Operation of a bubble chamber filled with argon, nitrogen, and argonnitrogen mixtures

Nuclear Instruments and Methods 187 (1981) 363-369 North-Holland Publishing Company

363

OPERATION OF A BUBBLE CHAMBER FILLED WITH ARGON, NITROGEN, AND ARGON-NITROGEN MIXTURES Gert HARIGEL, Gerhard LINSER and Ferdinand SCHENK CERN, Geneva 23, Switzerland

Received 10 February 1981

A 2.7 1 test bubble chamber was filled with argon, with nitrogen, and with two argon-nitrogen mixtures and exposed to a high-energy hadron beam at the SPS. Good track quality has been obtained in these liquids for the first time. The design, the performance and the operating conditions of the chamber are described, and an outlook on further tests is given.

1. Introduction Bubble chambers of many cubic meters as well as those with only a few hundred cubic centimeters volume are used in elementary particle research in beams up to 400 GeV/c momentum. The size of the bubble chamber, its liquid, the bubble size, the bubble density along the particle track, and the optical resolution are closely related, and their choise is governed largely by the kind of information one hopes to obtain from the interactions. Since the very beginning of physics with bubble chambers much emphasis has been put on their combination with electronic counters, mainly in order to improve particle identification and gamma detection: "hybrid systems" have been often in use, essentially with detectors outside the chamber liquid, such as External Muon Identifiers [1], External Particle Identifiers [2], External Gamma Detector [3], Internal Picket Fence [4], and various counter hodoscopes. However, in many cases it seems to be more desirable to obtain physics information from regions closer to the interaction vertex inside the liquid. To this end various "passive" elements have been put into the fiducial volume, such as Track-Sensitive Targets [5], Idled with hydrogen or deuterium and surrounded by heavy n e o n - h y d r o g e n mixtures; or simply metal plates, providing a higher conversion probability for gamma rays. Some of the present-day efforts are aiming at the application of electronic devices as calorimeters inside hydrogen or deuterium chambers, such as e.g. Solid Argon Ionization Chambers [6]. 0029-554X/81/0000-0000/$ 02.50 © North-Holland

A new and more direct way to the advantages of the above hybrid systems, especially at very high energies, is opened with liquid argon, which offers in addition to its track-sensitivity in a bubble chamber several unique properties: (1) electron drift over large distances may be possible during bubble chamber operation, allowing charge collection and, hence, immediate information about energy contained in very large electromagnetic or hadronic showers, which could otherwise only be obtained from tedious measurements of track curvatures; (2) liquid argon is known to be an excellent scintillator, a property, which could be employed to trigger the flash of the bubble chamber when a rare event occured, or even to get some information about the energy of an interaction; and (3) bubbles can be created in liquid argon by laser light in combination with the expansion, and these bubbles may serve as reference tracks or as a fiducial system. Furthermore, from a more technical and commercial point of view, liquid argon has the advantages of non-inflammability, that it can be cooled to its operating temperature by liquid nitrogen, and that both liquids are abundant and inexpensive, so that the construction of very large chambers can be envisaged. Following a proposal to build ARGONAUT - A Novel Detector for Very High Energy Neutrino Interactions - for Fermilab [7], as a first step towards the development of such a detector we studied the anticipated operating conditions for pure argon in a 2.7 1 test bubble chamber [8]. To the best of our knowledge this is the first time that track sensitivity in this

364

G. Harigel et al. / Bubble chamber filled with argon and nitrogen

liquid has been obtained. The quality of the bubble tracks at various temperatures is excellent and bubble size and density can be controlled to suit, in particular, very large bubble chambers. It had been demonstrated already that bubble tracks can be created by a laser beam during the chamber's expansion [9]. A charge coUection device is now installed and simultaneous bubble chamber operation and calorimetry will be tried. The application of a wavelength shifter on the chamber window should permit the collection of scintillation light produced by high-energy particles. Furthermore, liquid argon may show a measurable relativistic rise of ionization and, therefore, allow the identification of particles at high momenta with the help o f bubble counting. Because of the suitability of the test chamber - as far as pressure and the cooling system is concerned we tested also liquid nitrogen, and 75/25 and 50/50 volume percent nitrogen-argon mixtures. Besides some earlier indication of sensitivity of pure nitrogen to a radioactive source, which gave rise to single bubble ruptures (in a non-specified temperature and pressure range) in a Glaser Bubble Chamber [10], we have obtained for the first time excellent bubble tracks in

12

this liquid and in its mixtures with argon. We expect from our results that nitrogen-cooled chambers can be operated with all mixture ratios of nitrogenargon, covering a wide range of liquid densities, radiation, interaction and collision lengths, previously reserved for propane-freon mixtures in heavy liquid chambers or neon hydrogen mixtures in hydrogencooled chambers.

2. Experimental arrangement A 2.7 1 test bubble chamber (without a magnetic field) was built by the BEBC group at CERN using some surplus equipment and well-proven low-temperature techniques. The volume and the shape of the chamber were mainly determined by the performance of the hydraulic expansion system and the requirements for the calorimeter tests. A cylindrical chamber vessel with vertical axis, made out of stainless steel, with 16 cm inner diameter and 10 cm height, was chosen, being closed on the top by expansion bellows and on the bottom by a BK7 glass window (fig. 1). The servo-controlled hydraulic expansion system

I

,o /

/

8 6

9 1

4__

+/ SCALE

EXPERIMENTAL

ARGON

BUBBLE

iN

METER

CHAMBER

Fig. 1. Argon test bubble chamber. 1 - Chamber body, 2 - expansion piston with Scotchlite, 3 - chamber window, 4 - mirror, 5 - lens with annular flash, 6 - camera, 7 - expansion system, 8 - heat exchanger system, 9 - liquid argon reservoir, 10 - liquid nitrogen reservoir, 11 - vacuum tank, 12 - liquid nitrogen supply.

365

G. Harigel et al. /Bubble chamber filled with argon and nitrogen VENT "

~

I r -- -± ---1

I

--

I HEAT

II

L_q_

EXCHANGEI~ .~ ~. ~

vAcuu. r

-~----

I- =

-7:1 I

I, <: ~

~"

-

I

~&

~

__~-~,

I

~

vAc'ou,

vAcu'o.

I

I

ELECTRICAL 'HEATER

I ~,1!.

~,~_i~_=___n~ i i ~.i i..,

I I

I

<<->

=~

I

'

<>

"

~

I, - -

'- . . . . . . .

I

~

+

~,,~-~~7-~-~,,~, --

VACUUM TANK

I

'

[

II

~r -~ q I

II

I

i

o l,-

'-

~

I

÷G [~

1

I

II

L_T_"

_

~_ _ __-I>,:ff___ T __ +__ T__tx~__l_ ~" ~ e I ~ I ~ Z~ ~ 1 ,t~ WALLACE

. . . .

I -~ ~

. . . . . . .

I

~=

I

I

!

(Db-(~'_

GAUGE

F'-'-l

~

~-----q

1 /

/~ I

1

[ l

~1

I""- vAcuu.~iI

I i.----I-

I

/

I

I

I '

~/

!

VACUUM~ J

I

I I

.

[

/

g

[ I

. . . . . . . - - I

Bo..,,,,o, [

I

~-

~

J

k

II, I ~ I

y

I_~1

I 'I

I

j

---- - -L- - -I>~

Fig. 2. Cooling circuit and temperature control (schematic) Full line: nitrogen; dashed line: argon; dash-dotted line: Vent.

(a copy of a system built for LEBC [11]) allows for a maximum piston stroke of 3 mm, and with the bellows having a diameter of 14 cm, for a volume change of A V / V ~--1.5%. The shortest sinusoidal expansionrecompression cycle is 8 ms, and therefore the maximum repetition rate is about 120 Hz. The piston is operated with an oil pressure of 300 bar, acting on a piston area of 11 cm 2. The cooling of the chamber and its temperature control are done with a modified Amiot system [12] (fig. 2). The coolant is liquid nitrogen, provided from a 12001 dewar, which can be pressurized to 3.9 bar (--~91 K). Nitrogen is transferred continuously or in intervals during the operation of the chamber to an (upper) 9.5 1 reservoir inside the vacuum tar&. Inside the reservoir is a cooling loop (320 cm long, 0.8 cm outer diameter, i.e. -~-800 cm 2 active surface), which is connected to an (lower) 7.7 1 unit, which consists out of another 5.1 1 reservoir and the 2.6 1 cooling jacket of the chamber. During the cooldown about 3.5 standard cubic meters of argon gas (about 4.5 1 liquid argon) are condensed into this circuit, which is thereafter closed up. The temperature of the nitrogen

bath is kept above 85 K by limiting the pressure to >2 bar to prevent argon from being solidified at 83.7 K inside the cooling loop. A regulating valve in the argon cooling loop is controlled by a pressure transducer (Eckard-Regler) and tke amount of the valve opening determines the quantity of argon vapour to be recondensed by the nitrogen bath. This defines the vapour pressure and, hence, the temperature of the cooling circuit. To allow for a rapid change to higher chamber temperature, an electrical heater (about 100W) is incorporated in the cooling circuit. The short- and long-term temperature stability of the system is about -+0.1 K. The chamber is filled by condensation. The temperature of the chamber liquid is measured with a vapour pressure and a platinum thermometer. The absolute temperature is known with a precision of about -+0.3 K between the boiling and the critical point of argon. These values were checked at several temperatures against the vapour pressure in the cooling loop, and inside the chamber liquid via a capillary, connected to a pressure gauge (either Wallace and Tiernan, series 1500, maximum 35 bar; or Ashcroft Digigauge, 30 bar) within the pressure range

366

G. Harigel et al. / Bubble chamber filled with argon and nitrogen

cf these instruments. The typical temperature difference between the liquid in the chamber TL and in the cooling jacket Tcool is TL -- Tcool ~- 0.7 K. The static chamber pressure Pstat prior to the expansion is also measured via the capillary and the pressure gauges, with a precision of -+0.05 bar. During the expansion-recompression cycle the dynamic pressure is measured with a piezoelectric transducer (Kistler, type 701 A), which is at present calibrated at the operating temperature of the chamber to only +0.5 bar. An application of the calibration method described in ref. 13 will allow to improve on the value during a forthcoming run. The cooldown time of the chamber is about 30 h and is mainly limited by stress considerations for the glass window. The photography of the bubble tracks is done in bright-field illumination: a Scotchiite disc of 11 cm diameter is glued onto the expansion piston. The camera lens is surrounded by an annular flash tube, so that illumination and photography are done from the same side. With the given geometry of the vacuum tank (a former baffle of a BEBC diffusion pump) and the vertical chamber axis, we had to use a mirror inclined by 45 ° underneath the chamber window. A Schneider Componon-S lens, with a focal length of f = 180 mm was closed to an F-stop = 16 during most of the tests. The mean magnification over the chamber depth is 1/4. With this arrangement bubbles with diameters larger than about 100 /am can be photographed with an electrical flash power of 6 J with good contrast on standard microfile film (width 50 mm). At present only one camera has been installed.

3. Experimental results The chamber's expansion and photography are synchronized with the beam pulse. A short spill (>/2 /as) hadron beam, during the first part of the run with 70 GeV/c and later with 140 GeV/c negative pions and muons, was injected through a vacuum tank window into the chamber. The beam spread was 4 cm horizontally and 2 cm vertically. This beam was used parasitically together with other users in front of the bubble chamber. We aimed at a few tracks per expansion. The repetition rate of the chamber was 2 per 10 s accelerator cycle, dictated by the double ejection of the SPS into this radio-frequency separated beam line in the West Experimental Area. No systematic tests for higher repetition rates were attempted. In

total 6000 photos were taken under various conditions in the liquids mentioned above. 3.1. Results in argon

The main purpose of our tests was to prove that liquid argon can be made sensitive to ionizing particles in a bubble chamber, and to establish the working conditions, in particular the liquid temperature TL and the necessary expanded pressure /'exp. We used commercially available argon with a purity of 99.996%, which is of sufficient quality for bubble chamber operation. During the 5 d run period just prior to the long shutdown of the SPS we demonstrated that good tracks can be obtained in the temperature interval between ~132 and ~136 K, which corresponds to vapour pressures between Pv ~ 22 and 27 bar. The expanded pressures Pexp were between 9 and 15 bar, respectively. At these temperatures the liquid density d ~ 1.0 g/cm 3, the radiation length Xo--~20 cm, the absorption length X ab s ~'~ 116 cm, and the nuclear collision length ) t t o t a I "~ 76 cm. At the lower end of this temperature interval bubbles grew to diameters of about 400/am during 2.5 ms between particle injection and photography. Bubble densities of 15 bubbles/cm were obtained for the primary (minimum ionizing) particles, which could be measured with good accuracy on the beam tracks, which are perpendicular to the optical axis. Bubble densities are considerably higher for slower particles, as can be seen from the interaction shown in fig. 3. However, due to the lack of stereophotography we could not yet determine their densities as function of momentum (range). Over the whole temperature region a volume change of A V / V ~ - - I % appears appropriate to produce track sensitivity. With an expansion-recompression cycle duration of 15 ms, we obtained pressure drops of 2uD=Pv-Pexp = 14 bar with a piston stroke of about 2.2 mm. During the test run the operating conditions (TL and Pstatic) were not stable enough to determine with good precision bubble densities and growth rates as function of temperature and expanded pressure. Nevertheless, we found already that there is reasonable agreement between the measured maximum bubble sizes and the values expected from theory [14]. Extrapolation to expansion cycles of 100 ms, in use for big chambers like BEBC or the 15 Foot Bubble Chamber at Fermilab, let us expect for very large argon chambers bubbles of 0.8 mm diameter at flash delays of about ?9 ms [15].

G. Harigel et al. / Bubble chamber filled with argon and nitrogen

367

Fig. 3. Interaction o f a 70 GeV/c rr- in argon (section of the photograph). Liquid temprature ~136.5 K. Bubble density o f the 3 incoming beam particles --,10 bubbles/cm.

During the tests we were able to determine at two fairly high liquid temperatures the foam limit [16], which permits then an estimate of the lowest useful expanded pressure throughout the entire liquid temperature range. 3.2. Results in nitrogen

The chamber was ftlled by condensation with ordinary quality nitrogen. We obtained good tracks in the temperature interval between ~111 and ~115 K, with vapour pressures of Pv ~- 15.5 and 19.0 bar, and pressure drops of AP = Pv - Pexp ~- 9.5 and 5.0 bar, respectively. In this temperature interval the density of the liquid d ""0.6 g/cm 3, the radiation length Xo "~ 65 cm, the absorption length Xabs ~- 290 cm, and the nuclear collision lenght )ktota I "~ 210 cm. The foam limit has been measured in nitrogen at one temperature. 3.3. Results in nitrogen-argon mixtures

At the end of the run two nitrogen-argon mix-

tures were tested. With ~50/50 volume percent nitrogen-argon we got good tracks between ~119 and ~123 K, and for a ~75/25 volume percent nitrogenargon mixture between ~112 and ~ l 1 8 K . Fig. 4 shows a picture from the 50/50% mixture, where the minimum ionizing tracks have a bubble density of 16 bubbles/cm. The operation of the chamber seems to be somewhat easier with mixtures than with the pure liquids. We intend to continue the tests with mixtures. We have strong evidence that much higher bubble densities can be obtained in the mixtures than in the pure liquids. The thermodynamic data of nitrogen-argon mixtures as function of the mixture ratio are unknown to us. We may, however, assume the approximate validity of a Van der Waals' "b" rule for the liquid densit~ PL (and the vapour pressure Pv), related to the shor range repulsive forces between molecules of definit volume, so that 2

PI~ = ~ i=1

o i X ~ + 2012XIX2 ,

368

Fig. 4. Interaction of 140 GeV/c n- in a 50/50 volume percent nitrogen-argon mixture. Liquid temperature ~123 K. Bubble density of the ~ 12 incoming particles ~ 16 bubbles/era. 80-~-

J/L

Xo radiationl

~ /// I

4SS.+ole~

/

~J

/

nuclear collision

/~absorption / Z--300

z/

-200

40

I

I

~'-.~'~-...~

20+

~ %Ne

-100

"~"""

I I

+ I 0--~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N 2 at 113°K N 2 / A r - mixtures

0 Ar at t35°K

Fig. 5. Radiation, nuclear collision, and absorption length for liquid argon at 135 K and liquid nitrogen at 113 K. Radiation lengths for some often used neon-hydrogen mixtures are indicated.

G. Harigel et a L / Bubble chamber filled with argon and nitrogen

References

with

/312 = rtl ~* l

369

1/3

+p113)}3 ,

where X i is the mole fraction of the ith component. Then, one can put the appropriate scale on the abscissa in fig. 5. For comparison we indicated the radiation lengths for some often used n e o n - h y d r o g e n mixtures. This shows that one can overlap a wide range of n e o n - h y d r o g e n mixtures by the much easier to handle n i t r o g e n - a r g o n mixtures.

4. Conclusions It was demonstrated that argon, nitrogen and two n i t r o g e n - a r g o n mixtures can be used as liquids in nitrogen-cooled bubble chambers. In particular, liquid argon has attractive features which are superior to other liquids: it may be suited to allow calorimetry and triggering on rare events during bubble chamber operation. Tests on electron drift and the measurement of scintillation light are being pursued with the present experimental set-up. Systematic measurements of bubble densities and sizes at various temperatures in a m i n i m u m ionizing beam are scheduled. The authors are greatly indebted to many people in the BEBC-group at CERN, in particular to W. Bichler, A. Carrere, A. Dupenloup, J. Feyt, A. Herv6, J.-P. Orlic, P. Rada, E. Rusconi, W. Seidl, and D. Voillat for their dedicated help during construction and tests of the bubble chamber, and to the operators of the S3-beam line. Continuous support for the project from Dr. H.P. Reinhard is gratefully acknowledged.

[1] Hawaii-LBL CoUaboration, R.J. Cence et al., Proc. Int. Conf. on Instruments of high energy physics, Frascati (1973) p. 101; C. Brand, R.C.A. Brown, E. Chesi, H. Foeth, A. Gilgrass, H.J. Hilke, D. Jacobs, P. Lazeyras and Y. Sacquin, Nucl. Instr. and Meth. 106 (1976) 485. [2] M. Aderholz, P. Lazeyras, I. Lehraus, R. Matthewson and W. Tejessy, Nucl. Instr. and Meth. 123 (1975) 237; W.W.M. Allison, C.B. Brooks, J.N. Bunch, J.H. Cobb, J.L. Lloyd and R.W. Pleming; Nucl. Instr. and Meth. 119 (1974) 499. [3] A. Bettini et al. CERN/SPSC/75-24/P 45 (March5, 1975) (unpublished). [4] H.C. BaUagh et al., Proc. Topical Conf. on Neutrino physics at accelerators, Oxford (1978) p. 362; H. Foeth, Nucl. Instr. and Meth. 176 (1980) 203. [5] R. Florent, C. Geles, G. Harigel, H. Leutz, F. Schmeissner, J. Tischhauser, G. Horlitz, S. Wolff and H. Filthuth, Nucl. Instr. and Meth. 56 (1967) 160. [6] J.H. Cobb, and D.J. Miller, Nucl. Instr. and Meth. 141 (1977) 433. [7] G. Harigel, H. Kautzky, P. Mclntyre and A. van Ginneken, Fermilab Proposal 601 (1978) (unpublished). [8] G. Harigel, G. Linser and F. Schenk, Conf. on Neutrino physics and astrophysics, Erice (1980) (unpublished); CERN Courier (September, 1980) p. 251. [9] G. Harigel, H.J. Hilke, G. Linser and F. Schenk, submitted to Nucl. Instr. and Meth. [10] R.H. Hildebrand and D.E. Nagle, Phys. Rev. 92 (1953) 517. [11] C. Fisher, A. Herv6, H. Leutz, G. Passardi, J. Tischhauser and H. Wenninger, to be published in Nucl. Instr. and Meth. [12] P. Amiot, Nucl. Instr. and Meth. 3 (1958) 275. [13] G. Horlitz, S. Wolff and G. Harigel, Nucl. Instr. and Meth. 65 (1968) 353. [14] M.S. Plesset and S.A. Zwick, J: Appl. Phys. 23 (1952) 95. [15] G. Harigel and J.F. Nyborg, Internal Report CERN/EF/ BEBC/OP-81 (1981) (unpublished). [16] D.V. Bugg, Prog. Nucl. Phys. 7 (1959) 1.