CCSD3ZF0000100000001NJPL3IF0PDSX00000001 PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = ULY INSTRUMENT_ID = "COSPIN-HFT" OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "COSPIN-HIGH FLUX TELESCOPE" INSTRUMENT_TYPE = "CHARGED PARTICLE ANALYZER" INSTRUMENT_DESC = " (descriptions excerpted from [SIMPSONETAL1992A]) Instrument Overview =================== The HFT is designed: (a) to provide measurements of protons, helium, and heavier particles when the fluxes are too intense for the larger telescopes, and (b) to provide an instrument with exceptional azimuthal resolution so that highly collimated field-aligned particle distributions can be measured if they are encountered, especially over the solar poles and in Jupiter's magnetosphere. Detectors ========= The HFT, which is mounted on top of the HET, consists of a single 25 mm^2 x 18 [micro]m silicon surface-barrier detector, passively collimated by a fan-shaped aluminium collimator to give a viewing aperture of 17 [deg.] x 60 [deg.], with a geometrical factor of 0.033 cm^2-sr. The collimator imposes a low-energy cut-off of 50 MeV for protons and 5 MeV for electrons incident from outside the viewing aperture. The exposed surface of the detector carries an evaporated layer of aluminium 0.25 mg/cm^2 thick to exclude light and attenuate low- energy protons, which would cause serious pulse pile-up when the flux is large and the energy spectrum steep, as in the inner Jovian magnetosphere. The absorber excludes protons below 80 KeV and electrons below 10 KeV. There is a small sensitivity to directly incident sun light which slightly increases the detector leakage current when the instrument is viewing the Sun. The thinness of the detector, the high discriminator levels, and the fast shaping time constants used in the electronics make the HFT very insensitive to electrons. The detector output is amplified and shaped using double-differentiation time constants of 100 ns to provide bipolar pulses that can be handled without excessive loss or base-line shift at rates up to 10^6 per second. Five discriminator channels (designated F[1] - F[5]) set at levels corresponding to energy depositions between 0.20 and 36.0 MeV define the energy ranges given in Table 9. The F[1] discriminator can be set by command to any one of 16 threshold levels between 0.20 and 4.3 MeV. Alternatively, F[1] can be commanded into a stepping mode where the discriminator cycles continuously through the 16 levels at a rate of one step per data accumulation period (16 seconds at a telemetry data rate of 1024 bits/s). The stepped mode has been selected for normal mission operations. The F[2] discriminator can be set by command to two levels corresponding to 0.21 and 0.50 MeV, respectively. Channels F[3], F[4], and F[5] are set at levels 2.45, 8.36, and 36.0 MeV, respectively. F[1] levels 0-2 and F[2] levels 0-1 respond primarily to protons. F[1] levels 3-13 and F[3] respond primarily to helium. F[4] responds primarily to the CNO group. F[5] responds primarily to S and the Fe group. The F[2] output to the DPU can be scaled down by a factor four on command when it is expected that the rate may exceed the capability of the DPU interface unit, as during Jupiter encounter. The instrument includes an internal, dual range, 128-step pulse generator for in-flight calibration of the discriminator triggering levels. The instrument is mounted so that the collimator 'fan' plane lies parallel to the spacecraft spin axis. The central axis of the detector and the collimator is set at an angle of 45 [deg.] with respect to the Earth-pointing end of the spin axis so that the aperture extends from a spacecraft polar angle of 15 [deg.] to 75 [deg.]. All HFT counting rate accumulation intervals are spin synchronized using the Ulysses spacecraft sun reference pulse. Channels F[1] - F[5] are each accumulated for an integral number of spins every telemetry read out. The output of channel F[2] is also sector accumulated using 32 sectors per spacecraft rotation (11.25 [deg.] per sector), which permits the recognition of highly collimated angular distributions with a resolution of ~= 20 [deg.]. To conserve storage and telemetry facilities, eight equispaced sectors are successively accumulated for an integral number of spacecraft rotations having a nominal average period of 32 spacecraft minor frames. During the period over the solar poles and during an adjacent 18-month period, the angle between the assumed direction of the interplanetary magnetic field and the spin axis will lie in the range 40 [deg.] +/- 25 [deg.], so that the field direction will be included within the HFT aperture at some time during each spacecraft spin period. Operational Considerations ========================== At closest approach to Jupiter (~= 6 R[j]), from Pioneer 10/11 and Voyager 1/2 reports we estimate the electron intensity > 10 KeV to be ~= 10^8 cm^-2 s^-1 sr^-1, or 3 x 10^6 s^-1 within the acceptance angle of the telescope. This is on average ~= 0.3 particles per 100 ns, with an average deposit of 15 - 20 KeV per particle (the maximum energy deposit is ~= 60 KeV). The high order of pile-up required to trigger the 200 KeV discriminator makes electron contamination of this channel unlikely. Electron pile-up studies have been performed at NRC using a small electron accelerator facility. At L ~= 7 R[j] where the proton flux reaches a maximum, we estimate that for energies greater than a specified energy, E, and a dJ/dE [proportional to] E^-3 spectrum, the integral proton flux through the detector will be ~= 2 x 10^5 E^-2 cm^-2 s^-1 sr^-1 at energies around 1 MeV. If this energy dependence persists down to low energy (50 - 100 KeV), the proton pile-up would, in the absence of an absorber in front of the detector, saturate the 0.20 MeV discriminator, and this effect would be appreciable out to L = 10 - 12 (where L is the McIlwain parameter which is equivalent to R[j] at the equator). The presence of the 0.25 mg/cm^2 Al absorber significantly extends the intensity range where proton pile-up is unimportant. Since the alpha particle discriminator F[3] is set at 2.2 times the maximum energy deposition by protons, it requires a three- fold proton pile-up to simulate an alpha. At L = 7, the proton contamination of the F[3] alpha channel is estimated at less than 10% and for L > 9 it is negligible. In interplanetary space, the highest counting rates expected at the lowest discriminator levels are in the 10^3 - 10^4 s^-1 range, well below the level where pile-up or loss is significant. In-flight Performance ===================== The post-launch performance of the HFT has been nominal in all respects. Except for calibration periods, the HFT has been operated with the F[1] discriminator in stepped mode and the F[2] discriminator set low. Periodic spikes in the HFT detector current monitor channel were observed following instrument switch-on with a period of 4064 +/- 16 seconds. This effect is caused by sunlight incident on the detector. The period is a function of the HFT view cone, the spacecraft spin period and the 32 second sampling of the detector current. Some sensitivity to sunlight had been expected from bench tests prior to instrument delivery and it was a relief to find that the maximum observed amplitude (~ 80 nA) was well below the level which would cause any increase in system noise. It is possible to extend the proton and helium energy spectra derived from the other COSPIN sensors down to about 0.3 MeV/nucleon using the F[1] stepped discriminator channel. Using an assumed spectral form for the proton and helium differential energy spectra, the free parameters are determined by a best fit to the 16 F[1] counting rates. This analysis has been performed for the period ~ 1700 to 2400 UT on day 296 just after instrument switch-on and yields reasonably consistent flux values with the LET, HET, and KET. Measured Parameters =================== Table 9. High flux telescope data channels. Name Discr. Prim. Energy Species Avg. Time Sectors Discr. Part. Range* for Ener. Resolution Level Type (MeV(/n)) Range (s) J0 Z>=1 0.28-7.0 Protons J1 Z>=1 0.50-2.27 Protons J2 Z>=1 0.76-1.37 Protons 16/readout F1+ J4 Z>=2 0.36-3.9 Helium 256 for -- J6 Z>=2 0.51-2.26 Helium same disc. J8 Z>=2 0.64-1.66 Helium level J10 Z>=2 0.78-1.35 Helium F2 Lo Z>=1 0.29-6.7 Protons 16 -- Hi Z>=1 0.55-1.95 Protons F2s [same as for F2] 32 8 sampled from 32 128 32 F3 Z>=2 0.68-1.56 Helium 32 -- F4 Z>=3 0.8-4.9 Carbon 64 -- F5 Z>=12 0.8-34+ Iron 64 -- + F1 steps through 15 discriminator levels in sequence in standard flight mode * Energy ranges form range-energy relations" 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