CCSD3ZF0000100000001NJPL3IF0PDSX00000001 PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = ULY INSTRUMENT_ID = "COSPIN-KET" OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "COSPIN-KIEL ELECTRON TELESCOPE" INSTRUMENT_TYPE = "CHARGED PARTICLE ANALYZER" INSTRUMENT_DESC = " (descriptions excerpted from [SIMPSONETAL1992A]) Instrument Overview =================== The KET consists of two separate boxes, SIM3A and SIM3B, mounted on the other side of the spacecraft platform with respect to the SIM1 and SIM2 units. SIM3B is the telescope portion of the KET, and SIM3A contains the analogue and digital electronics for the KET. The KET is designed to measure electron fluxes between 2.5 and 6000 MeV, and to determine energy spectra in the range 7 - 170 MeV. The telescope also provides measurements of the proton and alpha-particle fluxes in several energy windows between 3 and > 2100 MeV/nucleon. In addition, two low-energy electron and proton channels provide anisotropy information in 8 sectors. The determination of electron spectra and their variation with distance from the sun and solar latitude provides vital information on the solar modulation of interstellar electrons as well as on acceleration and propagation of interplanetary and solar electrons. The measurement of protons and helium nuclei with the same telescope enhances confidence in the calibration and response of the telescope, and permits investigation of the dependence on particle species of the effects of solar distance and latitude. Detectors ========= To reduce mass and complexity of the instrument, the KET consists of two separate parts: the sensor (SIM3B) and the electronics box (SIM3A), both mounted beneath the spacecraft platform. The telescope is mounted to view perpendicular to the spin axis and has an acceptance angle of 44.6 degrees full cone with an auxiliary field of view of 106 degrees. Accordingly, for nuclei the geometric factor varies between 0.72 cm^2 sr for particles reaching D[2] and 6.5 cm^2 sr for particles stopping in D[1]. A more sophisticated evaluation of the geometric factor taking into account energy dependence and effects of scattering and shower production for electrons is shown in Table 10. The instrument incorporates several techniques in order to identify the particles and their energies: electron- photon cascades, Cherenkov thresholds, dE/dx versus E method, and discriminator settings. Functionally, the detector system divides naturally into two parts, consisting of an entrance telescope and a calorimeter surrounded by a guard counter. The entrance telescope is composed of a silica aerogel Cherenkov detector C[1] inserted between two surface-barrier semiconductor detectors D[1] and D[2]. Together with the guard counter A, it defines the geometry and selects singly-charged particles of high velocity ([beta] > 0.938) to discriminate between electrons and protons. In conventional designs the velocity discrimination is performed using a high-pressure-gas Cherenkov detector. The availability of a low- density solid material like silica aerogel, which can have a refractive index as low as 1.02 to 1.2, which is translucent and which shows a negligible scintillation contribution to the signal, allows us to circumvent the disadvantages of a gas Cherenkov detector, which include a significant weight penalty and additional material in the acceptance cone. For use in the KET, we have chosen an aerogel with a refractive index of n = 1.066. Since no use is made of the directionality of the Cherenkov light, the block of aerogel is placed in a diffusion box with millipore walls. The light signal of C[1] is viewed by the phototube PM1 through a hole in the guard counter, A. To prevent particles from hitting the photocathode of PM1 directly, and thus simulating an aerogel detector response, a scintillator disc S[1] has been introduced in front of PM1 as a veto counter. S[1] gives a signal for minimum ionizing particles 6 sigma above the signature of relativistic alpha particles in aerogel. The calorimeter consists of a lead-fluoride crystal C[1] in which the electron shower develops, and a scintillator cup S[2] to detect particles not absorbed in C[2]. C[2] has a thickness of 2.2 cm, corresponding to 2.5 radiation lengths. Lead-fluoride was chosen for its short radiation length (X[0] = 6.6 g/cm^2), its high density ([rho] = 7.7 g/cm^3), and its convenient refractive index (n = 1.885). Furthermore, it does not scintillate and is thus relatively insensitive to the RTG background. The Cherenkov light of C[2] is viewed through a hole in S[2] using a diffusion-box design. To prevent particles from escaping undetected through the hole in S[2], the planar part of the cup has been extended. The scintillator A not only helps to define the sensor geometry, but also protects from background caused by nuclear interactions produced in the telescope by cosmic rays and by neutrons from the RTG. If experience in flight shows that shower development in the calorimeter reduces the effective geometry factor to an unacceptably low value, the discriminator threshold of A can be raised by telecommand for penetrating electrons only (channel E[300]). To compensate for a possible phototube gain loss during the long mission lifetime, the high voltages can be stepped up by telecommand in eight steps. In the case of complete failure of a detector, it can logically be switched off by telecommand using the onboard failure reconfiguration logic. Stimulation of the onboard electronics for checkout purposes is also provided. The data produced by KET consist of pulse height analysis data and coincidence counting rates corresponding to broad energy ranges for incident particles. The ranges and time resolutions for the counting rates are shown in Table 10. Pulse height analysis is performed on a sample of the incident particle events to provide good energy resolution (between 60 and 100 % for electrons) and clear identification of particle species. Single-detector counting rates and housekeeping data are also telemetered to monitor instrument performance. Calibration =========== The KET instrument measures cosmic ray electrons with energies from a few MeV to several GeV, which have a flux in the ecliptic plane 3 to 4 orders of magnitude lower than the proton flux. The instrument response was therefore extensively calibrated with both electrons and protons. The most important goals of these calibrations were: - to determine the response of the individual detectors as a function of particle species and energy - to determine the detection thresholds for the individual detectors, especially the Cherenkov thresholds of C[1] and C[2] - to determine the energy thresholds for the coincidence counting rate channels - to provide correction tables for the counting rate data (response matrix) - to determine the electron detection efficiency of individual counting rate channels and of the complete instrument as a function of energy - to determine the proton rejection rate for electron channels - to use the calibration data to verify and adjust the mathematical model of the instrument - to determine the background in the various counting rate channels due to the RTG Results of proton calibrations ------------------------------ The main goals of the proton calibrations were to determine proper settings for i) the Cherenkov thresholds of C[1] and C[2], and ii) the discriminator threshold values C[10], C[20], and S[20], this last one being actually determined by the range of the particles in the telescope. In addition, these calibrations provided measurements of key parameters such as angular response and sensitivity of the anticoincidence which define the effective geometry factor of the instrument. Results of electron calibrations -------------------------------- These calibrations resulted in the measurements of: i) the response of the calorimeter C[2] as a function of energy, the monoenergetic response of C[2] being of fundamental importance to determine the electron spectrum from the flight data, and ii) the variation of the efficiency and response of the telescope as a function of energy and incidence angle. It was possible to determine relative variations of the response of the detector with energy and incidence angle of the beam. However, it proved to be extremely difficult to measure the absolute efficiency for electrons, particularly at low energies, where the use of external counters to monitor the beam is impossible because of the large scattering they induce. Therefore, only the relative efficiency could be reliably measured. Results of the RTG calibration tests ------------------------------------ The RTG calibration of the KET was performed in 1985 at Mound Laboratories using the Galileo Flight RTG (F[3]) and in 1986 and 1990 at KSC during the RTG compatibility tests as part of the two launch campaigns. The main results can be summarized as follows: - RTG induced background in the instrument leads to a deadtime increase from 0.003 % to 0.5 %, which is a tolerable value. The main contribution to the dead time stems from the scintillation guard counter with its large field of view. - From a scientific point of view, only the channels P[1] and E[4] are adversely affected, as the RTG background is of the order of the cosmic ray quiet flux in these KET particle channels, e.g. 1 x 10^-3 counts/s in E[4]. They will only produce meaningful data during solar flares and near Jupiter. Use of Cherenkov detectors and multifold coincidences guarantees the low susceptibility of all the other KET scientific channels. - Between the RTG tests in 1986 and 1990 the KET observed an increase of the RTG background by about a factor of 2. This is attributed to time variations in the plutonium decay chain. - Flight data measured during quiet times confirm the same background level in the RTG-sensitive channels P[1] and E[4] as observed during the 1990 RTG-test on ground. Simulation of the telescope --------------------------- In order to determine the absolute efficiency of KET for electrons, it was necessary to use a Monte-Carlo simulation of the instrument. The GEANT program, developed and widely used by particle physicists, was adapted for this purpose. In a first step, the different parameters of the telescope, namely thresholds and detector resolutions, were adjusted in order to reproduce the calibration data obtained at the different accelerators, with a parallel beam in the axis of the instrument. An excellent agreement between the simulation and calibration results was achieved. This agreement ensures that we can reliably determine the efficient geometry factor of KET for an isotropic flux, which was done according to the method given by Sullivan (Sullivan J.D., Nucl. Inst. and Meth. 95, 5, 1971). In-flight calibration --------------------- On COSPIN switch-on the currents and high voltage values were checked and found to be nominal. The in-flight test generator was used to check the amplifier gains and the functioning of the coincidence logic and of the failure mode reconfiguration. All of these were found to be nominal. To measure the performance of the detectors, the linearity of the analogue chains, and the discriminator thresholds, it is necessary to use the cosmic ray data themselves. Protons and helium nuclei with an energy above 2.1 GeV/n can be used for this purpose since they are minimum ionizing particles that penetrate the telescope while producing almost no nuclear interactions in the detectors, and are above the Cherenkov threshold C[10]. Their large flux allows collection in a few days of a sample with sufficient statistical accuracy to monitor all the detector responses and amplifier gains except for C[1] (for which no PHA value is registered). For P[4000] the in-flight responses are identical within a few percent to the responses derived from calibrations with a beam of 5 GeV/c protons. The position and width of the helium response for these detectors are also compatible with the expected values derived from the proton response. The C[1] PHA are registered only in the E[4] and E[12] channels, and therefore can be monitored only in these channels. The C[1] response for E[12] particles is compatible with the calibration results for electrons of 7.5 and 10 MeV. Measured Parameters =================== Table 10. Electron telescope data channels. Name Primary Energy Geometric Avg. Time Sectors PHA Particle Range Factor Resolution Type (MeV(/n)) (cm^2sr) (s) K1(P1) proton 2.7-5.4 6.5 128 -- -- K21-28(P4) proton 5.4-23.1 6.5 128 8 -- K3(P32) proton 34.1-125 0.72 128 -- P1 K34(P116) proton 125-320 1.2 128 -- P1 K12(P190) proton 320-2100 1.7 128 -- P0 K10(P4000) proton >2100 1.7 128 -- P0 K2(A4) He 6.0-20.4 6.5 128 -- -- K33(A32) He 34.2-125 0.72 128 -- P1 K29(A116) He 125-320 1.0 128 -- P1 K31(A190) He 320-2100 1.4 128 -- P0 K30(A4000) He >2100 1.4 128 -- P1 K13-20(E4) electron 2.5-7 0.26 128 8 P1 K11(E12) electron 7-170 0.40 128 -- P3 K32(E300) electron >170 0.38 128 -- P2" 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