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Specialized detector techniques and electronics of the ERNE instrument onboard SOHO
PROCEEDINGS SUPPLEMENTS ELSEVIER
Nuclear Physics B (Proc. Suppl.) 61B (1998) 6-11
Specialized detector techniques and electronics of the ERNE instrument onboard SOHO E. Valtonen, J. Peltonen', T. Eronen, M. Louhola, M. Lumme, E. Riihonen, M. Teittinen, and J. Torsti Space Research Laboratory, Department of Physics, FIN-20014 University of Turku
ERNE is designed to study the composition and energy spectra of particles encountered in interplanetary space in the energy range from 1 MeV/n to well beyond 500 MeV/n. Several innovative ideas had to be incorporated in the design of the instrument in order to fulfill the scientific requirements. Position-sensitive strip detectors are used in the High Energy Detector (HED) for determining particle trajectories. Integrated interstrip capacitances were designed to allow for a very simple read-out technique from the two edges of each detector. The event recognition, signal multiplexing and control of the pulse height analysis are based on a gate array technique. The versatility of the gate array also allowed an elegant realization of a number of other functions and instrument control tasks. By using the gate array a very compact structure of the digital control electronics was achieved.
I. INTRODUCTION SOHO is a cooperative mission between ESA and NASA. The SOHO spacecraft carries twelve scientific instruments with the common primary goal of investigating the Sun and the heliosphere by various techniques, from the deep interiors of the Sun to the far-out boundaries of the heliosphere [1]. ERNE is one of the three particle instruments onboard. ERNE measures the composition and energy spectra of particles encountered in interplanetary space in the energy range from 1 MeV/n to well beyond 500 MeV/n. Ions from hydrogen to zinc, originating from the Sun, local interstellar space or distant galactic sources, will be identified. Abundances of isotopes of many elements not measured before, will be determined. The main scientific objectives of the ERNE experiment are to investigate the particle acceleration processes at the Sun and in interplanetary space, to study the heating mechanisms of the solar corona, and physical properties and processes taking place in the solar atmosphere. A general description of the instrument and its scientific goals has earlier been given by Torsti et al. [2]. * Presently at the Centre for Biotechnology, University of Turku 0920-5632/98/$19.00 © 1998 Elsevier Science B.V All rights reserved. PII S0920-5632(97)00532-X
ERNE instrument is a new design without a heritage from previous space experiments. Therefore, in many respects unique solutions have been used both in the sensors and in the electronics. The design of ERNE was started in 1986. The main development phase took place through 1988 to 1990. All the basic solutions in the architecture of the sensors and the electronics were adopted at that time. In early 1991 the design was frozen, after which only minor modifications were possible.
2. DESIGN OVERVIEW
2.1. General Several innovative ideas had to be incorporated in the design of the instrument in order to fulfill the scientific requirements. New detectors were developed and high integration levels of analog and digital electronics were employed by hybridization and by a gate array technique. The technical prerequisites arising from the scientific objectives combined with the limitations in the allocated power (7 W), mass (9 kg) and telemetry rate (0.8 kb/s) made the design particularly demanding.
E. Valtonen et aL /Nuclear Physics B (Proc. Suppl.) 61B (1998) 6-11
2.2. Sensors Taking into account the ion species and energy ranges to be covered, the simplest solution was to construct two particle telescopes. One of the sensors (LED) is a relatively simple 3-layer silicon telescope operating roughly in the energy range 1-10 MeV/n. The sensor operating in the high-energy range (HED) consists of altogether nine detector layers, eight of which are used for energy measurements and one as an anticoincidenece detector distinguishing particles penetrating through the entire sensor. Both silicon detectors and scintillators are employed. Position-sensitive silicon strip detectors are used in the first four layers. A special read-out technique was developed, based on increased interstrip capacitance, requiring only two amplifier channels per detector. In each of the four layers there are two detectors in parallel increasing the solid angle of the view cone and providing redundancy. In addition to measuring the energy losses, an important: task of the strip detectors is to determine the trajectories of incident particles. The four strip detector layers are configured to give the coordinates of two well separated points along the particle tracks, from which the angles of incidence are determined. This is essential information in order to achieve a good mass resolution, and at the same time gives a directional sensitivity for the sensor. The details of the strip detector structure are presented in Section 3. 2.3. Electronics The charge created by a particle in the detectors is read out by charge-sensitive preamplifiers. The charge amplifiers are followed by filter amplifiers providing signal shaping with two time constants, referred to as slow and fast. The slow signals are used for pulse height analysis. Due to the large dynamic range of the instrument, amplifier channels with three different gain ranges are required. This makes up altogether 60 slow signal channels in HED (Fig. 1), from which 11 for each acceptable event are to be selected for pulse height analysis. The fast signals, created from the pre-amplifier pulses by shaping with a short time constant, are used for controlling the slow signal selection for pulse height analysis. These signals are also used to indicate the rough level of energy loss in various
7
detectors or combinations of detectors, which again results in a wide dynamic range requirement. Amplifier channels with two gain ranges are applied. In HED this sums up to 26 fast signal channels (Fig. 1).
HED
DISC
__8i ..' s2Ls2J-%>: i MUX 4:
i F---q DR
AT'
i i ICTRL
AMP1&2
I I to ~
,coco
SMPLI&2
Dl=discriminators CTRS=counters DR=dataregister MUX CTRL=analog mux control PD=peak detectors SMPL=sampling A/D=analog-to-digital converter i~P=microprocessor Figure I. Basic functional structure of the HED electronics. The dashed lines indicate division in different printed circuit boards. In HED the total number of amplifier channels thus rises to 86. Due to limitations in space and mass, inclusion of such a high number of amplifiers would not have been possible without hybridization. The charge-sensitive preamplifiers used in ERNE are commercial types (Amptek A-250), whereas the filter amplifier hybrids were designed and manufactured within the ERNE project (Technical Research Centre of Finland). As a result, all the HED amplifiers fit on two printed circuit boards with dimensions of 234 mm x 138 mm, as indicated in Figure 1 (AMPI&2). All functions of the HED electronics related to the event recognition and pulse height analysis are located on one printed circuit board. Due to the vital role of the board, it is fully redundant, but with only one board powered at a time (SMPLI&2 in Fig. 1). Concentrating all the control functions on one board was only possible by using a gate array containing all the logic circuitry. The logic pulses obtained from the discriminators (DISC in Fig. 1) are received and processed by the gate array. These pulses correspond to the presence or absence of various detector signals or indicate exceedings of certain
8
E. Valtonen et al./Nuclear Physics B (Proc. Suppl.) 61B (1998) 6-11
pre-determined energy loss levels in a detector or in a combination of detectors. The basic tasks of the gate array are the event recognition and the control of slow signal selection and multiplexing for pulse height analysis. The decision procedure and signal selection is based on the event pattern generated from the discriminator signals. The purpose is to recognize a valid event and to select active detectors with appropriate gain ranges for analog-to-digital conversion. For acceptable events, the pattern is also stored in a status register and associated with the final pulse height information. In addition to these basic tasks, the versatility of the gate array is utilized in several other ways, as described in Section 4. An efficient MAS-281 processor is used for the overall control of the instrument. After completion of the pulse height analysis of an accepted event the gate array sends an interrupt request to the processor (DPU in Fig. 1). The DPU then reads out the results of the analog-to-digital conversion and the associated status word from the gate array data/status register. The data is further analyzed by the on-board software, and the light particles up to 4He are identified and classified according to their energies. The on-board analysis is an essential function, allowing a much larger amount of information to be telemetered to ground within the allocated telemetry rate.
a guard ring connected to the detector bias voltage during operation. The contact pads are connected to the two edge strips (Fig. 2 b). Having the contact possibility at both ends makes the assembling more convenient. The electrical connections from the detectors to the preamplifiers are achieved by using conductive rubber. Therefore, relatively large contact pads were necessary to ease the assembling. Total length: 82000 Total width: 40500
Guard.n9 33 strips: a) I
Pitch 1000 Width 900 Length 70000
Guard ring
1 P
Scribe line
Resistor
3. S T R I P D E T E C T O R S In the initial design of the ERNE sensors, twosided strip detectors were planned to be used in HED. However, due to obvious difficulties and uncertainties in the production of such detectors in the late 1980's, the simpler one-sided devices were adopted for the final design. They were manufactured by Canberra Semiconductor N.V., Belgium. The simple pattern of the p+ implantation of the HED strip detectors is shown in Figure 2 a. The overall dimensions of the n-type substrate are 82.0 mm x 40.5 mm. Each detector has 33 p+ implanted strips with a length of 70.0 mm, width 0.900 mm and pitch 1.000 mm. Thereby, the dimensions of the active area of the detector become 33.0 mm x 70.0 mm. The active area, including the signal contact pads at both ends of the detector, are surrounded by
I--I p+ implantation i i i Contact to Si (SiO2 etch)
Aluminum m
Polysilicon resistor
b) Figure 2. a) p+ implantation pattern of the HED strip detector, b) a detail of one comer of the detector showing the various layers and structures. As a consequence of severe power limitations, the strip detectors had to be designed in a way to allow for a very simple read-out technique in terms
E. Valtonen et al./Nuclear Physics B (Proc. Suppl.) 61B (1998) 6 11
of the number of required amplifier channels. The method adopted was to employ capacitive charge transfer between the strips and perform the read-out from the two edge strips of a detector only. To reach efficient charge transfer, the capacitive coupling of the strips was increased by processing structures creating large interstrip capacitances. This was achieved by using an aluminum layer, in electrical contact with a p+ implantation strip, but for the most part extending over the area of the neighboring strip p+ implantation, and electrically insulated from it by a very thin silicone oxide layer. A detail of one corner of the detector is presented in Figure 2 b revealing the various structures. The black lines indicate the p+ implantation pattern, being the layer deepest in the detector bulk. The dark gray areas are the openings in the silicon oxide layer allowing for the contacts to the implantation layers. Excluding the first strip, these openings are mainly at the lower edges of the strip implantations in Figure 2 b. Over these structures, aluminum was evaporated in a pattern shown by the light gray areas in Figure 2 b. The goal was to reach as large a capacitance between the neighboring strips as possible. An isolating oxide layer thickness of 150 nm was used. In order to minimize the possibility of short circuits, e.g., through pin holes, the oxide processing was made in two phases. With this method a nominal, specified capacitance of 11 nF between the strips was expected. The measured values in the delivered detectors varied between 12.0 nF and 15.5 nF, with an almost constant value in a single detector and with relatively small variations in the same manufacturing batch. In addition to the integrated interstrip capacitances, the HED strip detectors have integrated polysilicon bias resistors as indicated in Figure 2 b. Between each strip and the bias line a resistor with a nominal value of 4 Mr2 has been processed. The tolerance was specified as + 20 %. A contact pad as shown in Figure 2 b allows testing of each resistor. In the incoming inspections the resistance values were generally found to be in the range 3.6 Mr2 - 4.4 MR with a few exceptions being at the limits of the tolerances. The bias line, made of evaporated aluminum, circles around the lower part of the detector to the other end of the strips. Similarly to the signal connections, conductive rubber is used to connect bias voltage to the
9
detector, and for convenience this can be done at either end of the detector. At the lower edge of the detector, not visible in Figure 2 b, the guard ring is connected to the bias line through its own bias resistor. In principle, with the read-out method used, the total energy loss in the strip detector is obtained as the sum of the signals from the two edges, and the position as the ratio of one of the edge signals to the sum. However, due to inefficiencies in the charge transfer in the detector, some nonlinearities are introduced. The ratio of interstrip capacitance to that between a strip and the backplane of the detector is about 700. This results in a reasonably efficient charge transfer from strip to strip, producing a smooth variation of the measured total energy with position having a minimum at the centre of the detector and deviating by about 15-20 % from the value measured at one of the edge strips. To obtain the best resolution in the measured position and total energy, the charge transfer characteristics of each individual detector has to be known. To achieve this, extensive simulations and beam tests of the detectors were carried out. The main emphasis was to develop correction algorithms for the measured total energy loss. The modeling was mainly performed in order to understand the effects of changing detector properties on the charge transfer, which might be encountered during the flight. The model developed was able to well reproduce the measured position dependence of the total energy loss. The beam tests of the HED strip detectors were carried out at the I.S.N. in Grenoble by using a nickel beam colliding in a target.. The results were used to optimize the model parameters describing the detector behavior. Tables were created and are used by the on-board software to convert the measured strip detector pulse heights to coordinates, and further to obtain correction factors by which the total energy loss signals are to be multiplied in order to compensate for the attenuation of the charge pulses in the detector. In the course of the analysis of the beam tests, methods were also developed, which can be used to re-calibrate the detector responses by using the flight data.
10
E. Valtonen et al./Nuclear Physics B (Proc. Suppl.) 6IB (1998) 6 11
4. G A T E ARRAY
combination is examined. If a valid event is recognized, a certain priority will be given to it. Subsequently, the analog multiplexer control sets multiplexer addresses selecting the active detectors with appropriate gain ranges. The sequence control reads the output of the priority control, and in the case of a valid event starts a sequence, which ultimately leads to a DPU interruption to read the pulse height data of the event from the gate array data register. The sequence control starts the sequencer, a state machine controlling all the functions of the peak detectors and the ADconverter. At the same time, the sequence control latches the status information in the status register and prevents further sampling of the discriminator signals during the active sequence. As the result of a complete sequence, the data register contains the digitized pulse heights, and the contents of the status register defines the active and passive discriminator signals describing the nature of the recorded event. The entire sequence takes about 13 ps. A new sequence can start after the DPU has read the contents of the data and status registers.
As briefly described in Section 2.3, all digital control functions of ERNE are concentrated in a gate array. The individual sensor electronics of LED and HED have their own gate arrays with similar functions. Only one gate array design was made, but with the ability to operate in two modes corresponding to the slightly different needs of LED and HED. The mode of operation can be selected by an external pin. The functions of the gate array can be studied from the block diagram shown in Figure 3. When the first signal exceeding the detection level in any detector is received by the discriminator control block, a time window of 1250 ns is opened. The signals from all the detectors received during this time window are stored in an input register and the status of all the discriminator signal lines fed in a status register. At the end of the time window, the recorded input data is sampled. A new time window can open 125 ns after the preceding one. In the priority block the validity of the recorded signal
Mux/Sample Controls
TT Analog Mux Control
Discr. I/F
IZ-"
Discriminator Control
.
Power Controls
Sequencer
T
Priority
J
I d Sequence . Control ~
I
"
h
l,
Control Register
1
Block
I
~ m
Counter functions and Counter Controls
Figure 3. Functional diagram of the gate array.
T
D, "1 Re, ter
I
,nteac i DPU I/F
E. Valtonen et al./Nuclear Physics B (Proc. Suppl.) 61B (1998) 6-.11
Employing the gate array technique has given the opportunity to easily include several useful feala~res in the electronics. In interplanetary space most of the particles are protons or helium nuclei. On the other hand, the abundances of heavier ions and rare isotopes have significant scientific interest. During solar flares the event rates are expected to be very high, and it might happen that due to their high intensity, only protons and some helium would be recorded. To prevent this, a three-level priority system was developed ensuring that during high flux times all heavy ions will be collected and pulse height analyzed. The system relies on an interactive operation of the gate array with the onboard software. The principle is very simple and is based on the recognition of the priority level of an event by the gate array, and on the division of the th low priority event rates, allowing only every n event to start the AD-conversion sequence. The value of the divider is determined by the on-board software based on the instantaneous flux level, and set by writing in the control register of the gate array. A further task of the gate array is to generate logic functions of the discriminator signals corresponding to protons, helium or heavier ions at selected energy ranges. The energy ranges are basically determined by the number of detectors traversed and the particles identified by their energy losses in successive detector layers, for which a number of discriminator thresholds have been defined. The logic functions can be combinations of either active or non-active detectors. The true or false status of the counter functions are sampled at the end of each initial 1250 ns time window started by the first detected detector signal, and the counters are incremented accordingly. There are 16 24-bit external counters in HED. The contents of the counters are read out serially to the gate array data register by the DPU command, and subsequently to the DPU buffers for further treatment. The advantage of having the hardware counters is that during high flux conditions the counters are updated during each open time window, irrespective of the status of an AD-conversion sequence. The gate array enables an easy and flexible reconfiguring of the sensors. The discriminator signals of any of the detectors can be disconnected
I1
from the rest of the electronics. This is achieved by writing in the control register by using ground commands, which then reprograms the discriminator control block. Thereby, a faulty or noisy detector can be isolated. After disconnection, the corresponding signal can be permanently set to either active or non-active state. In addition, in the counter functions a disconnected signal is replaced by a signal level, which best restores the original function in spite of the missing real signal. Another action that can be taken in case of noisy detectors, due to, e.g., increased leakage currents, is to raise the minimum signal level interpreted as a particle hit in a detector to three times the nominal value. This is also achieved by writing in the control registers of the gate array. Finally, in HED two different event conditions have been defined. The nominal event condition corresponds to protons or heavier ions penetrating through at least the three first strip detectors and giving a signal from the fourth. The additional trigger condition corresponds to electrons only, stopping in either of the scintillators. The two event conditions can be enabled simultaneously or the electron condition can be inhibited by the gate array. The gate array was designed by using the rules applicable for radiation hard devices. The design was made by Smartech Oy, Finland. The total number of gates needed was 7430 in a MC10K matrix. The processing and testing were performed according to the MIL-STD-833 Class B, Group D classification. The manufacturer was Matra MHS, France, by using a 1 micron/2 metal technology on an epitaxial latch-up free substrate. The process had been tested and guaranteed to stand a cumulative dose up to 85 krads without functional failures.
REFERENCES 1. V. Domingo, B. Fleck, and A. Poland, Solar Physics 162 (1995) 1. 2. J. Torsti, E. Valtonen, M. Lumme, P. Peltonen, T. Eronen, M. Louhola, E. Riihonen, G. Schultz, M. Teittinen, K. Ahola, C. Holmlund, V. Kelh~i, K. Leppala, P. Ruuska, and E. Str0mmer, Solar Physics 162 (1995) 505.
Nuclear Physics B (Proc. Suppl.) 61B (1998) 6-11
Specialized detector techniques and electronics of the ERNE instrument onboard SOHO E. Valtonen, J. Peltonen', T. Eronen, M. Louhola, M. Lumme, E. Riihonen, M. Teittinen, and J. Torsti Space Research Laboratory, Department of Physics, FIN-20014 University of Turku
ERNE is designed to study the composition and energy spectra of particles encountered in interplanetary space in the energy range from 1 MeV/n to well beyond 500 MeV/n. Several innovative ideas had to be incorporated in the design of the instrument in order to fulfill the scientific requirements. Position-sensitive strip detectors are used in the High Energy Detector (HED) for determining particle trajectories. Integrated interstrip capacitances were designed to allow for a very simple read-out technique from the two edges of each detector. The event recognition, signal multiplexing and control of the pulse height analysis are based on a gate array technique. The versatility of the gate array also allowed an elegant realization of a number of other functions and instrument control tasks. By using the gate array a very compact structure of the digital control electronics was achieved.
I. INTRODUCTION SOHO is a cooperative mission between ESA and NASA. The SOHO spacecraft carries twelve scientific instruments with the common primary goal of investigating the Sun and the heliosphere by various techniques, from the deep interiors of the Sun to the far-out boundaries of the heliosphere [1]. ERNE is one of the three particle instruments onboard. ERNE measures the composition and energy spectra of particles encountered in interplanetary space in the energy range from 1 MeV/n to well beyond 500 MeV/n. Ions from hydrogen to zinc, originating from the Sun, local interstellar space or distant galactic sources, will be identified. Abundances of isotopes of many elements not measured before, will be determined. The main scientific objectives of the ERNE experiment are to investigate the particle acceleration processes at the Sun and in interplanetary space, to study the heating mechanisms of the solar corona, and physical properties and processes taking place in the solar atmosphere. A general description of the instrument and its scientific goals has earlier been given by Torsti et al. [2]. * Presently at the Centre for Biotechnology, University of Turku 0920-5632/98/$19.00 © 1998 Elsevier Science B.V All rights reserved. PII S0920-5632(97)00532-X
ERNE instrument is a new design without a heritage from previous space experiments. Therefore, in many respects unique solutions have been used both in the sensors and in the electronics. The design of ERNE was started in 1986. The main development phase took place through 1988 to 1990. All the basic solutions in the architecture of the sensors and the electronics were adopted at that time. In early 1991 the design was frozen, after which only minor modifications were possible.
2. DESIGN OVERVIEW
2.1. General Several innovative ideas had to be incorporated in the design of the instrument in order to fulfill the scientific requirements. New detectors were developed and high integration levels of analog and digital electronics were employed by hybridization and by a gate array technique. The technical prerequisites arising from the scientific objectives combined with the limitations in the allocated power (7 W), mass (9 kg) and telemetry rate (0.8 kb/s) made the design particularly demanding.
E. Valtonen et aL /Nuclear Physics B (Proc. Suppl.) 61B (1998) 6-11
2.2. Sensors Taking into account the ion species and energy ranges to be covered, the simplest solution was to construct two particle telescopes. One of the sensors (LED) is a relatively simple 3-layer silicon telescope operating roughly in the energy range 1-10 MeV/n. The sensor operating in the high-energy range (HED) consists of altogether nine detector layers, eight of which are used for energy measurements and one as an anticoincidenece detector distinguishing particles penetrating through the entire sensor. Both silicon detectors and scintillators are employed. Position-sensitive silicon strip detectors are used in the first four layers. A special read-out technique was developed, based on increased interstrip capacitance, requiring only two amplifier channels per detector. In each of the four layers there are two detectors in parallel increasing the solid angle of the view cone and providing redundancy. In addition to measuring the energy losses, an important: task of the strip detectors is to determine the trajectories of incident particles. The four strip detector layers are configured to give the coordinates of two well separated points along the particle tracks, from which the angles of incidence are determined. This is essential information in order to achieve a good mass resolution, and at the same time gives a directional sensitivity for the sensor. The details of the strip detector structure are presented in Section 3. 2.3. Electronics The charge created by a particle in the detectors is read out by charge-sensitive preamplifiers. The charge amplifiers are followed by filter amplifiers providing signal shaping with two time constants, referred to as slow and fast. The slow signals are used for pulse height analysis. Due to the large dynamic range of the instrument, amplifier channels with three different gain ranges are required. This makes up altogether 60 slow signal channels in HED (Fig. 1), from which 11 for each acceptable event are to be selected for pulse height analysis. The fast signals, created from the pre-amplifier pulses by shaping with a short time constant, are used for controlling the slow signal selection for pulse height analysis. These signals are also used to indicate the rough level of energy loss in various
7
detectors or combinations of detectors, which again results in a wide dynamic range requirement. Amplifier channels with two gain ranges are applied. In HED this sums up to 26 fast signal channels (Fig. 1).
HED
DISC
__8i ..' s2Ls2J-%>: i MUX 4:
i F---q DR
AT'
i i ICTRL
AMP1&2
I I to ~
,coco
SMPLI&2
Dl=discriminators CTRS=counters DR=dataregister MUX CTRL=analog mux control PD=peak detectors SMPL=sampling A/D=analog-to-digital converter i~P=microprocessor Figure I. Basic functional structure of the HED electronics. The dashed lines indicate division in different printed circuit boards. In HED the total number of amplifier channels thus rises to 86. Due to limitations in space and mass, inclusion of such a high number of amplifiers would not have been possible without hybridization. The charge-sensitive preamplifiers used in ERNE are commercial types (Amptek A-250), whereas the filter amplifier hybrids were designed and manufactured within the ERNE project (Technical Research Centre of Finland). As a result, all the HED amplifiers fit on two printed circuit boards with dimensions of 234 mm x 138 mm, as indicated in Figure 1 (AMPI&2). All functions of the HED electronics related to the event recognition and pulse height analysis are located on one printed circuit board. Due to the vital role of the board, it is fully redundant, but with only one board powered at a time (SMPLI&2 in Fig. 1). Concentrating all the control functions on one board was only possible by using a gate array containing all the logic circuitry. The logic pulses obtained from the discriminators (DISC in Fig. 1) are received and processed by the gate array. These pulses correspond to the presence or absence of various detector signals or indicate exceedings of certain
8
E. Valtonen et al./Nuclear Physics B (Proc. Suppl.) 61B (1998) 6-11
pre-determined energy loss levels in a detector or in a combination of detectors. The basic tasks of the gate array are the event recognition and the control of slow signal selection and multiplexing for pulse height analysis. The decision procedure and signal selection is based on the event pattern generated from the discriminator signals. The purpose is to recognize a valid event and to select active detectors with appropriate gain ranges for analog-to-digital conversion. For acceptable events, the pattern is also stored in a status register and associated with the final pulse height information. In addition to these basic tasks, the versatility of the gate array is utilized in several other ways, as described in Section 4. An efficient MAS-281 processor is used for the overall control of the instrument. After completion of the pulse height analysis of an accepted event the gate array sends an interrupt request to the processor (DPU in Fig. 1). The DPU then reads out the results of the analog-to-digital conversion and the associated status word from the gate array data/status register. The data is further analyzed by the on-board software, and the light particles up to 4He are identified and classified according to their energies. The on-board analysis is an essential function, allowing a much larger amount of information to be telemetered to ground within the allocated telemetry rate.
a guard ring connected to the detector bias voltage during operation. The contact pads are connected to the two edge strips (Fig. 2 b). Having the contact possibility at both ends makes the assembling more convenient. The electrical connections from the detectors to the preamplifiers are achieved by using conductive rubber. Therefore, relatively large contact pads were necessary to ease the assembling. Total length: 82000 Total width: 40500
Guard.n9 33 strips: a) I
Pitch 1000 Width 900 Length 70000
Guard ring
1 P
Scribe line
Resistor
3. S T R I P D E T E C T O R S In the initial design of the ERNE sensors, twosided strip detectors were planned to be used in HED. However, due to obvious difficulties and uncertainties in the production of such detectors in the late 1980's, the simpler one-sided devices were adopted for the final design. They were manufactured by Canberra Semiconductor N.V., Belgium. The simple pattern of the p+ implantation of the HED strip detectors is shown in Figure 2 a. The overall dimensions of the n-type substrate are 82.0 mm x 40.5 mm. Each detector has 33 p+ implanted strips with a length of 70.0 mm, width 0.900 mm and pitch 1.000 mm. Thereby, the dimensions of the active area of the detector become 33.0 mm x 70.0 mm. The active area, including the signal contact pads at both ends of the detector, are surrounded by
I--I p+ implantation i i i Contact to Si (SiO2 etch)
Aluminum m
Polysilicon resistor
b) Figure 2. a) p+ implantation pattern of the HED strip detector, b) a detail of one comer of the detector showing the various layers and structures. As a consequence of severe power limitations, the strip detectors had to be designed in a way to allow for a very simple read-out technique in terms
E. Valtonen et al./Nuclear Physics B (Proc. Suppl.) 61B (1998) 6 11
of the number of required amplifier channels. The method adopted was to employ capacitive charge transfer between the strips and perform the read-out from the two edge strips of a detector only. To reach efficient charge transfer, the capacitive coupling of the strips was increased by processing structures creating large interstrip capacitances. This was achieved by using an aluminum layer, in electrical contact with a p+ implantation strip, but for the most part extending over the area of the neighboring strip p+ implantation, and electrically insulated from it by a very thin silicone oxide layer. A detail of one corner of the detector is presented in Figure 2 b revealing the various structures. The black lines indicate the p+ implantation pattern, being the layer deepest in the detector bulk. The dark gray areas are the openings in the silicon oxide layer allowing for the contacts to the implantation layers. Excluding the first strip, these openings are mainly at the lower edges of the strip implantations in Figure 2 b. Over these structures, aluminum was evaporated in a pattern shown by the light gray areas in Figure 2 b. The goal was to reach as large a capacitance between the neighboring strips as possible. An isolating oxide layer thickness of 150 nm was used. In order to minimize the possibility of short circuits, e.g., through pin holes, the oxide processing was made in two phases. With this method a nominal, specified capacitance of 11 nF between the strips was expected. The measured values in the delivered detectors varied between 12.0 nF and 15.5 nF, with an almost constant value in a single detector and with relatively small variations in the same manufacturing batch. In addition to the integrated interstrip capacitances, the HED strip detectors have integrated polysilicon bias resistors as indicated in Figure 2 b. Between each strip and the bias line a resistor with a nominal value of 4 Mr2 has been processed. The tolerance was specified as + 20 %. A contact pad as shown in Figure 2 b allows testing of each resistor. In the incoming inspections the resistance values were generally found to be in the range 3.6 Mr2 - 4.4 MR with a few exceptions being at the limits of the tolerances. The bias line, made of evaporated aluminum, circles around the lower part of the detector to the other end of the strips. Similarly to the signal connections, conductive rubber is used to connect bias voltage to the
9
detector, and for convenience this can be done at either end of the detector. At the lower edge of the detector, not visible in Figure 2 b, the guard ring is connected to the bias line through its own bias resistor. In principle, with the read-out method used, the total energy loss in the strip detector is obtained as the sum of the signals from the two edges, and the position as the ratio of one of the edge signals to the sum. However, due to inefficiencies in the charge transfer in the detector, some nonlinearities are introduced. The ratio of interstrip capacitance to that between a strip and the backplane of the detector is about 700. This results in a reasonably efficient charge transfer from strip to strip, producing a smooth variation of the measured total energy with position having a minimum at the centre of the detector and deviating by about 15-20 % from the value measured at one of the edge strips. To obtain the best resolution in the measured position and total energy, the charge transfer characteristics of each individual detector has to be known. To achieve this, extensive simulations and beam tests of the detectors were carried out. The main emphasis was to develop correction algorithms for the measured total energy loss. The modeling was mainly performed in order to understand the effects of changing detector properties on the charge transfer, which might be encountered during the flight. The model developed was able to well reproduce the measured position dependence of the total energy loss. The beam tests of the HED strip detectors were carried out at the I.S.N. in Grenoble by using a nickel beam colliding in a target.. The results were used to optimize the model parameters describing the detector behavior. Tables were created and are used by the on-board software to convert the measured strip detector pulse heights to coordinates, and further to obtain correction factors by which the total energy loss signals are to be multiplied in order to compensate for the attenuation of the charge pulses in the detector. In the course of the analysis of the beam tests, methods were also developed, which can be used to re-calibrate the detector responses by using the flight data.
10
E. Valtonen et al./Nuclear Physics B (Proc. Suppl.) 6IB (1998) 6 11
4. G A T E ARRAY
combination is examined. If a valid event is recognized, a certain priority will be given to it. Subsequently, the analog multiplexer control sets multiplexer addresses selecting the active detectors with appropriate gain ranges. The sequence control reads the output of the priority control, and in the case of a valid event starts a sequence, which ultimately leads to a DPU interruption to read the pulse height data of the event from the gate array data register. The sequence control starts the sequencer, a state machine controlling all the functions of the peak detectors and the ADconverter. At the same time, the sequence control latches the status information in the status register and prevents further sampling of the discriminator signals during the active sequence. As the result of a complete sequence, the data register contains the digitized pulse heights, and the contents of the status register defines the active and passive discriminator signals describing the nature of the recorded event. The entire sequence takes about 13 ps. A new sequence can start after the DPU has read the contents of the data and status registers.
As briefly described in Section 2.3, all digital control functions of ERNE are concentrated in a gate array. The individual sensor electronics of LED and HED have their own gate arrays with similar functions. Only one gate array design was made, but with the ability to operate in two modes corresponding to the slightly different needs of LED and HED. The mode of operation can be selected by an external pin. The functions of the gate array can be studied from the block diagram shown in Figure 3. When the first signal exceeding the detection level in any detector is received by the discriminator control block, a time window of 1250 ns is opened. The signals from all the detectors received during this time window are stored in an input register and the status of all the discriminator signal lines fed in a status register. At the end of the time window, the recorded input data is sampled. A new time window can open 125 ns after the preceding one. In the priority block the validity of the recorded signal
Mux/Sample Controls
TT Analog Mux Control
Discr. I/F
IZ-"
Discriminator Control
.
Power Controls
Sequencer
T
Priority
J
I d Sequence . Control ~
I
"
h
l,
Control Register
1
Block
I
~ m
Counter functions and Counter Controls
Figure 3. Functional diagram of the gate array.
T
D, "1 Re, ter
I
,nteac i DPU I/F
E. Valtonen et al./Nuclear Physics B (Proc. Suppl.) 61B (1998) 6-.11
Employing the gate array technique has given the opportunity to easily include several useful feala~res in the electronics. In interplanetary space most of the particles are protons or helium nuclei. On the other hand, the abundances of heavier ions and rare isotopes have significant scientific interest. During solar flares the event rates are expected to be very high, and it might happen that due to their high intensity, only protons and some helium would be recorded. To prevent this, a three-level priority system was developed ensuring that during high flux times all heavy ions will be collected and pulse height analyzed. The system relies on an interactive operation of the gate array with the onboard software. The principle is very simple and is based on the recognition of the priority level of an event by the gate array, and on the division of the th low priority event rates, allowing only every n event to start the AD-conversion sequence. The value of the divider is determined by the on-board software based on the instantaneous flux level, and set by writing in the control register of the gate array. A further task of the gate array is to generate logic functions of the discriminator signals corresponding to protons, helium or heavier ions at selected energy ranges. The energy ranges are basically determined by the number of detectors traversed and the particles identified by their energy losses in successive detector layers, for which a number of discriminator thresholds have been defined. The logic functions can be combinations of either active or non-active detectors. The true or false status of the counter functions are sampled at the end of each initial 1250 ns time window started by the first detected detector signal, and the counters are incremented accordingly. There are 16 24-bit external counters in HED. The contents of the counters are read out serially to the gate array data register by the DPU command, and subsequently to the DPU buffers for further treatment. The advantage of having the hardware counters is that during high flux conditions the counters are updated during each open time window, irrespective of the status of an AD-conversion sequence. The gate array enables an easy and flexible reconfiguring of the sensors. The discriminator signals of any of the detectors can be disconnected
I1
from the rest of the electronics. This is achieved by writing in the control register by using ground commands, which then reprograms the discriminator control block. Thereby, a faulty or noisy detector can be isolated. After disconnection, the corresponding signal can be permanently set to either active or non-active state. In addition, in the counter functions a disconnected signal is replaced by a signal level, which best restores the original function in spite of the missing real signal. Another action that can be taken in case of noisy detectors, due to, e.g., increased leakage currents, is to raise the minimum signal level interpreted as a particle hit in a detector to three times the nominal value. This is also achieved by writing in the control registers of the gate array. Finally, in HED two different event conditions have been defined. The nominal event condition corresponds to protons or heavier ions penetrating through at least the three first strip detectors and giving a signal from the fourth. The additional trigger condition corresponds to electrons only, stopping in either of the scintillators. The two event conditions can be enabled simultaneously or the electron condition can be inhibited by the gate array. The gate array was designed by using the rules applicable for radiation hard devices. The design was made by Smartech Oy, Finland. The total number of gates needed was 7430 in a MC10K matrix. The processing and testing were performed according to the MIL-STD-833 Class B, Group D classification. The manufacturer was Matra MHS, France, by using a 1 micron/2 metal technology on an epitaxial latch-up free substrate. The process had been tested and guaranteed to stand a cumulative dose up to 85 krads without functional failures.
REFERENCES 1. V. Domingo, B. Fleck, and A. Poland, Solar Physics 162 (1995) 1. 2. J. Torsti, E. Valtonen, M. Lumme, P. Peltonen, T. Eronen, M. Louhola, E. Riihonen, G. Schultz, M. Teittinen, K. Ahola, C. Holmlund, V. Kelh~i, K. Leppala, P. Ruuska, and E. Str0mmer, Solar Physics 162 (1995) 505.