Galileo Heavy Ion Counter Science Requirements Document California Institute of Technology, SPACE RADIATION LABORATORY Objectives and Overview The Galileo Heavy Ion Counter (HIC) was added onto the Galileo mission for the purpose of monitoring highly ionizing energetic particles capable of causing "single event upsets" in the memory chips, etc. of the spacecraft in the Jovian magnetosphere. The instrument is optimized to measure the energy spectra and charge composition of oxygen, sulfur, and sodium in the Jovian magnetosphere from ~ 5 MeV/nucleon to ~ 200 MeV/nucleon. We planned to monitor penetrating galactic carbon, nitrogen, and oxygen during the cruise phase and in the outer magnetosphere because these nuclei can also cause "upsets". The capability to cover a charge range up to Fe at low energies is desirable for the cruise phase and for the outer magnetosphere for scientific reasons. The radial distance range covered by Galileo extends from ~ 4 RJ to more than 100 RJ. The telescope is constrained to function in the flux maxima around 4 RJ and around 7-8 RJ. It should survive radiation damage from fluences calculated for mission lifetimes of more than a year. The instrument also had to be be built within cost, weight, power, volume, time, et cetera limitation imposed by its nature as an add-on to the existing mission. These constraints were met by using the existing Voyager CRS PTM (Proof-Test Model) instrument. The LET telescopes have demonstrated the ability to resolve O, Na, and S in the Jovian magnetosphere. Eight of these telescopes were exposed to the radiation environment with shielding of only 3 microns Aluminum and only one detector was lost. We adapted the instrument to extend its energy range and to improve its resolution by rerouting LET detector signals into the HET electronics and providing better collimation and a thicker window. To conserve telemetry and emphasize the heavies we raised all thresholds above proton and alpha signals. One of the parallel HET/LET "blocks" of electronics was extracted from the PTM and repackaged to meet volume and shielding constraints. Relevant flux levels are quite small. The sensitive area of one of the IC's used on Galileo is ~ 3.4 x 10^^-3 cm^^2. AW is ~ 0.01 cm^^2 sr. For a circuit to have a 2% chance of seeing an oxygen nucleus, the fluence is 2 Oxygen/cm^^2 sr. To observe 10 oxygens we must have a geometry factor of 5 cm^^2 sr. Thus one telescope, LET E, has a widened acceptance geometry. In order to interface signals to the Galileo spacecraft an adapter was added which receives the Voyager data and outputs Galileo data. This adapter also translates Galileo commands for the CRS Voyager interface. HIC is mounted on top of Bay 2 in the spinning portion of the spacecraft. This location is shown in the JPL Galileo Spacecraft Mechanical Configuration, drawing number 10084461. (See also JPL ICD 10086786.) The telescopes are oriented about 10 degrees "below" the normal to the spin axis, in order to keep the sunshade out of the field of view. In spacecraft coordinates, our boresight is TBD. Analysis Conditions Events are generated when particles generate signals satisfying the requirements shown below. Table 1 shows the "physically relevant" conditions for analysis of events of various types. Table 1 - Events Geometry Energy Range Event condition factor (MeV/nucleon) ____________________________________________________________________________ LETB LB1.LB2.LB3.LB4* 0.4293 ~4.8 to 17.5 for oxygen including L1.L2 DUBL LE1.LE2.LE3* 0.435 ~17-18 for oxygen TRPL LE1.LE2.LE3.LE4* 0.435 ~18-24 for oxygen WDSTP LE2.LE3.LE4.LE5* ~4.006 ~30**-48 for oxygen WDPEN LE2.LE3.LE4.LE5 ~4.006 ~48-185 for oxygen, cutoff at 185 due to LE1 threshold for oxygen but not sulfur. HGPEN LE2.LE3.LE4.LE5.HG ~4.006 Cosmic ray carbon and heavier >41MeV/nuc Read A.B.C* as A and B but not C **Energy assignment depends on whether L1 fired. The LET B Telescope The LET-B is a Voyager CRS spare LET with upgrading of the collimator. The cross section of the LET B telescope is illustrated in Figure 1. All detectors are silicon surface barrier devices. Signals from detectors LB1, LB2 and LB3 are analyzed, while LB4 is an anti-coincidence device. The geometry of LB1 and LB2 limits the viewing cone to 50 degrees; full angle. Detector areas and thickness are also given in Figure 1. LET B Detector Positions and Sizes height radius thickness path len thickness area Item (cm) (cm) mu-m (L1L2) material mu-m equiv Si (cm^2) ____________________________________________________________________________ WA 3.55 2.60 25 25.6 Kapton 20.0 21.237 LB1 0 0.9456 32.06 32.84 Si 2.8092 LB2 -4.08 0.9458 29.607 30.34 Si 2.8104 LB3 -4.3 1.114 421.24 432.21 Si 4.1151 LB4 -4.47 1.137 439.92 451.09 Si 4.0621 Height refers to the bottom of item The LET-E is modified from the Voyager LET by substitution of two thick surface barrier detector (LE4 and LE5) for the existing L4 and by much heavier collimation and shielding. The cross section of the telescope is shown in Figure 2. Detectors LE1, LE2 and LE3 are surface barrier devices; LE4 and LE5 are lithium drifted. Signals from all detectors are analyzed. LE1 and LE2 define a 50 degrees; viewing cone, while LE2 and LE5 define a 92 degrees; viewing cone in the wide angle mode. Detector areas and thicknesses are also given in Figure 2. LET E Telescope Detector Positions and Sizes height radius thickness path len thickness area Item (cm) (cm) mu-m (L1L2) material mu-m equiv Si (cm^2) _____________________________________________________________________________ WA 7.00 6.37 76 77.9 Kapton 60.0 127.48 WB 5.54 5.30 256.54 262.95 Al 289.5 88.247 LE1 4.08 0.9489 30.41 31.16 Si 2.8288 LE2 0 0.9491 33.41 34.24 Si 2.8302 LE3 -0.22 1.138 463.12 474.95 Si 4.0678 LE4 -0.65 2.19 2000 2050 Si 15. dead 54.2 54.5 Si LE5 -1.08 2.19 2000 2050 Si 15. dead 54.9 56.3 Si Height refers to the bottom of item Table 2 - Geometrical factors Items Geom half-angle (deg) ____________________________________________________ LB1,W 3.02 45 LB1,LB2 0.4293 24.87 LE1,WA 7.34 68 LE2,WA 4.006 46.27 LE1,LE2 0.435 24.95 LE2,LE3,LE4,LE5,WA 4.006 46.27 LE2,LE3,LE4,WA 4.006 46.27 Logic and Data Format The existing Voyager electronics was modified by rerouting preamp inputs and outputs so that LET E talks to the HET electronics and by making minor changes to some of the logic. In addition to this, the GEM (Galileo Element Monitor) adapter board (GAB) which interfaces the CRS-HIC to the Galileo spacecraft imposes some structure on the data. Figure 3 shows the structure of the electronics and the following sections document separately the changes and the new status of the electronics. Event Format Events consist of four 12-bit words: the tag word, PHA3, PHA2, and PHA1 (words 4 through 7 of the spacecraft minor frame, see Packets). All zero's are telemetered if no event is available. The format of the tag word is shown in Table 5 and contents of the PHA's is shown in Table and in Figure 4. Separate buffers are used for each of the five event modes, and readout of events progresses cyclically through the five buffers. The polling of buffers is done in the sequence LET B, WDPEN, DUBL, TRPL, LET B, WDPEN, WDSTP. Thus LET B and WDPEN events get more emphasis if all buffers are filling more rapidly than they can be readout. Table 5 Word Format HIC Contents of TAG word (word 4 of minor frame) bit number LET-E LET-B _________________________________________________________________ 4-1 LE4 slant (SLB) 4-2 LE1 LB3 4-3 LE5 LB2 4-4 LE3 LB1 4-5 slant(SB) 0 (DLA2) 4-6 LE2 DLB3(cmd 8-5) 4-7 0 DLB2(cmd 8-6) 4-8 HG 0 (DLA3) 4-9 buffer ind 1 4-10 buffer ind 0 4-11 1 (LET-E) 0 (LET-B) 4-12 caution flag caution flag (Note: original document contains no table 3 or 4) The caution flag indicates PHA overflow and/or gain switching in progress. The LET E buffer indicator (bits 9 and 10) has the following states: 4-9 4-10 4-9 4-10 0 0 DUBL 1 1 WDSTP 0 1 TRPL 1 0 WDPEN Most events will have a tag bit pattern from the following list: LB Triple & LET B cmd state Double DUBL TRPL WDPEN WDPEN wL1 WDSTP __________________________________________________ F48 B48 4C2 5C6 BCA FCA 9CE F08 B68 F68 Table 6 PHA Contents Mode PHA3 PHA2 PHA1 logic condition __________________________________________________ DUBL - LE1 LE2 LE1.LE2.LE3 TRPL LE3 LE1 LE2 LE1.LE2.LE3.LE4 WDSTP LE3 LE4 LE2 LE2.LE3.LE4.LE5 WDPEN LE3 LE4+LE5 LE2 LE2.LE3.LE4.LE5 LET B LB3 LB2 LB1 LB1.LB2.LB3.LB4 Rate Scalers Eight rate accumulators (numbered A through H) are used. In two of the accumulators (F and H), the input signals are subcommutated sixteen times. (The same subcom sequence controls status readout.) Table 7 shows rate readout as a function of accumulator letter and subcom state. Table 7 Rate Readout N = Readout Accumulation Subcom State A B C D E G F H State _______________________________________________________________________ 0 DUBL TRPL WDSTP WDPEN LETB LE1 SB SLB 2 1 DUBL TRPL WDSTP WDPEN LETB LE1 SB SLB 3 2 DUBL TRPL WDSTP WDPEN LETB LE1 SB SLB 4 3 DUBL TRPL WDSTP WDPEN LETB LE1 SB LBTRP 5 4 DUBL TRPL WDSTP WDPEN LETB LE1 SB SLB 6 5 DUBL TRPL WDSTP WDPEN LETB LE1 SB SLB 7 6 DUBL TRPL WDSTP WDPEN LETB LE1 SB SLB 8 7 DUBL TRPL WDSTP WDPEN LETB LE1 SB SLB 9 8 DUBL TRPL WDSTP WDPEN LETB LE1 LE5 LB1 10 (A) 9 DUBL TRPL WDSTP WDPEN LETB LE1 LE3 LB2 11 (B) 10(A) DUBL TRPL WDSTP WDPEN LETB LE1 LE4 LB3 12 (C) 11(B) DUBL TRPL WDSTP WDPEN LETB LE1 LE2 LB4 13 (D) 12(C) DUBL TRPL WDSTP WDPEN LETB LE1 SB SLB 14 (E) 13(D) DUBL TRPL WDSTP WDPEN LETB LE1 SB SLB 15 (F) 14(E) DUBL TRPL WDSTP WDPEN LETB LE1 SB SLB 0 15(F) DUBL TRPL WDSTP WDPEN LETB LE1 SB SLB 1 The first five rates are rates of "events" (an event is a triggering of the HIC by an energetic particle or the PHA and TAG data generated by such a triggering) as defined below, the remainder are singles rates from the detectors (LE1 - LE5, LB1 - LB4) or slant discriminators (SLB in LET B, SB in LET E). LBTRP is the rate of LB1.LB2.LB3.LB4* coincidences. The requirements for the various types of events are discussed in the event section. The spin of the spacecraft at 3 rpm nominal will allow calculation of anisotropies from the rate scalers after the fact. Note that the eight rate scalers are read every 2 seconds; ten times per spin. It will be necessary to understand the buffering delay to get the phase right (or perhaps vice-versa). Rate Compression Rate counts are accumulated in a 24-bit accumulator which is reset to the all-one's state. The first count (if any) increments the accumulator into the all-zero's state; the next increments to a single one; and so forth. At the end of the appropriate time interval (3 minor frames = 2 sec on Galileo) the contents of the accumulator are up-shifted until the most significant one bit is in position 24 (the MSB of the accumulator) or until 31 shifts have been done. The instrument then transmits five bits which specify the number of shifts and 7 bits which specify the 7 less significant bits of the 8 most significant bits in the up-shifted accumulator. The single MSB is known to be a 1. For example, zero counts leaves the accumulator at all-one's. Zero shifts are required to up-shift a 1 into position 24. All 8 MSB's are 1's. The transmitted number has 5 leading 0 bits and 7 trailing 1 bits (octal 0177). One count leaves the accumulator at all-zero's. Thirty-one up-shifts are done searching for a 1 but none is found. This case is the exception to the MSB = 1 rule. The transmitted number has 5 leading 1's and 7 trailing 0's (octal 7600). Two counts leave the accumulator at 1. Twenty-three shifts are required. The transmitted number is octal 5600. For 7200 counts (the internal calibrator) the accumulator state will be binary 1-1100000-11111 ; eleven shifts will be required and the italicized (between dashes) bits will form the mantissa. The result is 010111100000 in binary, 5E0 in hex, 2740 in octal. To decompress rates, the 7-bit mantissa is picked up and put in position 17 through 23 of a computer word with at least 24 bits (numbered 1 to 24). Bit number 24 is set to 1. The word is then down-shifted the number of times indicated by the 5-bit exponent. The word is then incremented to compensate for the all-one's reset state of the accumulator. In pseudo-FORTRAN notation, rate = (128+mantissa)*(2**16) rate = rate/2**exponent rate = rate + 1 Two exceptions must be checked for -- if rate = 0 or 1 (very common on the ground) then the algorithm fails. These cases are recognized and handled as indicated in the examples above. If the resulting rate is greater than 256, a better estimate for many data processing applications is obtained by using 128.5 +/- 0.5 instead of 128 in the formula above. For example, hex 5E0 decodes as 7185 +/- 16. Table 8 specifies examples for the smaller numbers likely to be encounted in flight. Table 8 Rate Compression Examples Raw Compressed Compressed Decompressed counts octal hex counts resolution __________________________________________________________________ 0 177 07f 0 1 1 7600 f80 1 1 2 5600 b80 2 1 3 5400 b00 3 1 4 5500 b40 4 1 5 5200 a80 5 1 6 5240 aa0 6 1 7 5300 ac0 7 1 8 5340 ae0 8 1 9 5000 a00 9 1 10 5020 a10 10 1 11 5040 a20 11 1 12 5060 a30 12 1 16 5160 a70 16 1 17 4600 980 17 1 32 4770 9f8 32 1 33 4400 900 33 1 34 4404 904 34 1 64 4574 97c 64 1 65 4200 880 65 1 128 4376 8fe 128 1 129 4000 800 129 1 130 4001 801 130 1 256 4177 87f 256 1 257 3600 780 257 2 258 3600 780 258 2 Command/Status Data A Galileo command consists of two 8-bit bytes sent to the GAB as documented in the JPL IRD 512335. These 16 bits are decoded by the GAB. The first bit is spare, the second indicates "cal start" (BC28CAL), the third indicates "high voltage on" (BC28HVON), and the fourth indicates that the following 12 are a serial command. These 12 bits are sent on to the CRS electronic as was done on Voyager. As before, these 12 bits are interpreted as a four-bit column number or register address and eight bits of data for that column. Table 10 shows the interpretation of each bit. Recall that commandable functions are also shown in the rate definitions in Table 6 by brackets. The JPL nomenclature indicated in parentheses above and in Table 9 must be used when speaking with them. The prefix BC is bus command; GEM/HIC is experiment number 28. When printing status data JPL/MTS uses hex. When specifying commands to be sent, they usually use binary for the eight bits of "data". Table 9 Command / Status Data Column # -> 0 2 6 8 12 13 bit # status only BC28E BC28PHA BC28ANAL BC28BP BC28MISC ______________________________________________________________________________ 5 (MSB) redundant LE1 preamp Delete LB3 High Voltage polling power off terms redundant enable ------------------------------------------------------------------------------ 6 LE2 preamp Delete LB2 Cal Stim power off terms disable ------------------------------------------------------------------------------ 7 High Voltage LE3 preamp Disable redundant enable power off WDSTP mode polling ------------------------------------------------------------------------------ 8 HET 2 gain LE4 preamp Disable Delete LB3 power off TRPL mode terms ------------------------------------------------------------------------------ 9 Delete LE4 LB4 preamp terms power off ------------------------------------------------------------------------------ 10 Cal Status LE5 preamp Disable Delete LE1 LB3 preamp Q3 (MSB) power off DUBL mode terms power off ------------------------------------------------------------------------------ 11 Cal Status Disable Delete LE2 LB2 preamp auto gain Q2 LET B terms power off ------------------------------------------------------------------------------ 12 (LSB) Cal Status Disable Delete LE1 LB1 preamp high gain Q1 WDPEN terms power off ______________________________________________________________________________ Analog Data Analog data is readout by the spacecraft via a multiplexed line. In the telemetry we receive an eight-bit "data number" (dn). The multiplexor is stepped by the adapter board once each 7 minor frames, but this sequence is not synched to the spacecraft major frame (rim) structure. Note that two step signals are required to step the multiplexer; it may well take 14 minor frames to switch states. The reset signal cannot be sent. Synchronism must be achieved by inspecting the data. The data consists of power supply voltages and temperatures as listed in Table 11. Table 11 Analog Signals number name nominal value description ______________________________________________________ 1 V+10 234 +10 volt power supply voltage 2 ZERO 0 unused and grounded 3 V+6 251 +6 volt power supply voltage 4 V+3 250 +3 volt power supply voltage 5 V-3 52 -3 volt power supply voltage 6 V-6 59 -6 volt power supply voltage 7 V-12 91 -12 volt power supply voltage 8 ZERO 0 unused and grounded 9 ZERO 0 unused and grounded 10 LOW 16 unused and held about 0.2 volts 11 LOW 16 unused and held about 0.2 volts 12 LOW 16 unused and held about 0.2 volts 13 LOW 16 unused and held about 0.2 volts 14 TLB 61 LET B temperature 15 TLE 60 LET E temperature 16 TPC 56 power converter temperature 17 ZERO 0 unused and grounded 18 TBP 58 baseplate temperature 19 TPHA 57 PHA electronics temperature 20 TTP 61 top plate temperature 21 ZERO 0 unused and grounded 22 ZERO 0 unused and grounded 23 ZERO 0 unused and grounded 24 ZERO 0 unused and grounded (Note: original document has no Table 10) The temperature calibrations for the multiplexed analog data are roughly given by Deg C = A0 + A1*(dn) + A2*(dn)^^2 + A3*(dn)^^3 where A0 ~ 67. A1 ~ -1. A2 ~ 5. * 10^^-3 A3 ~ -11. * 10^^-6 There is also a separate temperature transducer on the telescope housing which is not multiplexed and which is readout by the spacecraft. The JPL acronym is TTEMP. Its calibration is given by A0 ~ -102.45 A1 ~ +0.674666 A2 ~ 90.524 * 10^^-6 A3 ~ 0.0 Packets Do not confuse HIC instrument packets, described here, with CDS 02 telemetry packets, described in Galileo Project Doc. 625-205: 3-280, Phase 2 (available from JPL). The GAB issues a fixed sequence of word gates to CRS to create a particular mixture of rates, status, and PHA's which are then sent on to the Galileo spacecraft by GAB or HIC. A HIC instrument packet consists of three minor frames. Each minor frame consists of eight 12-bit words. The third minor frame of the three in a packet contains subcommutated rate and status data, with a subcom depth of 16. Thus an instrument cycle consists of 16 packets, numbered 0 through 15 by the four MSB's in the status word. Figure 6 illustrates the packet format. Error Protection Encoding The CRC word consists of 8 bits of actual CRC (the CRC character) and four trailing bits of zero's. The CRC character is generated in a 8-bit shift register which applies the encoding polynomial x**8 + x**7 + x**6 + 1. On the ground, where the error rate is negligible, the CRC should be checked but no data correction is necessary. The encoding circuit is illustrated in Figure 7. The following "subroutine" will perform the same encoding. initialize 84-element array x() to zero loop for n = 1 to 84 input = nth bit of 84-bit data stream x(0) = x(8) XOR input x(8) = x(7) XOR x(0) x(7) = x(6) XOR x(0) x(6) = x(5) x(5) = x(4) x(4) = x(3) x(3) = x(2) x(2) = x(1) x(1) = x(0) end of loop CRC = 128*x(8) + 64*x(7) + 32*x(6) + 16*x(5) + 8*x(4) + 4*x(3) + 2*x(2) + x(1) Synchronism There are some values which the 12-bit log compressed rate words never have (including F00 and 020) and others which are impossible in the Galileo application because the rapid readout (every 3 minor frames or every 2 seconds) means that we cannot accumulate more than about 100,000 counts in the accumulator. Thus if the first four bits of an rs word have value 1, 2, 3, 12, 13, or 14 the rs word is a status word and the index SCN must be 2 (possible values are 0, 1, 2) and mux state N is the value found. The following "subroutine" will determine all the HIC pointers after less than 30 minor frames. N is the value of the first four bits, and MUXN is the mux state, if ([N < 1] or [N >14]) return if ([N > 3] and [N < 12]) return if (LCN==3*N+2) return SCN = 2 MUXN = N LCN = 3*N + SCN return PHA Gain and Discriminator Levels The nominal gain and discriminator values are given below. Actual values should be requested from Thomas L. Garrard. Table 12 PHA Gain and Discriminator Values Detector Normal full Normal name scale (MeV) Discriminator (MeV) __________________________________________________________ LE1 307 9.6 LE2 307 2.0 LE3 2048 26. LE4 6144 120. LB1 307 0.5 LB2 307 0.4 LB4 50* 2.0 *Pre-amp full scale; not connected to analyzer LET E -- Slant SB low gain -> LE1 + LE2/2 + LE3/10 + (LE4 + LE5)/25 = 9.6 MeV LET B -- Slant SLB LB1 + 0.42*L2 + 0.20*L3 = 9.6 MeV Buffering Both events and rates go though one stage of buffering in the instrument and another in the GAB. Study the cal stim printouts to see the 2 minor frame delay in the readout here. More buffering must be done in the CDS (spacecraft); this is almost certainly one minor frame. This must be understood so that correlation or rate readout and angle from the AACS can be done. Figure 9 shows accumulation time and readout time of rate data. It also shows two extreme possibilities for event time and event readout; one event readout with minimal buffering delay and one with a maximum delay due to all six event polling buffers being full. The AACS is readout with no delay as shown in the figure.