a passive aluminum collimator shield which defines the 70 deg. full-opening angle of the telescope. The front detector has a nominal thickness of 30 [micro]m and a sensitive area of 2.0 cm^2, while detectors B and C are each of thickness 300 [micro]m and have areas of 4.5 cm^2 each. The exposed surface of the front detector has an evaporated layer of aluminium of 60 [micro]m/cm^2 on it to reduce optical sensitivity, and has an 8 micron thick foil of aluminized kapton over it to eliminate optical effects and provide physical protection. It also eases the thermal balance problems of the telescope. All three detectors are operated at depletion voltage plus 50% to minimize radiation damage effects. The C detector is operated in anti-coincidence with the A and B detectors and therefore provides active shielding over the reverse cone of the telescope. The forward acceptance cones of the telescopes are defined by the passive aluminium collimator shield which imposes a minimum low energy cut-off of 20 MeV for protons and 1.35 MeV for electrons. This reduces contamination of the energy channels to < 10% for an omni-directional E^-2 differential spectrum, which is reasonable at these low energies. Should a differential spectrum of E^-1 be observed at energies below ~ 100 MeV, a contamination correction may be necessary. Inside the collimator are a series of matt black anodized baffles to reduce scattered sunlight and to scatter away energetic electrons which could be reflected onto the front detector. The electron and proton responses of the telescope are discussed below. Electronics =========== The outputs of the 3 detectors are combined to define a series of energy channels. There are two specific features that need to be commented on. One is the In-Flight Test Generator (IFTG) and the other is the Reconfiguration Logic. The IFTG can be activated by command to produce a series of pulses which check the functioning and stability of the discriminator levels via the amplifier chain. It has an automatic switch-off mode as well as a commanded-off mode. The Reconfiguration Logic is designed to minimize the effects of possible electronic failures during the long time scale of the Ulysses mission, and permits some modification of the channel logics. Since the A1 discriminator plays a key role in enabling the logic, if it should fail its role is automatically assumed by the A2 discriminator, albeit at the trigger level of A2. As a part of the independent electronics chains, each telescope has an independent command-receive system. The purpose of the digital data system is to organize the data suitably prior to it being read into the CPU. [In] essence, during any one spin of spacecraft the data are routed into the appropriate one of the eight equiangular sector accumulators, and a data sample is composed of an integral number of spacecraft spins. Thus the data-collection time corresponds to this integral number of spin periods and is asynchronous with the (fixed) telemetry sampling rate. The data system deals with this problem by suitably adjusting the integral number of spins per sample period to remain 'in-step' overall with the telemetry sampling period. This system is used to give some of the energy channels both spin averaged and sectored outputs. The sampling times shown are the average sampling periods, since the actual sampling periods corresponding to an integral number of spin periods. The sectoring for the Z >= 2 (3.1 to 7.2 MeV) channel is reduced to quadrants to reflect the expected lower counting rates at these energies. After processing by the data system, the data for each telescope is routed by its independent interface to one of the two processing units in the CPU for processing into the COSPIN data format. In contrast to other data channels in COSPIN, each AT is served exclusively by one of the redundant CPUs in the DPU. Calibration =========== The electron response of the front detector of a telescope is minimized by the use of a thin detector and a high discriminator level (equivalent to 300 keV) for channel A1. The 30 micron detector corresponds to an effective range for an electron of 66 keV. The 300 keV discriminator can only be triggered by a 5-fold 'pile-up' effect at this energy, or by electron 'straggle' effects of electrons of 300 keV. Calculation indicates that a 30 micron detector has an efficiency of < 10^-3 for 300 keV electrons. This is compatible with electron accelerator tests using 200 to 400 keV electrons from the Van de Graaf accelerator at Harwell which indicate a detection efficiency of ~ 2 x 10^-5 at these energies. Additional electron accelerator tests have been done on the electron accelerator at the Herzberg Institute in Ottawa. These were performed at an energy of 65 keV to try to measure 'pile- up' effects for electrons just coming to rest in the front detector. Interpretation of these results has proved complex, and a computer simulation of the electronics system is being used to produce a model that relates observed rates to actual rates. This is obviously an important tool for interpreting results during the Jupiter fly-by. To calibrate channels A1 to A4 for particles with Z >= 1, tests were done on the IBIS accelerator at AERE, Harwell, which gave protons up to 3 MeV. The minimum beam energy obtainable was 0.65 Mev. The 'mid-point' energies of the measured thresholds have been used to define the energy ranges given in Table 5. The differences between calculated and measured values were small and above 2 MeV the measured edge agreed with calculated value. Pre-launch calibration and test of the AT amplifiers was done using calibrated test generators, and the amplifiers are checked using the IFTG discussed above. In-flight Performance ===================== The instrument performance since switch-on has been good. The highest background counting rate, due to system noise, RTG induced counts and the cosmic ray background is in channel 1 (0.7 to 0.9 MeV, Z >= 1), and corresponds to 0.05 particles cm^-2 sr^-1 s^-1 for either AT1 or AT2. The background rates in the other channels are between a factor of 1.7 to 6 less than this. Overall the AT telescopes are providing satisfactory performance. During [a] solar particle event starting on day 296, 1990, once particle isotropy had been established, the spin averaged counting rates observed by the two telescopes tracked each other closely. Similarly the spin averaged energy spectra and sectored counting rates also compared closely. This is also true of the other events observed. Of more interest is the behavior during the initial stages of an event. Figure 14 shows the sectored counting rates at a time when the particle intensity was still rising. AT2 (looking sunward, in the direction of the spin axis) saw a markedly anisotropic distribution and a significantly higher intensity than telescope AT1 (viewing 'backscattered' particles) which saw a lower intensity, largely isotropic distribution. For proton spectra derived from a fit to the integrated fluxes in the energy ranges of the AT channels, neither a power law nor an exponential in energy gives a good fit for this event. Measured Parameters =================== TABLE 5. Anisotropy telescopes data channels (identical channels defined for each telescope). Name Primary Energy Geometric Avg. Time Sectors Particle Range Factor Resolution Type (MeV) (cm^2sr) (s) A1 Z>=1 0.7-0.9 0.75 16 -- A2 Z>=1 0.9-1.3 0.75 16 -- A3 Z>=1 1.3-2.2 0.75 16 -- A4 proton 2.2-3.6 0.75 16 -- A5 proton 3.6-6.5 0.75 16 -- A38 Z>=2 3.1-7.2 0.75 128 -- A39 Z>=2 7.2-12 0.75 128 -- A40 Z>=2 12-23 0.75 128 -- A6-A13 Z>=1 0.7-1.3 0.75 16 8 A14-A21 Z>=1 1.3-2.2 0.75 64 8 A22-A29 proton 2.2-3.6 0.75 64 8 A30-A37 proton 3.6-6.5 0.75 64 8 A41-A44 Z>=2 3.1-7.2 0.75 128 4 A45-A47 -- -- -- 128 --" 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