Construction and performance of a large-area multiwire ionization hodoscope for use in a cosmic-ray detector

N U C L E A R I N S T R U M E N T S AND METHODS

0977) 569-576;

I40

© N O R T H - H O L L A N D P U B L I S H I N G CO.

C O N S T R U C T I O N AND P E R F O R M A N C E OF A L A R G E - A R E A M U L T I W I R E I O N I Z A T I O N H O D O S C O P E F O R USE I N A C O S M I C - R A Y DETECTOR* P.L. LOVE?, J. TUELLER, J.W. EPSTEIN, M.H. ISRAEL and J. KLARMANN Department of Physics and McDonnell Center for the Space Sciences, Washington University, St. Louis, Missouri 63130, U.S.A.

Received 12 October 1976 The mechanical construction and electronics of a large-area multiwire ionization hodoscope are described. Each chamber is 0.83 m by 1.65 m with wire spacing of 2.5 cm. The chambers are assembled to compose a hodoscope for a 6.6 m2"sr cosmic ray detector. Operating characteristics and in-flight performance from two 30 h high-altitude balloon flights are presented.

1. Introduction

the elemental composition of U H cosmic rays wil determine whether they were produced by the r-process, a mixture of both r- and s-processes, or possibly a mixture including some other process. Such a determination will put constraints on the physical conditions of the astrophysical site where these nuclei originate1).

We report on the design, construction and operation of a multiwire ionization hodoscope (MWIH) used in our very-large-area balloon-borne detector for determining the charge composition of the ultraheavy (charge, Z, greater than 30) cosmic-ray nuclei. The M W I H is similar to a multiwire proportional counter (MWPC) but takes advantage of the larger ionization produced by these highly charged nuclei and so operates in the ionization mode rather than in the proportional mode. Nuclei in the solar system with charge larger than 30are principally synthesized in either a rapid, r-process, or a slow, s-process, capture of neutrons on iron seed nuclei. The processes are distinguishable by the location of peaks in the elemental distribution, with r-process peaks typically occurring 2-4 charge units below corresponding s peaks. A measurement of

The measurement is difficult due to the low flux of these nuclei (about 2/m 2- sr. h) and the close proximity of the r and s features in the elemental distribution. Thus the experiment requires both large area and individual-element charge resolution. We have flown in two 30 h balloon flights a 6.6 m 2. sr electronic detector composed of two identical systems of ionization chambers, a Cherenkov counter and the M W I H . A cross-section of one system is shown in fig. 1. The charge and velocity of each incident nucleus are determined by the d E f d x - C method wherein d E / d x , the rate of energy loss of the nucleus, is measured by the ionization in the ion-chamber gas (90% argon, 10% methane) and C is the intensity of Cherenkov radiation produced by the nucleus in passing through a Lucite radiator. The signals in both ioniza-

* This work was supported in part by NASA Grant NGR 26-008-001. i Supported in part by Fellowships from Washington University and from the McDonnell Center for the Space Sciences. 2.5cm

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tion and Cherenkov light depend on the square of the particle's charge, on different functions of the particle's velocity, and on the pathlength of the particle's trajectory in the counter. The hodoscope measures the particle's trajectory by locating its x-y position to 2.5 cm at the top and bottom of the stack. In this paper we will first discuss experimental considerations for our MWIH and then describe its operation, construction, signal characteristics and inflight performance.

2. Experimental considerations The primary role of the hodoscope in this experiment is to measure the pathlength of each particle's trajectory in the ionization and Cherenkov counters. This is necessary because our large-area detector allows zenith angles as big as 65 °, resulting in pathlength differences up to a factor of two. This is to be contrasted with the fractional separation in ionization-chamber pulse heights between two charges, Z and Z + I of equal path length and velocity, of 2/Z for large Z, which is a few percent in the charge region of interest. In fig. 2 we have plotted the standard deviations in charge units for the main contributions to the charge resolution for a typical angle and energy. These are, statistical fluctuations in the ionization deposited in the ion chambers2), electronic noise generated by the ion chamber preamplifiers, and the uncertainty in the ionized pathlength. We have also plotted the total of these errors. This figure assumes that all other uncertainties proportional to the signal can be made

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small compared to the pathlength uncertainty. For example, a gain drift in the ion-chambers due to temperature induced gas density changes has been observed in the data and corrected using the abundant cosmic-ray iron nuclei. The uncertainty in charge due to pathlength uncertainty (see appendix) is given (in charge units) by az=

sin 0 cos 0 sZ 2 \/6

lo '

where s = interwire spacing, 1o = separation of the hodoscope planes, Z = c h a r g e of the nucleus, and 0 = zenith angle of the trajectory of the nucleus. In our case s = 2.5 cm and 10 = 79 cm. Beyond charge sixty the total resolution is strongly influenced by the pathlength uncertainty which increases linearly with Z. Eecause of the steep decrease in elemental abundance with increasing charge our balloon exposure does not require a finer hodoscope. In a 30 h balloon flight we expect to see only 3 or 4 particles with charge greater than sixty. The other important role of the hodoscope is to provide location in the counters so that corrections may be made for areal non-uniformities. These are only a few percent of the signal in the ion chambers but are as big as 20% in the Cherenkov counters due mainly to light-collection non-uniformities.

3. Description Fig. 3 is a block diagram of one layer of the MWIH consisting of 62 parallel wires in one plane. The parallel planes (cathodes) on either side of the wire plane are held at negative high voltage (600 V) with respect to the collecting wires. The detector volume is filled with Research Grade P-10 Proportional Counting Gas, a mixture of 90% argon and 10% methane. A cosmic ray traversing the detector ionizes the gas along its path from which free electrons move toward the collecting wire closest to their site of production, generating a charge signal on that wire. The wires are operated in the electron-pulse mode where the preamplifier differentiation time of 10/Ls is longer than the electron collection time of less than 0.5/~s, but is much shorter than the positive-ion collection time of several milliseconds. The 600 V operating point is on a wide plateau in signal response such that varying the high voltage from 100 to 1000 V produced a change in signal of less than 3% for 0.025 cm (0.010") diameter wire. Each wire is connected to a four-stage HodoscopeAmplifier-Discriminator (HAD) which amplifies the

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571

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Fig. 3. Block diagram of one of four layers composing the MWIH.

signal and compares it to a preset discriminator level equivalent to 3 x IO- 3 pC at the HAD input. When this level is exceeded the discriminator fires and a “one” is held on the parallel inputs of a shift register for 30~s. Provided the same cosmic ray has traversed all four layers of ionization chambers in the stack, a four-fold coincidence signal is produced which parallel-loads the shift registers and activates the address logic. It generates the addresses of any two wires fired that are at least five wires apart and holds the states of the four following wires adjacent to each a.ddressed wire. It also flags an event with a third a.ddressable wire in one layer as an overflow and calculates a parity bit for the first address. These 22 bits for each of the four hodoscope layers are recorded with the ionization and Cherenkov pulse heights on a seventrack incremental tape recorder in the flight instrument. charge

4. Signal characteristics The charge signal induced on a wire when an ionizing particle traverses the hodoscope is produced by the separation of the electrons, as they move to the closest wire, from the essentially motionless positive ions which remain distributed along the particle’s trajectory. We first treat the problem of the charge signal induced on a wire due to a single charge q at position P in the bodoscope. We wiil then generalize to the track of electrons ionized by the particle. The induced charge qk on wire k due to a charge q at position P in the hodoscoFe is derived as follows: Green’s Reciprocation Theorem of Electrostatics3) states that with a given set of conductors 1, 2, . . , N if charges ql, q2, . . . , qN result in potentials V, , V, , . . . ,

V,, then charges q; , q; , . . . , q2; will result in potentials v;, v;, . ..) Vh such that

To relate qk to q it is convenient to make the following choices for the charges and potentials. The (qi, Vi) are chosen (q, V) at P and (ql, 0), . .., (qN, 0) on the electrodes. (Setting the Vi = 0 ignores the dc charges resulting from the dc applied voltage.) The (qi, Vi) are chosen (0, V’) at P and (q; , 0), . . . , (q;_ 1 , 0), (qi, Y3,(Y;+,,O), ...? (41;) 0) on the electrodes. Green’s theorem then gives qv+q,v;

= 0.

Thus the charge qk induced on wire k by a charge q at point P is calculated from the potential V’ at P with all the other wires and the two cathodes grounded and with wire k held at VL. This potential can be found by expanding in unit line charges between plane parallel electrodes with the static charges qi as the expansion parameters4). We have used the numerical results from ref. 4 for our geometry to obtain the potential. The net signal on wire k due to a single electron-ion pair formed at point P after the electron has drifted to point Q is thus.

where the electron charge is-e. In particular, after the electron is collected, but before the ion has moved

572

P.L. LOVE et al.

from point P,

qk

=

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if the electron has been collected on wire k. If the electron has been collected on any other wire qk

=

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Note that the polarity of the signal on the wire which collects the electron (the nearest wire to P) is opposite the polarity on any other wire. Let the wires of the hodoscope be located at x = na (n an integer) and the cathodes at y = +_b. Assuming the passage of a vertical particle (x = constant) of constant ionization the charge signal S on the wire closest to the ionization is given by

e fo I

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1

FET-input charge-sensitive pre-amplifier, a voltage shaping amplifier, a voltage gain amplifier, and a comparator discriminator. The charge sensitive stage is a discrete amplifier using a 2N4416 input FET with cascode and emitter follower transistors. The shaping stage integrates the output of the charge-sensitive stage with a l0 #s time constant, differentiates with a 25 #s time constant, and supplies a gain of six. The voltage gain stage inverts the signal and has a gain of ten making the overall conversion gain about 38 V per pC with no input load. The comparator discriminator as well as the two voltage amplifiers use Motorola Mc1712C1 commercial operational amplifiers. The properties of the H A D are illustrated in fig. 4 which shows signal versus percent firing level of the i

vo J

where N is the total number of electrons produced, e is the electronic charge, V(x, y) is the potential at (x, y) with the nearest wire at Vo and all other electrodes grounded, and e(x) is defined as the charge collection efficiency. The integral is done numerically, and e(x) varies from 0.80 at x = 0 to 0.91 at x = ½a, a 13% maximum areal variation in the signal amplitude. The signal rise time depends on the electron collection time which also varies with location in the hodoscope due to the non-uniformities of the actual electric field. An excellent approximate solution to this field can be obtained from the potential of an infinite wire grid in free space, which is nearly an equi-potential at the cathodes. Using the field dependence of the drift velocity 5) we find collection times varying from 0.3 fls for an electron produced at y = 0, x-~ ½a to 0.6 ys for one at y = b, x ~- ½a.

5. Hodoseope--amplifier--discriminator We wish to operate the hodoscope in flight so that it will provide adresses for all nuclei with Z>__12. In 2.5 cm of P-10, a minimum path length contributing to the signal on at least one wire, a minimum ionizing nucleus (energy > 2 GeV/nucleon) of Z = 12, loses 0.92 MeV. This energy deposited in the chamber is converted into 3.0 x 104 electrons at 27 eV per electronion pair. From the expression for S along x = ½a we expect a signal of 4 . 3 × 1 0 - 3 p C . We also want to address at least Z = 70 in 5 cm of gas so we can expect rare signals as large as 200 x 10-3 pC. The H A D must amplify this range of signals and discriminate against more abundant lower charge nuclei as well as noise. The H A D is a four-stage circuit composed of a

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discriminator for various temperatures (a) and capacitive input loads (b). The thermal coefficient of the discriminator level (50% firing point) is - 0 . 1 % / ° C while the expected signal variation due to gas density changes is - 0 . 3 3 % / ° C . Thus the circuit is more temperature stable than the inherent detector signal. The increase with detector input capacitance in the noise (slope of the line) as well as the increase of the discriminator level are effects caused by capacitive loading of the charge-sensitive preamp. With a 15 pF load on the detector input the standard deviation in firing level is 0.2 × I0-3 pC. The capacitance of one wire to the world is about 10 pF/m almost independent of the geometry of the hodoscope*). For our 1.65 m wires this implies a detector input load of 17 pF and therefore at threshold (3 x 10 - 3 p C ) the signal is about 15 times the r.m.s, noise. 6. Mechanical construction

Fig. 5 is a cutaway view of half of a hodoscopg layer containing thirty-one 0.25 mm diameter wires (Malins 1 Music Wire) in a gas-tight box with inside dimensions 1.65 m by 0.83 m by 5 cm. The electrodes are 0.006 mm thick pieces of Mylar aluminized on both sides with surface resistance less than 1 0 per square. The gas is contained on top and bottom with 0.012 mm thick pieces of clear Mylar. The walls of the box are made of Lucite-stainlesssteel I-l~eams t-.eld together with epoxy (Scotch Weld ~2216a-B) fillets. The assembled frame was strung with wires at 2.5 cm intervals and both 0.83 m ends ~ere bowed 3 mm at their center to produce 0.5 kgf

(1 lb) of tension in each wire. The wires are attached to the end walls with a feedthrough (Cambion #1019-06) which also serves as the electrical contact for the preamplifiers. The Mylar electrodes were prestretched and taped to the top and bottom of the frame with the 1.65 m sides bowed 3 mm at the half-center points. The tension caused by this bow holds the electrodes flat such that they produce excellent mirror images. Gas circulation holes were cut through the stainless and the mylar" so that the gas pressure would not bow the cathodes causing areal signal variations. Finally, the clear Mylar gas seal was laid over the top; and the edges of the clear Mylar, aluminized Mylar, and stainless were bonded with silicon rubber (RTV 1200, G.E. Construction Sealent). The finished chambers were checked for gas leaks and for electrical leakage resistance. We applied enough over-pressure to bow the gas covers about 8 cm at their center. If this bow stayed for a week we judged any gas leak that might remain small enough that the flow system would easily compensate for it. During the balloon flight and in the lab we replace the gas volume of the chamber about once every twelve hours. The leakage resistance, measured at 1000V, was always greater than 10 GQ between the two cathodes or between the wires and one cathode. The hodoscope-chambers and ion-chambers above the Cherenkov counter are glued together into one unit and wrapped in 0.016g/cm 3 (1 lb/ft 3) density, 1.27 cm thick Dorvon (expanded polystyrene) foam, and then in aluminum screen wire. The screen wire is

574

P.L. LOVE et al.

grounded to the amplifier grounds and acts as an electromagnetic shield. This light-weight construction makes the instrument especially well suited for experiments measuring highly charged nuclei because of the small amount of matter introduced in the particle beam, thus minimizing nuclear interactions. The total thickness of the MWIH including electrodes, gas seal, gas and shield is 0.13 g/cm 2 1% of an interaction length for a nucleus with charge fifty, which is much less than that due to the typical residual atmosphere at balloon altitudes of 3-4 g/cm 2.

7. Flight performance In our sixty hours of balloon exposure the MW1H has provided trajectory information for one million nuclei with essentially 100% recording efficiency for Z>__ 12. It had ambiguous trajectories due to multiple particles in the hodoscope on less than one percent o f these events, consistent with the expected rate of accidental coincidences in the 30 #s resolving time. Fig. 6 is a histogram of wire firings for one system from a portion of our first flight. It shows that the HAD discrimination level of each wire was below the event acceptance threshold set by the ion chamber discriminator. The drop off in counts at the edges of the upper Y histogram is because upper Y is the interior hodoscope layer on top and particles near its edge have a high probability of missing the X layer

completely and were thus rejected. Two wires in the lower Ylayer have few counts due to a low-temperature failure mode in their HADS which was prevented in the second flight. The hodoscope responds to nuclei such that an appreciable signal is produced only on the wire closest to the ionization track. This cellular nature is shown graphically in fig. 7 where the small circles representing wires have been separated by vertical dashed line cellboundaries which are not part of the physical structure. Adjacent wires have a small induced charge on them which is negative (opposite to the signal on the wire in the ionized cell) after the electron collection time (0.5 #s). For trajectories near a cell boundary, the charge on an adjacent wire swings slightly positive (less than 1%) before going negative; the width of the positive swing is less than the collection time. Since the integration time of the preamplifier (10 #s) is much larger, the resultant adjacent wire signal is not seen by the discriminator. We thus get the cellular response evidenced by fig. 8. Fig. 8 shows histograms of the tangent of the projected zenith angle of the particle's trajectory for events in which one, two, three, four and five adjacent wires have fired. The numbers in parentheses are the tan

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575

MULTI-WIRE IONIZATION HODOSCOPE total number of particles in that histogram. In the histogram for three adjacent wires for example, we expect no events with Itan 01<0.5 since particles of smaller angle can penetrate only one or two cells (fig. 7). The rest of fig. 8 can similarly Ice understood in terms of cellular hodoscope response. Only 380 events (0.3%) of the 125657 events plotted in fig. 8 fire more wires than is consistent with their angle, even though the figure is composed of raw data with no selection of events. Fully one third of these 380 are clear cases of two particles in the detector at once where the hodoscope trajectory calculation is inconsistent with the ion-chambers sections that had pulse heights above the noise (fig. 1). For the remaining 250 background events the recorded information is insufficient to determine the proper scenario. Also, 1230 of the events plotted in fig. 8 fire fewer wires than is consistent with their angle (the angle requires a pathlength larger than 2.5 cm in more cells than fired). O f these 1141 are edge wire events where particles left or entered the detector volume at that hodoscope plane. Since during a fraction of the flight the event

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acceptance telescope discriminators were set just below Z = 12 minimum ionizing these results are consistent with near perfect efficiency for Z >i 12.

8. Conclusion We conclude that a M W I H functions efficiently, with few background events, in an experiment to measure high-charge nuclei. It has several characteristics important to such an experiment; the small amount of matter introduced into the particle beam, the uniform areal response, and the constancy of the discriminator level which is dominated by gas density changes alone. A M W I H of similar design will be flown in the Heavy Nuclei Experiment on N A S A ' s third High Energy Astronomy Observatory (HEAO-C) scheduled for launch in July 19796). The multiwire ionization hodoscope developed from a proposal by E.C. Stone and R.E. Vogt of California Institute of Technology for an ionization hodoscol;e on the H E A O Heavy Nuclei Experiment. The hodoscope-amplifier-discriminators which we built are similar to those designed by W.E. Althouse and W.G. 131odgett (Caltech) and W.A. Gneiser

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Fig. 9. Schematic drawing of a particle trajectory which intersects the upper hodoscope at (xi,yl) in the hodoscope element centered at (X1, Y1) and intersects the lower hodoscope at (x~, Y2) in the element centered at (X2, Y~).

576

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(Ball Brothers Research Corporation) for the H E A O experiment. We are grateful to the balloon-flight crew o f Raven Industries for two successful flights under difficult conditions, and also acknowledge the balloon-flight supervision by the Office o f Naval Research.

I f Xl lies in an upper hodoscope element centered at X 1 , and x2 lies in a lower hodoscope element centered at X2, then 6, the error in A x , is related to A x by AX = X2--X l = X2--X

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The probability distribution o f the error 6 is given by

Appendix The ionization signal I for a cosmic ray o f charge Z and constant velocity can be written I = AZ 2 l,

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which yields

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This distribution deviation

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2 x/6

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(Ay)2,

If xa is the position along the X-axis in a hodoscope element o f width s, the probability o f a nucleus passing between xa and x~ +dXl is given by

p ( x O dxl = d x l / s .

References 1) M. H. Israel, P. B. Price and C. L. Waddington, Phys. Today 5 (1975) 23. 2) j. W. Epstein, J. I. Fernandez, M. H. Israel, J. Klarmann and R. A. Mewaldt, Nucl. Instr. and Meth. 95 (1971) 77. a) W. R. Smythe, Static and dynamic electricity (McGraw-Hill, New York, N.Y., 1950) p.34. 4) G. A. Erskine, Nucl. Instr. and Meth. 105 (1972) 565. 5) W. N. English and G. C. Hanna, Can. J. Phys. 31 (1953) 768. 6) The Heavy Nuclei Experiment is a collaboration among scientists at California Institute of Technology, McDonnell Douglas Research Laboratories, University of Minnesota, and Washington University with major portions of detailed design and fabrication by Ball Brothers Research Corporation.