Nuclear Instruments and Methods in Physics Research A323 (1992) 380-385 North-Holland

NUCLEAR INSTRUMENTS

& METHODS IN PHYSICS RESEARCH

Sechon A

A ring imaging Cherenkov telescope for observations of high energy cosmic rays

J. Buckley, J. Dwyer, D. Mfiller, S. Swordy and K.K. Tang 1 Enrico Fermi Institute and Department of Physics, Unit~ersity of Chicago, IL 60637, USA

We describe a ring imaging Cherenkov counter for high altitude balloon flights designed to measure the energy spectra of cosmic ray nuclei above 40 GeV/amu. The instrument has a 3 m long nitrogen gas radiator at 1 atm, a spherical and planar mirror system, and = 2.2 m 2 of TMAE/ethane photon detecting drift chambers with fused silica windows in the focal plane. The mirror system is designed such that cosmic ray particles which trigger the instrument cannot pass through the photon detectors. Cosmic ray trajectories through the instrument are determined by eight low mass drift chambers, each of 2.2 m e area, which predict the ring image center to _+0.76 ram. Ground tests with cosmic ray muons prior to the flight show that the detectors see about two photoelectrons from a high energy singly charged particle. Special efforts were made to ensure stable operation and constant temperatures and pressures under remote control during the balloon flight. In September 1991, this instrument flew for 31 h at an altitude of ~ 37 km and collected cosmic ray events which were transmitted to a ground station for recording.

1. Introduction

In tegra t ing Cherenkov counters or, at much higher energies, t ransi t ion radia t ion detectors , have been used successfully in recent years to directly measure the energy of highly relativistic nuclei in cosmic rays [1,2], These devices, however, have limited energy resolut ion and exhibit large signal f luctuat ions for light nuclei because of thei r low nuclear charge. The accurate de te rmina t ion of the shape of the energy spectra of these nuclei remains an essential goal in this field of research. Requi red is a de tec tor tha t can be exposed near the top of the a tmosphere , with a well def ined energy cal ibrat ion, and small, quantif iable, f luctuat ions in response.

To improve this s i tuat ion we have const ructed a large aper tu re (0.26 m 2 sr) de tec tor for energy meas- u rement s of cosmic ray nuclei over the range of 40-500 G e V / a m u . The major new fea ture of this ins t rument is the use of a ring imaging Cherenkov ( R I C H ) counter , which de te rmines the Lorentz factor by measur ing the angle of Cherenkov emission. This R I C H detects about two photoe lec t rons for a high energy singly charged particle when ni t rogen is used as the rad ia tor gas. For heavier nuclei the n u m b e r of de tec ted photoe lec t rons scales with the nuclear charge, Z, as Z 2. Thus, even for hel ium nuclei, the yield is already four t imes that of a singly charged particle, providing an obvious signa-

1 Now at the University of Utah, Salt Lake City, UT 84112, USA.

ture for a high energy event. As the flux of particles declines steeply with energy ( d N / d E ct E 2.7), the su- per ior energy resolut ion and good background rejec- t ion proper t ies of a R I C H are extremely important .

For appl icat ion of a R I C H to cosmic ray work there is one clear difference from accelera tor physics; the pr imary cosmic rays arrive isotropically and there is no prefe r red " b e a m " axis. Consequent ly , the optical axis of the R I C H will in general not coincide with a particle trajectory. However, s imulat ions show that the angle be tween the optical axis and the part icle trajectory can be up to ~ 30 ° in our ins t rument before significant aber ra t ions occur which cannot easily be corrected. In fact, the aper ture is const ructed so that part icle trajec- tories are l imited to within ~ 15 ° of the optical axis.

2. Instrument description

2.1. Ot:erall description

The cosmic ray ins t rument is shown schematically in fig. 1. The plastic scintil lation counters are used as a counte r telescope to tr igger the ins t rument by the passage of a cosmic ray nucleus. The ampl i tude of the scintillation light pulse is recorded and used to identify the charge, Z, of an incoming nucleus. For relativistic nuclei the energy loss per unit pa th length ( d E / d x ) is propor t iona l to Z 2, but i n d e p e n d e n t of energy due to the density effect. There fo re a measu remen t of the scintillation light ou tpu t normal ized for light collection

0168-9002/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

J. Buckley et al. / RICH counter for high energy cosmic rays 381

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scope

d r i f t

gas

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Fig. 1. A schematic cross section of the RICH high altitude balloon instrument.

efficiency and path length readily identifies the ele- ments at high energy. The active material in the two scintillation counters is a 1.5 × 1.5 m 2 of 1 cm thick acrylic scintillator. These are enclosed in 15 cm high light-integration boxes, painted on the inside with a high reflectivity white paint (Kodak #6080). Each box is viewed by 12 photomultiplier tubes (PMTs) with 13 cm diameter photocathodes (type RCA 4525). The anode signals from each box are summed to provide a fast coincidence signal for the trigger which also pro- vides a timing reference for the drift chambers. The last dynode signals are each digitized by 8-bit pulse height analyzers to provide a light yield signal. In a perfect light-integration device the collected light would be independent of the track position, while in practice the light collection exhibits significant spatial variation across a box of this type. In particular, hot-spots form close to the front of each PMT because of enhanced light collection for that particular tube. In order to reduce this effect in the data analysis, it is advanta- geous to pulse height analyze all PMTs individually.

To provide a time-of-flight measurement (TOF), eight fast PMTs with 5 cm diameter photocathodes (type RCA 8575) also view each plastic scintillator. These are mounted in groups of four on opposite sides of the counter to generate two independent TOF measurements from the left side and the right side of the instrument, The sum of these two signals is inde- pendent of the light travel time within the detectors.

J

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Fig. 2. Schematic view of a pair of drift chambers in the hodoscope assembly.

VIII. RICH DETECTORS

382 J. Buckley et al. / RICH counter for high energy cosmic rays

2.2. Hodoscope

The hodoscope consists of a pair of drift chamber assemblies, one at each end of the RICH counter. These are used to find the trajectory of the cosmic ray nucleus through the instrument. From the trajectory information, we generate counter response maps, nor- malize the counter signals for angle of incidence, and most importantly predict the position of Cherenkov rings. Each hodoscope assembly consists of two orthog- onal pairs of drift chambers, which uniquely determine the position of the track. A schematic riew of one of the pairs is shown in fig. 2. Each drift chamber is filled with a 95/5% A r / C H 4 mixture and divided into seven cells each with a maximum drift length of 10 cm. The chambers are strung with gold-plated tungsten wire (25

txm) and the cells are separated by 75 ~m field shaping wires. All these wires are held in place by springs, which make the wires less susceptible to breaking from shocks occuring during launch and recovery of the instrument. A major consideration in the design of this drift chamber was to make the mass in the cosmic ray path low, since heavy nuclei are rapidly lost by nuclear interactions in thick material. The windows of the drift chambers are therefore made from thin aluminized mylar etched in strips. These are stepped in potential to produce a uniform drift field and to minimize drift loss due to field distortions to < 3% over the entire detector volume. The net resultant mass of the ho- doscope is ~ 200 mg/cm 2.

Each sense wire is read out in 35 ns time intervals through amplifier/discriminator combinations. How-

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J. Buckley et al. / RICH counter for high energy cosmic rays 383

ever, we have to take into account that heavy nuclei may produce a copious number of knockon electrons. To distinguish signals produced by the nucleus from those due to knockon electrons, we use three ampli- f ie r /d iscr iminator pairs with different thresholds on each of the sense wires. The system provides a position resolution of +0.76 mm (lo-). In addition, the analog signals from the cathodes are digitized to obtain a measurement of d E / d x in the gas chambers. This improves the overall charge resolution of the instru- ment.

2.3. R I C H mirrors and radiator

The radiator volume is enclosed in a lightweight structure consisting of an aluminum frame covered with a skin of 300 Ixm copper clad mylar. The radiator is filled with pure nitrogen with a Lorentz factor threshold of ~ 40. To ensure proper transparency be- low 190 rim, oxygen and water vapor contamination must first be removed by flowing the gas through an oxisorb filter (MG Industries). Because it is impractical to flush such a large volume during the balloon flight, the radiator is sealed shortly before launch. To prevent pressure differentials, an expandable 500 litre plastic bag is attached to the radiator volume.

The spherical mirror has a 6 m radius of curvature and is composed of nine 0.95 cm thick square seg- ments, each measuring 50 cm on a side. These mirrors were produced by Glass Mountain Optics in Austin, Texas. Each segment was constructed from a pyrex blank which was drape formed into approximately the correct shape and then ground and polished to the exact figure. We measured the blur circle diameter to be less than 4 mm at the radius of curvature for all segments. The front surface of each mirror was coated under high vacuum with aluminum followed quickly by an overcoat of magnesium fluoride. The mirrors have been measured to be approximately 90% reflective at 220 nm decreasing to roughly 70% at 170 nm. Each of these mirrors is mounted with flexible rubber spacers to a stiff glass composite (G-10) ring. The spacers prevent deformation of the mirror due to differential thermal expansion between the pyrex and the G-10. The ring is held with three adjustable mounts which allow each segment to be aligned to __ 100 ixrad.

The flat 45 ° mirrors, shown in fig. 1, enable us to place the photon detectors out of the cosmic ray "beam" . These mirrors are constructed from twelve pieces of 3 mm thick float glass, each selected to be flatter than 10 f r inges / in at 633 nm (supplied by Acton Research, Acton, MA). Again, the front surfaces are coated with aluminum and magnesium fluoride to pro- vide reflection down to 170 nm. The mirrors were mounted by placing the segments face down on a flat surface table and glueing a stiff glass composite (G-10)

support to their backs. The differential thermal expan- sion in the assembly is negligible for float glass.

2.4. Photon chambers

The two photon chambers are large area (1.5 m × 0.75 m) position sensitive detectors capable of measur- ing the positions of individual U V photons to within + 1.2 mm (~). A schematic drawing of a photon cham- ber is shown in fig. 3a. Photons enter through fused silica windows and photoelectrically convert in the 5 cm depth of the chamber which is filled with ethane gas loaded with T M A E (tetrakis (dimethylamino) ethy- lene), An electric field lets the photoelectrons drift to sense wires at the edge of the chamber. The drift t ime and the wire location give the x and y coordinate of the photon. As the use of T M A E as a photosensitive agent has been described elsewhere [3,4], it suffices to say that the design of our detector was based on the first R I C H detector using T M A E at the C E R N O M E G A spectrometer [5]. However, we have made several improvements to the original design. These are illustrated in fig. 3b which shows in more detail a section of the drift chamber. We place sense wires (25 ~m gold-plated tungsten) every 4 mm inside brass walled cells to prevent photon mediated cross talk between the wires. Since a gain over 105 is required for efficient detection of photoelectrons, additional steps must be taken to minimize optical feedback produced by avalanching electrons. This was done by the system of blinds shown in fig. 3b above the sense wires. Addit ional field shaping wires are used to "focus" drifting electrons into the slot between the blinds, allowing electrons to reach the sense wire with high efficiency from the drift region. With this system, we have found the net probability for a photon feedback signal to be below 10%.

Field shaping in the drift region was accomplished on the back side by strips etched on copper clad fiberglass (G-10) at 2.5 mm intervals connected to resistive dividers, and at the front by thin wires at 3 mm distance on both sides of the 3 mm thick fused silica window. The chamber uses a drift field of ~ 150 V / c m and a voltage of ~ 2200 V between sense wire and surrounding walls.

The chamber uses ethane gas purified with an ox- isorb filter and then bubbled through clean liquid TMAE. Initial impurities in T M A E (supplied by R S A corp., Ardsley, NY) had to be removed by washing it with high purity oxygen free water, and drying over 3 molecular sieves and silica gel. To make sure that T M A E does not condense inside the photon detector, the bubbler is cooled to a temperature slightly below that of the photon chambers which operate at the ambient temperature of ~ 20°C. A typical bubbler temperature of 12°C corresponds to a vapor pressure

VIII. RICH DETECTORS

384 Z Buckley et al. / RICH counter/br high energy cosmic rays

of about 0.2 Torr and an absorption length of roughly 5 cm comparable to the thickness of the chambers.

2.5. Gas system and thermal control

During the balloon flight gas is continuously purged through the hodoscopes and the photon detectors, The gas flow to each of the detectors can be commanded on or off separately from the ground. All gas from these systems exits into the volume of the balloon gondola. A system was devised to periodically vent the gondola to the outside. This consisted of a pressure transducer coupled by an electrical threshold circuit to a series of redundant solenoid valves which maintained the gondola pressure to 670_+ 5 Torr throughout the flight.

The hodoscope gas system consisted of a supply tank and pressure regulator and eight needle valves adjusted to provide ~ 20 l / h through each drift cham- ber. The flight gas system used for the photon cham- bers in shown in fig. 4. During operation e t h a n e / T M A E flows at ~ 40 l / h through each photon cham- ber. There are two major problems associated with constructing a T M A E gas system for a high altitude balloon instrument. First, electrical power is at a pre- mium. Second, used T M A E / e t h a n e must be disposed

TMAE bubbler Photon Chamber FI . . . . ter . _ _ ~ ~ - ~

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Fig. 4. Photon chamber TMAE/ethane gas system and con- trol.

of safely. As T M A E is highly corrosive, it cannot simply be released into the pressure shell. Therefore, a dewar containing a mixture of dry ice and ethyl alcohol is used with a cold trap to freeze out the T M A E emerging from the photon chambers (see fig. 4). In

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x (ore) Fig. 5. Superposition of rings from ~ 5000 cosmic ray muons and electrons with center locations determined by the hodoscope. The

density corresponds to the frequency of hits in each pixel.

J. Buckley et al. / RICH counter for high energy cosmic rays 385

addition, cold fingers extending into the bath provide a thermal sink to keep the T M A E bubblers (one for each photon chamber), with auxiliary electric heaters at a temperature of 12°C. This system was found to be a reliable way of removing T M A E from the system. It also ensured that in case of an electrical power failure, the T M A E system would go into a "safe" mode where all the vapor pressures remain low. One filling of dry ice lasted for about 50 h. To prevent the possibility that the accumulation of ethane produced an explosive mix- ture inside the gondola, the gondola was purged with nitrogen before the flight and after recovery.

Accurate temperature control of the main instru- ment mass is critical for this instrument. The photon chambers must stay above ~ 20°C for the T M A E not to be in danger of condensing. This was accomplished by thermal insulation consisting of expanded poly- styrene boards, which enclose the whole gondola. A thermal model was used to calculate the thickness of these boards, based on the power dissipation of the instrument and the various conductive and radiative losses and gains. The temperature remained at 23 _+ 3°C throughout the flight as predicted by this model.

2.6. Electronics

the thermal control system for the T M A E bubblers, consumes ~ 340 W.

3. Tests and balloon flight

We have tested the instrument with cosmic rays on the ground. This provides an extremely useful calibra- tion. The result of a ground run is shown in fig. 5. Here the ring images formed in the photon chambers from arriving muons and electrons are shown. This image was obtained by superimposing the predicted ring cen- ters of ~ 5000 events, using the hodoscope. From this image we estimate ~ 2 photoelectrons from a high energy muon, giving our instrument a Cherenkov qual- ity factor [4], N 0 = 11 cm l

The R I C H instrument was launched in September 1991 from Ft. Sumner, New Mexico. It flew for a duration of 31 h and attained a float altitude of ~ 37 kin. During this t ime it collected cosmic ray events at a rate of ~ 100/s as expected. The R I C H instrument including gondola, batteries, telemetry systems, etc. led to a total payload weight of ~ 2700 kg. At present, the data from this flight are being analyzed.

Signals from the R I C H photon detectors are ana- lyzed in time slices up to the maximum drift t ime to allow the detection of multiple hits on any wire. The sense wires are fed into ampl i f ie r /d iscr iminator pairs digitized into 83 ns time slices and then stored in fast, high density, CMOS first-in-first-out (FIFO) memory devices, The acquisition of data, initiated by the pri- mary trigger from the scintillators, begins by storing all the discriminator states in FIFOs. To keep dead time to a minimum, the data are immediately transferred into a second set of F IFOs where the data are com- pressed by encoding time and position of the hits. Fur ther processing organizes the data, event by event, in a storage buffer which is used to feed the telemetry system. This consists of a serial radio link running at 455 kbi t s / s to the ground station where the data is recorded on magnetic tape. All the electronics were designed to minimize the power required. The entire payload, including 806 channels of timeslicing electron- ics for the photon chambers and the hodoscopes, and

Acknowledgements

This work would not have been possible without the devoted efforts of the staff of our laboratory, in partic- ular Messrs. D. Bonasera, E. Drag, B. Lynch and W. Johnson. This work was supported through N A S A grants NSG 7464, NSG W-1995 and N A S A GSRP grants N G T 50272 and N G T 50463.

References

[1] J. L'Heureux, J.M. Grunsfeld, P. Meyer, D. Miiller and S.P. Swordy, Nucl. Instr. and Meth. A295 (1990) 246.

[2] S.P. Swordy, J.M. Grunsfeld, J. L'Heureux, P. Meyer, D. Miiller and K.K. Tang, Phys. Rev. D42 (1990) 3197.

[3] D.F. Anderson, IEEE Trans. Nucl. Sci. NS-28 (1981) 842. [4] T. Ekel6f, IEEE Trans. Nucl. Sci. WS38 (1991) 424. [5] R.J. Apsimon et al., IEEE Trans. Nucl. Sci. NS-32 (1985)

674.

VlIl. RICH DETECTORS