Engler, J.

Keim, H.

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KfK 3638 Dezember 1983 A Liquid lonizati n Chamber Using Tetramethylsilane J. Engler, H. Keim Institut für Kernphysik Kernforschungszentrum Karlsruhe KERNFORSCHUNGSZENTRUM KARLSRUHE Institut für Kernphysik KfK 3638 A LIQUID IONIZATION CHAMBER USING TETRAMETHYLSILANE Joachim Engler and Heinrich Keim Kernforschungszentrum Karlsruhe GmbH, Karlsruhe Als Manuskript vervielfältigt Für diesen Bericht behalten wir uns alle Rechte vor Kernforschungszentrum Karlsruhe GmbH ISSN 0303-4003 A LIQUID IONIZATION CHAMBER USING TETRAMETHYLSILANE ABSTRACT First results with a liquid ionization chamber using Tetramethyl-silane (TMS) are presented. A stack of iron plates was tested with cosmic ray muons and the charge output for minimum ionizing particles was measured. BETRIEB EINER IONISATIONSKAMMER MIT TETRAMETHYLSILAN ZUSAMMENFASSUNG Es werden erste Ergebnisse über den Betrieb einer Ionisationskam-mer gegeben, die als Medium Tetramethylsilan (TMS) benutzt. Ein Satz aus Edelstahlplatten wurde mit kosmischen Myonen getestet und die Ladungsausbeute für minimal ionisierende Teilchen bestimmt. - 1 -1. INTRODUCTION In high energy and nuclear physics the use of liquid ionization chambers with electron charge transport was restricted up to now to cryogenic liquids only. Liquid Xenon and Argon are widely used; for a recent review see c. Brassard1 ). There are two obvious reasons to use these liquids: 1) In the liquid noble gases Ar, Xe, Kr, the charge output for minimum ionizing particles traversing the liquid is relative-ly high, and 2) they are cryogenic. The low temperature facili tates cleaning the liquids and maintaining the contamination below the necessary level of 1 ppm o2 equiva1ent. Nevertheless, for practical use, cryogenic liquids have severe drawbacks. The necessity of a croystat in many cases prevents a compact detector design, and the cryogenic operation makes hand-ling very cumbersome. Hence, for many applications, it is very desirable to have liquid ionization chambers operating at room temperature. First hints of the presence of free electrons in a room temperatu-re liquid were obtained by Tewari and Freeman2 ) for neopentane in 1968. Independently, and nearly at the same time, Schmidt and Allen3 ) observed a high mobility and a high yield of charge carriers in tetramethylsilane. Both effects were interpreted as being due to free electrons carrying the charge. Stimulated by these observations, we intend to use warm liquids in large scale high . energy detectors, for instance in calorimeters. Here we report on the first operation of a small test chamber using tetramethylsilane (TMS). The size of the chamber was chosen to represent the typical dimensions of a tower in a calorimeter, namely approximately 10 x 10 cm2 and seven planes ganged together in the longitudinal direction. The test was done with cosmic ray muons. - 2 -2. ELECTRON CHARGE TRANSPORT IN LIQUIDS When searching for candidates for free electron charge transport among the non-polar liquids, it is useful to look up the mobility and the electron affinity. These two quantities are extensively studied and give a good indication for free-moving electrons if the mobility is high and the electron affinity is negative. 2.1 Mobility The mobility is defined as ll = vD/E, where vD is the drift velo-city and E the electric field. A compilation of mobilities for various room temperature liquids has been given by W.F. Schmidt 4). An extract is shown in Fig. 1, where the mobilities are plotted I Krypton Xenon • • 1000 Argon - FIG. 1: IJ ,.Methane ................ i TMS Mobility in 100 --- .A>X TMT I - several li---Neopentane -----=--- quids. The ---u 10 mean free path At Neohexane • dashed line Q) Isooclane • tll separates > <)\ de Broglie the region -... "' Cyctopentane • -E between free u n- Pentane • and bound >. Ql n-Hexane - electrons. ...... GI :..0 •Helium 0 :::!: 0.01 -Neon 0Ethane • 100 200 300 Temperature . ( K) versus the temperature at which the mobili ty was measured. One observes high mobilities for the noble gases and methane and a large scale of mobilities for room temperature liquids. A plausi-ble condition for electrons to be able to move freely in a liquid is that the mean free path A between elastic collisions with the surrounding molecules be greater than the de Broglie wavelength of thermalized electrons. In the theory of Cohen and Lekner5 ) on free electron charge transport, a connection between A and ll is given by - 3 -e and m are the electron charge and mass, respectively, k is the Boltzmann constant and T the temperature. Inserting in this formula the de Broglie wavelength for A, one obtains a boundary limit as indicated by the dashed line in Fig. 1. Above the line, ve expect free electrons to he present, whereas below it, the electrons are supposed to be bound in localized states. In these latter liquids, the electrons are inuuobilized in a physical void or hy polarization forces and, therefore, move only slowly. 2.2 F.lectron Affinity The second indication for free electrons is a negative electron affinity, v0• This quantity is also interpreted as the potential of a conduction band of an "extended e.lectron state" in the 1 iquid, similar to the conduction bands in metals. It is usually determined as the di fference of the work function <!> of a photo-cathode in contact with the liquid and with the vacuum: V o <l>uqu. - <~>vac. • A negative value is an indication that the liquid does not attract electrons and, therefore, should be a hint for free conduction electrons. A collection of V0 -values for typical liquids as com-piled by W.F. Schmidt 6 ) is given in Table 1. One observes that a high mobility corresponds in many cases with a negative affinity. When scanning through the literature more thoroughly, one finds as candidate, among the room temperature liquids, spherical molecules with the following tetrahedral structure: X(CH 3) 4 • On the corners of the tetrahedron, there are four methyl radicals and, in the center, a four valent atom. So far, molecules with X = C (neopen-tane), Si, Ge, and Sn have been investieated. Neopentane has a boiling poi.nt of 9 Oe and is, therefore, not a real room tempera-ture liquid. Ge is too expensive to he used in large quantities. Hence, for practical applications, Tf'.1S and tetramethyltin (niT) remain. A feH interestine properties of these two liquids are listed in Table 2. Due to its toxic character, we have abandoned THT and have chosen TIIS for these tests. - 4 -TABLE 1: Values for Electron Affinity V0 for Various Liquids Liquid T [K J vo [ ev J Helium 4 + LOS Neon 25 + 0.67 Argon 84. - o.zo Krypton 116 -0.40 Xenon 161 -0.67 CH4 100 - 0.25 TMS 295 - 0.57 Neopentane 295 - 0.43 Cyclopentane 295 - o. 28 Isooctance 295 - 0.18 N-Pentane 295 o.oo N-Hexane 295 + 0.04 TABLE 2. Physical Properties of TMS and TMT Property Density (g/cm3) 0.65 1.31 Boiling point at 1 bar 26.5 ° c 78 ° c Melting Point -100 ° c -54 ° c Dielectric Const. (zoo c) 1.92 2.3 Rad. Length X0 (cm) 52.2 9.2 Interact. Length A.o (cm) 130 87 Ep min. ioniz. (MeV/ern) 1.03 1.72 Flammahle yes yes Toxic no yes - 5 -2.3 Drift Velocity The drift velocity for TMS in shown in Fig. 2, where it is plotted versus the applied fieldas measured by u. Sowada7). It is linear up to 20 kV/cm, giving a constant mobility. At higher fields, the mo bili ty decreases due to inelastic collisions, since the elec-trons begin to lose the energy which they pick up from the field between collisions. We . find that a field of 20 kV / cm can be realized without major complications. At this value, the drift velocity is 2 x 106 ern/sec which corresponds to a drifttime of 50 nsec/mm. This is a factor 4 faster than in LAr where the corres-ponding value is 200 nsec/mm. X 200 K o 296 K Electric field (V/cm) FIG. 2: The drift velocity in TMS (Ref. 7) - 6 -3. TEST APPRATUS 3.1 Ionization Chamber The ionization chamber is shown schematically in Fig. 3. A small stack of electrodes was ~hosen with dimensions which represent a typical tower arrangement in a high energy calorimeter. In order to avoid poisoning of the liquid, the only materials used for construction are stainless steel, polyethylene, teflon rods and ceramic washers. The materials used as seals are indium and PTFE-like compounds (Kalrez8)). L_ polyethylen insulation HV ---- ~130 mm ---Scint. ~lnseal electrodes stainless steel ~Scint. FIG. 3: Arrangement of the cham-ber The stack is built up of 7 pairs of stainless steel plates of hexagonal shape, 1.5 mm thick and about 130 mm across. The gap height is 3.5 mm, resulting in a total of 4.9 cm liquid to be transversed by a cosmic muon. The total amount of liquid contained in the chamber is approximately 3 .R.. Before assembly, the iron pla tes, the chamber enclosure and all parts of the support structure are electropolished and cleaned in an ultrasonic bath of methanol. After assembly, the chamber is baked out by goüc hot water flowing through a coil soldered onto the outside of the chamber body. Bake-out time is about 20 h at a pressure of about 10-4 mbar. I - 7 -The high voltage plates are ganged together and the signal fed to a common preamplifier. Below and above the chamber, scintillators serve to trigger on cosmic muons. 3.2 Electronic Circuit The readout electronics is shown in Fig. 4. The low noise charge amplifier is placed wi thin a few cm of the chamber body and shielded in a copper housing. The total capaci tive load corres-ponds to 800 pF and is rather high. We, therefore, use a transfor-mer to optimize the matehing to the amplifier input capacitance. An amplifier of the CELLO LAr-calorimeter was taken. For integra-tion, we use a time constant of 800 nsec. After the input stage, the signals are amplified and processed by an integrating and differentiating circuit (ORTEC 572) with both time constants ap-proximately 1 f.LS• Finally, the signals are analyzed in a multi-channel analyzer which is triggered by NIM-based electronics. Hq lokn H lOnF r-o:= Calibration 20MQ _L Ce= 2pf ~~~_L--i,.~~ Co 800pF Charge Preamp. •1st Filter Main- Analyser amP,. • 2 n~ Filter FIG. 4: Principle of the elec-tronic circuit Calibration is performed with a step voltage fed via a capacitor of 2.2 pF onto the detector capacitance. The rms noise corresponds to 0.8 fC. The high voltage is filtered carefully by RC networks, the later being located directly at the chamber entrance. As high voltage power supply we use a Spellmann RHSR 25 PN. - 8 -3.3 Purification System The purchased liquid has a puri ty of only 99.9% and has been in contact with air. A first step in purification, therefore, is vacuum distillation. The system used is shown in its principal parts in Fig. 5. It is designed to purify quantities of 5 1 of liquid. All parts in contact wi th the TMS liquid or vapour are made of stainless steel. co2 ice trap turbo pump molecular sieve 13x TMS Purification System FIG. 5: Principle of the purifi-cation system In the first step, the TMS is evaporated from the lower vessel into the upper one where it is recondensed by a water cooled heat exchanger coil. Part of the vapour phase, probably mainly air, is pumped away by a turbo pump. This is done very cautiously by observing in a glass ice trap whether TMS begins to condense. The vacuum distillation is repeated until no more residues are obser-ved in the evaporation vessel. The residues normally found look different for different batches of liquid. But typically, in the first two distillation passes, we observe on oily liquid, maybe water, remaining on the bottom and dust-like particles on the walls. We find three distillation passes to be sufficient. We let the last one proceed very slowly with a temperature difference of only z0c between the lower and the upper vessel. - 9 -After vacuum distillation, the TMS vapour passes through two chimneys filled with molecular sieves ( zeolite) of type 13X. We observe electric signals already after the first passage. Normally two passes are sufficient to give full signal output. The oxygen and maisture contamination is below the sensitivity of the monitors, which - in both cases - is approximately 1 ppm. At room temperature, the vapour pressure of TMS is around 0.9 bar. In order to avoid air leaking into the system, we apply a slight overpressure in the measuring cell by pressurizing wi th purified argon gas. 4. RESULTS We have measured the charge output for cosmic ray muons wi th respect to the applied voltage. The yield is shöwn in Fig. 6. As usual, G is defined as the yield of escaped electrons per 100 eV deposited ionization energy. For comparison, the yield obtained by H. Jungblut 11 ) is given for TMS and for liquid methane. Jungblut made his measurements in a small cell of a few cubic centimeters. Our findings agree well with his results. (!) "0 Q) >= c 0 .._ ...... u Q) üJ Q) Q) .... u.. 2.0 10 • TMS, this mE'as. - OnsagE'r, lhE'or . 20 30 Eleclric Field (kV/cm) FIG. 6: Number of free electrons G yielded for 100 eV deposited ionization energy - 10 -The straight line in the figure represents the prediction of 12) Onsager' s theory • Onsager assumes germinate recombination of the electrons with their parent positive ion. After being ther-malized by inelastic collisions wi th the surrounding molecules, the Coulomb attraction recombines the electron with its positive parent ion. For low fields, Onsager's calculation predicts a linear dependence with a slope-to-intercept ratio equal to e 3/2Ek2T2, where e is the electron charge and E the dielectric constant of the medium. Our result agrees well with this predic-tion. It is due to the Boltzmann factor that, for room temperature liquids, high fields are needed to reach saturated charge collec-tion. E u ....... u -<ll 21 I 2 I u -o I t; , I ..!!! 8 I I I I /~~:-;.,--/ Liquid ( 2ppm 01 equ.) 10 20 Electric Field ( kV I cm) FIG. 7: Collected charge for minimum ionizing par-ticles traversing 1 cm of liquid. The straight line through the mea-sured points corresponds to Onsager's theory. In Fig 7, the charge output is plotted in a more practical form given for a minimum ionizing particle traversing 1 cm of liquid. At a typical valtage of 20 kV/cm about 1 fC/cm are collected. For comparison, the yield in LAr is given tagether with a typical pu-rity level in large detector systems assumed to be 2 ppm o2 equivalent. One finds that, for a given detector configuraton, the pulse height in TMS is reduced by a factor 4-5 as compared to LAr. - 11 -We could typically raise the high valtage to 22 28 kV/cm without observing additional noise in the preamplifier. Beyend these values, small discharges start to augment the preamplifier noise. We have not yet localized these discharges in the liquid, the decoupling capacitor or the feedthrough cable We have repeated the measurements with two other fillings of liquid. During the measurements, the cell was decoupled from the purification system. For a period of two weeks, we could not observe any change in charge collection wi thin our accuracy of about 5%. 5. CONCLUSION In a small test chamber, we could detect cosmic ray muons wi th rather simple methods and obtained reasonable charge output. For the treatment of the chamber, only normal methods used in vacuum technology were applied. The purification of the liquid with a few vacuum distillations and a passage over molecular sieves is also relatively simple and straightforward. The charge output is reduced by a factor 4 ••• 5 with respect to liquid argon. But for many applications - such as high energy calorimetry, where the energy deposition is dense - this is no severe hindrance. We, therefore, feel confident that warm liquid ionization chambers will add a new tool to high energy instrumentation. We wish to express our appreciation of the skillful help of R. Dittmann in setting up the apparatus. We would like to thank Profs. G. Flügge and B. Zeitnitz for continuous interest and encouraging support. - 12 -REFERENCES 1) C. Brassard, Nucl.Instr. and Meth. 162 (1979) 29 2) P.H. Tewari and G.R. Freeman, J. Chem.Phys. 49 (1968) 4394 3) W.F. Schmidt and A.o. Allen, J~ Chem.Phys. 50 (1969) 5037 4) W.F. Schmidt, Can. J. Chem. 55 (1977) 2197 5) M.H. Cohen and J. Lekner, Phys.Rev. 158 (1967) 305 6) W.F. Schmidt in "Charge Storage, Charge Transport and Electrostaticswith their Applications", edited by Y. Wada, M.M. Perlman, H. Koado, Elsevier Scientific Publishing Company, Amsterdam-Oxford-New York (1979) 7) u. Sowada, Thesis at the Freie Universität Berlin, Fachbereich Physik (1976) · 8) Kalrez, trade mark Du Pont de Nemours S.A., France 9) Oxygen monitor Teledyne 311, Bestobell Comp., USA 10) Maisture monitor Aquanal, Gerhard GmbH, Blankenbach, Germany 11) H. Jungblut, Diploma work at the Freie Universität Berlin, not published 12) 1. Onsager, Phys.Rev. 54 (1938) 554 

A liquid ionization chamber using tetramethylsilane

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A liquid ionization chamber using tetramethylsilane

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