- Email: [email protected]
A large liquid xenon time projection chamber for the study of the radiative pion decay
s
Nuclear Instruments and Methods in
Physics Research A 376 (1996) 149-154
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH
.- ____
BB EISEYIER
SectlonA
A large liquid xenon time projection chamber for the study of the radiative pion decay G. Carugnoa’*, G. BressiC, S. Cerdoniodse, E. Conti”, A.T. Meneguzzoavb, R. Onofrio”‘b, D. Zanellod, U. Beriotto”‘b, S. De Biasia”‘b, M. Nicolettoa, R. Pedrotta”, N. Toniolof “INFN, sez. di Padova, hDip. di Fisica
“G. Galilei”, ‘INFN,
Via F. Marzolo
Universitri di Padova,
Fisica,
Italy 8, Padova,
Italy
sei. di Pavia, Via U. Bassi 6. Pavia. Italy
“INFN, se:. di Roma I, P.le A. More, ‘Dip.‘di
8. Padova. Via Marzolo
Universitli di Roma “La Sapienza”,
‘INFN, Lab. Naz. Lqnaro,
Roma, Italy P.le A. More.
Via Romea 4. LPgnaro
Received 19 December 1995; revised
(PD),
Ram,
Italy
Italy
form received 30 January 1996
Abstract A 64 1 liquid xenon TPC has been operated for the detection and measurement of the low energy y rays from the decay 7~+ --+ pf + Y + y. We present a detailed description of the chamber, the readout electronics and data acquisition together with the measurement of the liquid xenon purity and time stability obtained during a period of about two months. The electron lifetime was always longer than 2.7 ms (95% CL.) and no deterioration of the liquid purity has been observed. Such a purity level and stability have never been previously reported for a large volume liquid xenon chamber, making this technique mature for physical applications.
1. Introduction
In this paper we report on the technical performance of a large liquid xenon (LXe) Time Projection Chamber (TPC) which was used in the RAPID experiment [1] for the measurement of the y-rays from the rare decay n++p+
+v+y.
(1)
For pions at rest, the y’s from Eq. (1) have energies ranging from zero to 30 MeV, with a distribution strongly peaked at zero. The LXe is an optimal medium for y-ray calorimetry due to its high atomic number, high density, high electron yield. Moreover, it scintillates at I75 nm with a decay time -20 ns and a light yield comparable to the yield of a NaI(T1) crystal. An exhaustive review of its main properties can be found in Refs. [2,3]. The exploitation of the fast scintillation light was essential for our experiment not only to determine the TPC drift time but also in order to set a physics trigger for the wanted events. The LXe scintillation light was detected by *Corresponding
author.
Tel.
+39
[email protected]. 016%9002/96/$15.00 PII
Copyright
SOl68-9002(96)00245-8
01996
49
8277071,
e-mail
12 UV photomultipliers which worked at cryogenic temperature (T- 180 K). The ionization signal was used in order to measure the energy released in the LXe. Since the ionization charges must drift to the collecting anodes for as long as 30 ps, the liquid purity and, above all, its long-term time stability is a crucial point in order to achieve an accurate energy measurement. No data were available on the LXe purity and time stability for large volume chambers, the only reports referring to volumes 5 5 1 (4-61. Great care has been dedicated to the proper choice of the materials inside the chamber, to their cleaning procedure and to the xenon purification. The purity time stability can be obtained by cleaning continuously the Xe flowing through the purifier by means of a recirculation pump. The recirculation has been demonstrated to be very effective in the case of liquid argon [7]. A simpler approach is possible. A time stability of several weeks can be reached provided that a very good cleaning and outgassing of all materials is obtained, as demonstrated in Ref. [6]. This was the approach that we followed successfully. Here we report on the LXe purity of the TPC during the two months period of data taking of process (I). During the run the electron lifetime was periodically measured with a dedicated on-line monitor mounted inside the TPC. The method, already described in Ref. [8], consists on the
Elsevier Science B.V. All rights reserved
150
G. Cuncpzo et 01. I Nucl. Instr. nnd Merh. in Ph,vs. Res. A 376 (1996) 149- 154
comparison between the electric charge photoextracted from the cathode and the charge arriving to the anode after a drift of some cm in the LXe. The TPC is described in detail in Section 2. the purity monitor in Section 3 and the chamber temperature control system in Section 5. The cleaning procedure and the xenon purification are presented in Section 4 and the results on the LXe purity in Section 8. Section 6 deals with the TPC electronics and data acquisition. The procedure for the PM plateau setting is described in Section 7. Section 9 presents our conclusions.
(4
2. The chamber The LXe TPC was contained in a safety vacuum tank which also housed the dewar for the liquid nitrogen (Fig. 1). The tank volume is about 9 m7 and was evacuated down to IO-’ mbar by mean of a diffusion pump. In case of a total escape of the Xe from the chamber, the tank is capable to contain all the gas with a reasonable pressure on its walls. On the y-rays entrance side the tank is closed by a titanium window, 1.2 mm thick, 20 cm diameter. The pumping system and the purifier were placed on a platform above the tank and connected to the TPC by two pipes. Both pipes could be closed outside the tank by two all-metal valves, in order to insulate the cold side from the room temperature. The TPC vessel (see Fig. 2a) was a stainless steel cylinder of 40 cm diameter and 44 cm length. It was covered with a 1 cm thick copper jacket and kept in
To pump and purifier
Ti window
Fig. 1. A view of the vacuum liquid nitrogen dewar
safety tank with the TPC and the
(b)
Fig. 2. (a) A view of the chamber.
(b) A cross section of the TPC.
thermal contact with the liquid nitrogen dewar through two large copper bars. Three heating jackets were fixed on the copper sheet. The y entrance side of the TPC was a titanium window of 1 mm thickness, 25 cm diameter, fixed on the front flange by a Helicoflex [9] O-ring. The titanium was chosen due to its low atomic number and high resistance to mechanical stress. The back flange held the electrical high voltage and signals feedthroughs and the purity monitor chamber. On the vessel 12 quartz windows were mounted, grouped into 4 sets. Each group consists of three windows, whose axes lay on a plane perpendicular to the TPC axis. The three window-axis are in a symmetrical configuration. which means they form 120” angles respect to each other. The total volume of the chamber was about 64 I. A cross section of the TPC is drawn in Fig. 2b. The detector sensitive volume was a cylinder, 35 cm diameter and 39 cm length, divided into 6 drift region, 6.3 cm deep, corresponding to about 38 I. Each drift volume was delimited by a cathode and an anode. Each cathode was in
G. Curugno et al. I Nucl. Instr. und Meth. in Ph?;s. Res. A 376 (1996) l-19- I54
common for two consecutive drift regions. Four of the 6 anodes were grouped into pairs (see Fig. 2b). The problem was to minimize the dead space between the electrodes taking care that they also need to be electrically independent. In particular, their capacitive inter-coupling must be zeroed. It was necessary to put a ground plate between the two anodes consisting in an aluminium disk, 2 mm thick, on which the anodes were mounted, isolated by small ceramic spacers. The electrodes were in titanium, 0.5 mm thick, at 0.5 mm from the plate. The total thickness of the pair of anodes was therefore 4 mm. Due to the large capacitance of the anodes with the ground plate, we expected the electronic noise of those channels to be higher than the noise of the first and last anode, which were single. The first anode was at 13, mm from the y entrance titanium window. In order to minimize the inactive LXe, the aluminium anode was 10 mm thick, so that the xenon is replaced by a material with lower density and atomic number. Each anode was divided in two circular concentric pads, the inner one (28 cm diameter) delimiting the fiducial volume of the detector. Such a geometry did not allow us to reconstruct the three-dimensional tracks of the events. On the other hand, a very segmented anode would imply a loss in energy resolution because of the electronic noise contribution of each channel. At 3 mm in front of each anode there was a screening grid. Several efforts were done to develop grids with a large diameter (37 cm) which do not suffer mechanical deformations after successive thermal cycles. Trials with nickel or copper meshes and wires were all unsuccessful. A good solution was to drill a 0.2 mm thick stainless-steel foil, with a computer controlled CO, laser. The hole diameter was 2 mm and the pitch 2.1 mm. Grid transparency and screening inefficiency were measured with a small ionization chamber filled with gaseous Xe and irradiated by an o. source. We compared the grid and the anode signals when the grid was a metallic mesh. whose properties were known, or a laser perforated foil, as described above. The screening inefficiency of two grids was the same, 5 4%, and the grid transparency was nearly 100% when the ratio of the electric fields before and after the grid is l/4. In order to guarantee the electric field uniformity 6 guard rings, 35 cm internal diameter, were placed between anode and cathode, each connected to the following one by a 50 MR ceramic resistor. All the mechanical elements were supported by three ceramic bars [IO], 1 in. diameter, mounted on three stainless-steel bars. They were fixed on the back flange which supported the overall detector. The 175 nm wavelength LXe scintillation light was detected by 12 UV photomultipliers [ 111, 10 cm diameter. They had a UV transparent quartz window and a nominal quantum efficiency at 175 nm and room temperature of
151
about 10%. The PMs were optically coupled to the TPC by quartz windows, 10 cm diameter and 5 mm thickness, sealed on CFlOO flanges [ 121. Their transmission at 175 nm was about 70%.
3. The purity monitor The purity monitor was a double grid ionization chamber which has been described in detail in Ref. [8]. Here we recall its main features. The chamber consisted in a brass cathode and an anode, each 35 mm diameter and screened by a grid placed 5 mm from the electrode. The drift space between the two grids was 5 cm and the electric field was kept uniform by means of 4 guard rings. 30 mm internal and 35 mm external diameter. separated by I cm. They were connected to each other by 50 Ma ceramic resistors. All the mechanical elements of the monitor chamber were supported on 3 ceramic bars [IO] fixed on a CFI 50 flange which was in turn mounted on the back flange of the TPC. The two grids and the electrodes were accessible from the outside to be set at the desired voltage. The signals induced on the cathode and the anode by the drifting electrons were read by two low-noise charge amplifiers with an integration constant of about 340 us. The electrons were extracted from the cathode by a IO ns UV pulse, generated by a Nd:Yag laser quadrupled in frequency [13], at the wavelength of 266 nm. The light was injected into the liquid by mean of a quartz optical fibre. Two special feedthroughs were used for the passage of the fibre inside the vacuum tank and from the tank to the chamber.
4. The cleaning procedure and xenon purification Before being assembled almost all the TPC components were cleaned with the standard procedure described in Ref. [14]. Ceramic resistors, kapton wires and quartz windows were cleaned with acetone and n-hexane. The xenon purifier consisted of two Oxisorb cartridges [15] and two getter filters (working temperature 450°C) [16] working in parallel. The chamber and the purifier were helium leak tested with a sensitivity of 10m9 mbar l/s. pumped and baked at about 120°C under vacuum for a week. Both the purifier and the TPC were pumped by oil-free turbo pumps allowing to reach a vacuum level = 1Omx mbar. As already mentioned in Refs. [6.17] the recirculation of gaseous xenon was a crucial operation for the ultimate cleaning of all the surfaces. The technique consists on recirculating the xenon through the hot chamber (at about 80°C) and the purifier by means of a small double membrane compressor for at least 3 days. Then the chamber was again evacuated at lo-’ mbar. The recirculation before the liquefaction is
very effective in removing all the molecules (02, H,O, co,;. . ), which survived to the baking, from the surfaces of the vacuum system. Moreover. this operation causes the absorption of xenon atoms on the surfaces themselves and the substitution of the dangerous molecules with “clean” atoms. A spectrometer gauge placed above the TPC could measure the partial pressure of the components of the residual gas before and after the recirculation procedure. While the total outgassing rate remained almost unchanged, the contribution of each molecule changed significantly. For example, the H,O and 0, lines decreased by more than two orders of magnitude, while the xenon atoms, absent before the recirculation, turned out to be the only responsible for the outgassing rate. Another precaution was to isolate the cold TPC from the room temperature pipes by mean of two all-metal valves to avoid the cryogenic pumping of the outgassed molecules. The initial gas impurity concentration was about 50 ppm. Starting from room temperature about I4 hr were needed to stabilize the chamber temperature at = 180 K. The purifier could stand a flow of 20 gaseous I/min. but the actual rate was 5.5 l/min. With this rate the liquefaction procedure lasted nearly 5 days.
5. The temperature
control system
The temperature of the TPC was monitored in IO different position by PTIOO platinum resistors and one was inside the chamber in contact with the LXe. The values were digitized every 5 min and read by a Macintosh Quadra 950 computer via an IEEE488 interface bus. The
140
-
120
-
100
-
80
-
60
-
kg. During the run the LXe average temperature was 180 K-t2%. where the error is due to the absolute PTIOO calibration.
6. The front-end electronics
and data readout
The TPC electric circuit, front-end electronics and acquisition system are schematically depicted in Fig. 4. The three cathodes were biased by a no-switching high voltage power supply filtered by a low-pass frequency filter. The electric field I?~~,~, in the drift region was E,,,,rt = 500 V/cm, corresponding to a total drift time a32 IJs. The grids were fed to a low-noise charge amplifier (QA) with an integration constant ro_, = 340 t.~s via a 4.7 nF capacitor to decouple the amplifier from the high voltage. The signal was then filtered by an active filter with a low-frequency cutoff at I kHz. The anodes were fed directly to similar QAs and then filtered by 5 kHz cutoff frequency active filters. In order to match the finite range of our ADC with the wider range of the signal amplitudes, each anode channel was split in two and fed to two amplifiers with different gains. The QA front-end box was mounted outside the safety tank about 2 m from the TPC feedthroughs. Such approach is not the best from the point of view of the electronic noise but allows a practical replacement of the amplifiers. The PMs were independently supplied by a HV source. Each output current signal was split in two, one for a disc~minator for the trigger logic, and the other fed into a
QA.
180.5
Fig. 3. Dist~bution days.
computer was used to control if the LXe temperature is in the desired range and to consequently change the power dissipation in the three heating jackets. The average power dissipated with the chamber tilled was about 220 W. In such a way the temperature could be kept constant to within i0.5 K. In Fig. 3 the dist~bution of the LXe temperature during a time interval of 3 days is shown. The total mass to be controlled in temperature was about 400
181
181.5
182
LXe temperature (K) of the LXe temperature for a period of three
All the signals (24 from the anodes, 6 from the grids and 12 from the PMs) were fed to an &bit fast waveform analyzer (FADC) (CAEN mod. E426) for their digitization at 5 MHz frequency. A VME bus linked the FADC to a RTX module (CAEN mod. E427) which collected the data. A manager board, connected to the RTX by a CAENET bus, transferred the data to a circular memory. When a trigger signal occurred the data were unloaded from the circular memory to the computer memory (a Macintosh Quadra 700) for their display and recording on tape. The acquisition rate was limited to 5 Hz by the writing procedure on the exabyte tape but did not represent any limitation for our trigger rate.
153
G. Carugno et al. I Nucl. hzstr. and Meth. in Phys. Res. A 376 (1996) 149-154
-HV UV PM I\
IR,
Cil,‘=r
-HV e-/
current
-i
I I-
monitor
Y
,c
1
/A~______@!______.
Q
4.7z-l9
Act& filter
5 kHzcutoff
I .
7
Computed exabyte
4rtive tiltm
Fig. 4. Scheme of the TPC electric circuit, front-end
7. Operations
with the UV PMs
The 12 PMs for the detection of the LXe scintillation light worked at cold temperature. Under those conditions there was a strong reduction of the dark current. On the contrary the gain stability was reached only after 5-6 hr of continuous operation. Moreover, even a short interruption of the voltage supply could modify the PMs working conditions. The working points were established tagging the collinear 511 keV y-rays emitted by a “Na source. The y direction was along the chamber axis. One y ray was detected by a 4X4 inch cylindrical NaI(TI) detector and
PM30
0
.
and acquisition
the other by the LXe chamber. The 511 y ray of the NaI was defined by mean of a window discriminator. The threshold for the LXe PMs was -40 mV. We counted the number of coincidences between NaI and TPC as a function of the voltage applied to the PMs. An example of the resulting curves is plotted in Fig. 5. The plateau value defines the right working point. The working voltages ranged from - 1350 V to - 1700 V.
8. Results We measured the charge Q, extracted from the cathode and the charge Q, arriving to the anode after the drift and the time interval T,, T, and T, which are respectively the cathode-to-grid, the grid-to-grid and the grid-to-anode drift times. The relationship among these quantities and the electron lifetime 7 is
QC
T, 1 - exp( - T,/T)~~~
b :
K=+exp c
.
0 i II,I,,,,,,,,,,,,,,I,,,, 1100 1150 1200
I ,,,,I,,,.,.,,,: 1250
(
TD+ Tc 7
>’
PM1
which can be approximated,
loo0
system.
R _ Q, _ T, 1 - exp(- Ld
.
0 0
:
’
0
electronics
1300
1350
1400
1450
1500
Hv (Volts)
Fig. 5. Plateau curve for two different PMs based on the 511 keV y-y tagging method.
if T,, T, e T, as
( _r> (
7
where T = T, +(T, + T,)/2. haviour of R as a function of field of 10 V/cm. Each point measurements and the error
(2) The Fig. 6 shows the bethe run-time for an electric is the average of about 10 bars reflect both the in-
Acknowledgments
!
0.6 0.4
= 0.986 f 0.032
-
X9Y = 0.53
0.2 t
t
ot”“““‘i”“““‘I”” 0
10
20
30
40
50
Days
Fig. 6. Ratio of the charge arriving to the anode over the charge extracted from the cathode vs time. The electric drift field was 10 V/cm.
strumental and the statistical errors. The hypothesis that R was constant during the run gives a ,$/d.o.f. = 3.7/7. The average value is R = 0.986kO.032, corresponding to r = 15.2 ms. The uncertainty introduced by the error in T is 9%. The evaluation of the error on 7 requires some care since the x2 versus T plot is highly asymmetric when R is compatible with the unity within the errors. In our case T is even consistent with being infinity. On the left side of the minimum however the x2 curve behaves more regularly so that in genera1 one can determine a lower limit on P. We get r 2 2.7 ms
(95% C.L.).
9. Conclusions A 64 I LXe TPC was for the first time brought to an experimental floor and worked for physics, showing that this technique is ready for applications. There are several points we would like to stress: 1) we liquefied thrice in the TPC with the purification technique described above and we always obtained an electron lifetime some milliseconds long. One of those results has been reported in Ref. [18]; 2) although we feel that the operation can be speeded up, the liquefaction was a rather lengthly procedure (5 days); 3) the liquid purity was stable through the whole period of two months without any need for recirculation; 4) thermal stability was obtained and kept under control without any problem; 5) the operations with the UV PMs at cryogenic temperature was reliable.
The authors wish to thank S. Marigo. M. Negrato, L. Ziomi (Legnaro), D. Maniero, G. Greggio, G. Salvato. M. Turcato and P. Zatti (Padova) for their skilful technical assistance. Many design problems would not have been solved without the help of C. Fanin (Padova). The authors are also very grateful to S. Ventura (Padova) for his help during the data taking at the PSI. The assistance of P Gheno at the PSI. is acknowledged. Finally, the authors thank the I.N.F.N. National Laboratory in Legnaro and the Paul Scherrer Institute (Villigen, Switzerland) for their hospitality and support.
References G. Boca et al., Preprint Universita di Roma “La Sapienza” no. 733 (1990); G. Carugno et al., Proc. Int. Conf. on Liquid Radiation Detectors. Waseda University, Tokyo, 1992, ed. T. Doke. r21 T. Doke, Portugal. Phys. I2 (1981) 9. 131 T. Doke, Nucl. Instr. and Meth. 196 (1982) 87. 141 P. Benetti at al., Nucl. Instr. and Meth. A 329 (1993) 361, 151 M. Ichige, E. Aprile, T. Doke, K. Hasuike. K. Itoh, J. Kikuchi and K. Masuda. Nucl. Instr. and Meth. A 333 (1993) 355. R. [61 G. Carugno, S. Cerdonio. E. Conti, A. Meneguzzo. Onofrio, G. Bressi and D. Zanello, Nucl. Instr. and Meth. A 335 (1993) 338. VI P. Cennini et al., Nucl. Instr. and Meth. A 345 (1994) 230. 181 G. Carugno, B. Dainese. F. Pietropaolo and F. Ptohos. Nucl. Instr. and Meth. A 292 (1990) 580. [91 Cefilac, Paris, France. r101 MACOR, a machinable ceramic made by Coming, U.S.A. [Ill Mod. 9531 QB, Thorn Emi, London. U.K. 1121 Ceramaseal, New Lebanon, New York, U.S.A. [I31 Spectron Laser System, Derby, U.K. [I41 A.G. Methewson, CERN-&R-VA 74-10 and 74-30. [151 Messer Grieheim GmbH, Dusseldorf, Germany. I161 Saes Getter S.p.A., Milano. Italy. [I71 E. Conti, A. T. Meneguzzo, R. Onofrio, Cl. Carogno. S. Cerdonio, G. Bressi and D. Zanello, Nucl. Instr. and Meth. A 356 (1995) 286. 1181 G. Carugno, S. Cerdonio, E. Conti. A. Meneguzzo and R. Onofrio. in: Gamma-ray detector physics and applications, ed. E. Aprile. Proc. SPIE 2305 (1994) p. 63.
Nuclear Instruments and Methods in
Physics Research A 376 (1996) 149-154
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH
.- ____
BB EISEYIER
SectlonA
A large liquid xenon time projection chamber for the study of the radiative pion decay G. Carugnoa’*, G. BressiC, S. Cerdoniodse, E. Conti”, A.T. Meneguzzoavb, R. Onofrio”‘b, D. Zanellod, U. Beriotto”‘b, S. De Biasia”‘b, M. Nicolettoa, R. Pedrotta”, N. Toniolof “INFN, sez. di Padova, hDip. di Fisica
“G. Galilei”, ‘INFN,
Via F. Marzolo
Universitri di Padova,
Fisica,
Italy 8, Padova,
Italy
sei. di Pavia, Via U. Bassi 6. Pavia. Italy
“INFN, se:. di Roma I, P.le A. More, ‘Dip.‘di
8. Padova. Via Marzolo
Universitli di Roma “La Sapienza”,
‘INFN, Lab. Naz. Lqnaro,
Roma, Italy P.le A. More.
Via Romea 4. LPgnaro
Received 19 December 1995; revised
(PD),
Ram,
Italy
Italy
form received 30 January 1996
Abstract A 64 1 liquid xenon TPC has been operated for the detection and measurement of the low energy y rays from the decay 7~+ --+ pf + Y + y. We present a detailed description of the chamber, the readout electronics and data acquisition together with the measurement of the liquid xenon purity and time stability obtained during a period of about two months. The electron lifetime was always longer than 2.7 ms (95% CL.) and no deterioration of the liquid purity has been observed. Such a purity level and stability have never been previously reported for a large volume liquid xenon chamber, making this technique mature for physical applications.
1. Introduction
In this paper we report on the technical performance of a large liquid xenon (LXe) Time Projection Chamber (TPC) which was used in the RAPID experiment [1] for the measurement of the y-rays from the rare decay n++p+
+v+y.
(1)
For pions at rest, the y’s from Eq. (1) have energies ranging from zero to 30 MeV, with a distribution strongly peaked at zero. The LXe is an optimal medium for y-ray calorimetry due to its high atomic number, high density, high electron yield. Moreover, it scintillates at I75 nm with a decay time -20 ns and a light yield comparable to the yield of a NaI(T1) crystal. An exhaustive review of its main properties can be found in Refs. [2,3]. The exploitation of the fast scintillation light was essential for our experiment not only to determine the TPC drift time but also in order to set a physics trigger for the wanted events. The LXe scintillation light was detected by *Corresponding
author.
Tel.
+39
[email protected]. 016%9002/96/$15.00 PII
Copyright
SOl68-9002(96)00245-8
01996
49
8277071,
12 UV photomultipliers which worked at cryogenic temperature (T- 180 K). The ionization signal was used in order to measure the energy released in the LXe. Since the ionization charges must drift to the collecting anodes for as long as 30 ps, the liquid purity and, above all, its long-term time stability is a crucial point in order to achieve an accurate energy measurement. No data were available on the LXe purity and time stability for large volume chambers, the only reports referring to volumes 5 5 1 (4-61. Great care has been dedicated to the proper choice of the materials inside the chamber, to their cleaning procedure and to the xenon purification. The purity time stability can be obtained by cleaning continuously the Xe flowing through the purifier by means of a recirculation pump. The recirculation has been demonstrated to be very effective in the case of liquid argon [7]. A simpler approach is possible. A time stability of several weeks can be reached provided that a very good cleaning and outgassing of all materials is obtained, as demonstrated in Ref. [6]. This was the approach that we followed successfully. Here we report on the LXe purity of the TPC during the two months period of data taking of process (I). During the run the electron lifetime was periodically measured with a dedicated on-line monitor mounted inside the TPC. The method, already described in Ref. [8], consists on the
Elsevier Science B.V. All rights reserved
150
G. Cuncpzo et 01. I Nucl. Instr. nnd Merh. in Ph,vs. Res. A 376 (1996) 149- 154
comparison between the electric charge photoextracted from the cathode and the charge arriving to the anode after a drift of some cm in the LXe. The TPC is described in detail in Section 2. the purity monitor in Section 3 and the chamber temperature control system in Section 5. The cleaning procedure and the xenon purification are presented in Section 4 and the results on the LXe purity in Section 8. Section 6 deals with the TPC electronics and data acquisition. The procedure for the PM plateau setting is described in Section 7. Section 9 presents our conclusions.
(4
2. The chamber The LXe TPC was contained in a safety vacuum tank which also housed the dewar for the liquid nitrogen (Fig. 1). The tank volume is about 9 m7 and was evacuated down to IO-’ mbar by mean of a diffusion pump. In case of a total escape of the Xe from the chamber, the tank is capable to contain all the gas with a reasonable pressure on its walls. On the y-rays entrance side the tank is closed by a titanium window, 1.2 mm thick, 20 cm diameter. The pumping system and the purifier were placed on a platform above the tank and connected to the TPC by two pipes. Both pipes could be closed outside the tank by two all-metal valves, in order to insulate the cold side from the room temperature. The TPC vessel (see Fig. 2a) was a stainless steel cylinder of 40 cm diameter and 44 cm length. It was covered with a 1 cm thick copper jacket and kept in
To pump and purifier
Ti window
Fig. 1. A view of the vacuum liquid nitrogen dewar
safety tank with the TPC and the
(b)
Fig. 2. (a) A view of the chamber.
(b) A cross section of the TPC.
thermal contact with the liquid nitrogen dewar through two large copper bars. Three heating jackets were fixed on the copper sheet. The y entrance side of the TPC was a titanium window of 1 mm thickness, 25 cm diameter, fixed on the front flange by a Helicoflex [9] O-ring. The titanium was chosen due to its low atomic number and high resistance to mechanical stress. The back flange held the electrical high voltage and signals feedthroughs and the purity monitor chamber. On the vessel 12 quartz windows were mounted, grouped into 4 sets. Each group consists of three windows, whose axes lay on a plane perpendicular to the TPC axis. The three window-axis are in a symmetrical configuration. which means they form 120” angles respect to each other. The total volume of the chamber was about 64 I. A cross section of the TPC is drawn in Fig. 2b. The detector sensitive volume was a cylinder, 35 cm diameter and 39 cm length, divided into 6 drift region, 6.3 cm deep, corresponding to about 38 I. Each drift volume was delimited by a cathode and an anode. Each cathode was in
G. Curugno et al. I Nucl. Instr. und Meth. in Ph?;s. Res. A 376 (1996) l-19- I54
common for two consecutive drift regions. Four of the 6 anodes were grouped into pairs (see Fig. 2b). The problem was to minimize the dead space between the electrodes taking care that they also need to be electrically independent. In particular, their capacitive inter-coupling must be zeroed. It was necessary to put a ground plate between the two anodes consisting in an aluminium disk, 2 mm thick, on which the anodes were mounted, isolated by small ceramic spacers. The electrodes were in titanium, 0.5 mm thick, at 0.5 mm from the plate. The total thickness of the pair of anodes was therefore 4 mm. Due to the large capacitance of the anodes with the ground plate, we expected the electronic noise of those channels to be higher than the noise of the first and last anode, which were single. The first anode was at 13, mm from the y entrance titanium window. In order to minimize the inactive LXe, the aluminium anode was 10 mm thick, so that the xenon is replaced by a material with lower density and atomic number. Each anode was divided in two circular concentric pads, the inner one (28 cm diameter) delimiting the fiducial volume of the detector. Such a geometry did not allow us to reconstruct the three-dimensional tracks of the events. On the other hand, a very segmented anode would imply a loss in energy resolution because of the electronic noise contribution of each channel. At 3 mm in front of each anode there was a screening grid. Several efforts were done to develop grids with a large diameter (37 cm) which do not suffer mechanical deformations after successive thermal cycles. Trials with nickel or copper meshes and wires were all unsuccessful. A good solution was to drill a 0.2 mm thick stainless-steel foil, with a computer controlled CO, laser. The hole diameter was 2 mm and the pitch 2.1 mm. Grid transparency and screening inefficiency were measured with a small ionization chamber filled with gaseous Xe and irradiated by an o. source. We compared the grid and the anode signals when the grid was a metallic mesh. whose properties were known, or a laser perforated foil, as described above. The screening inefficiency of two grids was the same, 5 4%, and the grid transparency was nearly 100% when the ratio of the electric fields before and after the grid is l/4. In order to guarantee the electric field uniformity 6 guard rings, 35 cm internal diameter, were placed between anode and cathode, each connected to the following one by a 50 MR ceramic resistor. All the mechanical elements were supported by three ceramic bars [IO], 1 in. diameter, mounted on three stainless-steel bars. They were fixed on the back flange which supported the overall detector. The 175 nm wavelength LXe scintillation light was detected by 12 UV photomultipliers [ 111, 10 cm diameter. They had a UV transparent quartz window and a nominal quantum efficiency at 175 nm and room temperature of
151
about 10%. The PMs were optically coupled to the TPC by quartz windows, 10 cm diameter and 5 mm thickness, sealed on CFlOO flanges [ 121. Their transmission at 175 nm was about 70%.
3. The purity monitor The purity monitor was a double grid ionization chamber which has been described in detail in Ref. [8]. Here we recall its main features. The chamber consisted in a brass cathode and an anode, each 35 mm diameter and screened by a grid placed 5 mm from the electrode. The drift space between the two grids was 5 cm and the electric field was kept uniform by means of 4 guard rings. 30 mm internal and 35 mm external diameter. separated by I cm. They were connected to each other by 50 Ma ceramic resistors. All the mechanical elements of the monitor chamber were supported on 3 ceramic bars [IO] fixed on a CFI 50 flange which was in turn mounted on the back flange of the TPC. The two grids and the electrodes were accessible from the outside to be set at the desired voltage. The signals induced on the cathode and the anode by the drifting electrons were read by two low-noise charge amplifiers with an integration constant of about 340 us. The electrons were extracted from the cathode by a IO ns UV pulse, generated by a Nd:Yag laser quadrupled in frequency [13], at the wavelength of 266 nm. The light was injected into the liquid by mean of a quartz optical fibre. Two special feedthroughs were used for the passage of the fibre inside the vacuum tank and from the tank to the chamber.
4. The cleaning procedure and xenon purification Before being assembled almost all the TPC components were cleaned with the standard procedure described in Ref. [14]. Ceramic resistors, kapton wires and quartz windows were cleaned with acetone and n-hexane. The xenon purifier consisted of two Oxisorb cartridges [15] and two getter filters (working temperature 450°C) [16] working in parallel. The chamber and the purifier were helium leak tested with a sensitivity of 10m9 mbar l/s. pumped and baked at about 120°C under vacuum for a week. Both the purifier and the TPC were pumped by oil-free turbo pumps allowing to reach a vacuum level = 1Omx mbar. As already mentioned in Refs. [6.17] the recirculation of gaseous xenon was a crucial operation for the ultimate cleaning of all the surfaces. The technique consists on recirculating the xenon through the hot chamber (at about 80°C) and the purifier by means of a small double membrane compressor for at least 3 days. Then the chamber was again evacuated at lo-’ mbar. The recirculation before the liquefaction is
very effective in removing all the molecules (02, H,O, co,;. . ), which survived to the baking, from the surfaces of the vacuum system. Moreover. this operation causes the absorption of xenon atoms on the surfaces themselves and the substitution of the dangerous molecules with “clean” atoms. A spectrometer gauge placed above the TPC could measure the partial pressure of the components of the residual gas before and after the recirculation procedure. While the total outgassing rate remained almost unchanged, the contribution of each molecule changed significantly. For example, the H,O and 0, lines decreased by more than two orders of magnitude, while the xenon atoms, absent before the recirculation, turned out to be the only responsible for the outgassing rate. Another precaution was to isolate the cold TPC from the room temperature pipes by mean of two all-metal valves to avoid the cryogenic pumping of the outgassed molecules. The initial gas impurity concentration was about 50 ppm. Starting from room temperature about I4 hr were needed to stabilize the chamber temperature at = 180 K. The purifier could stand a flow of 20 gaseous I/min. but the actual rate was 5.5 l/min. With this rate the liquefaction procedure lasted nearly 5 days.
5. The temperature
control system
The temperature of the TPC was monitored in IO different position by PTIOO platinum resistors and one was inside the chamber in contact with the LXe. The values were digitized every 5 min and read by a Macintosh Quadra 950 computer via an IEEE488 interface bus. The
140
-
120
-
100
-
80
-
60
-
kg. During the run the LXe average temperature was 180 K-t2%. where the error is due to the absolute PTIOO calibration.
6. The front-end electronics
and data readout
The TPC electric circuit, front-end electronics and acquisition system are schematically depicted in Fig. 4. The three cathodes were biased by a no-switching high voltage power supply filtered by a low-pass frequency filter. The electric field I?~~,~, in the drift region was E,,,,rt = 500 V/cm, corresponding to a total drift time a32 IJs. The grids were fed to a low-noise charge amplifier (QA) with an integration constant ro_, = 340 t.~s via a 4.7 nF capacitor to decouple the amplifier from the high voltage. The signal was then filtered by an active filter with a low-frequency cutoff at I kHz. The anodes were fed directly to similar QAs and then filtered by 5 kHz cutoff frequency active filters. In order to match the finite range of our ADC with the wider range of the signal amplitudes, each anode channel was split in two and fed to two amplifiers with different gains. The QA front-end box was mounted outside the safety tank about 2 m from the TPC feedthroughs. Such approach is not the best from the point of view of the electronic noise but allows a practical replacement of the amplifiers. The PMs were independently supplied by a HV source. Each output current signal was split in two, one for a disc~minator for the trigger logic, and the other fed into a
QA.
180.5
Fig. 3. Dist~bution days.
computer was used to control if the LXe temperature is in the desired range and to consequently change the power dissipation in the three heating jackets. The average power dissipated with the chamber tilled was about 220 W. In such a way the temperature could be kept constant to within i0.5 K. In Fig. 3 the dist~bution of the LXe temperature during a time interval of 3 days is shown. The total mass to be controlled in temperature was about 400
181
181.5
182
LXe temperature (K) of the LXe temperature for a period of three
All the signals (24 from the anodes, 6 from the grids and 12 from the PMs) were fed to an &bit fast waveform analyzer (FADC) (CAEN mod. E426) for their digitization at 5 MHz frequency. A VME bus linked the FADC to a RTX module (CAEN mod. E427) which collected the data. A manager board, connected to the RTX by a CAENET bus, transferred the data to a circular memory. When a trigger signal occurred the data were unloaded from the circular memory to the computer memory (a Macintosh Quadra 700) for their display and recording on tape. The acquisition rate was limited to 5 Hz by the writing procedure on the exabyte tape but did not represent any limitation for our trigger rate.
153
G. Carugno et al. I Nucl. hzstr. and Meth. in Phys. Res. A 376 (1996) 149-154
-HV UV PM I\
IR,
Cil,‘=r
-HV e-/
current
-i
I I-
monitor
Y
,c
1
/A~______@!______.
Q
4.7z-l9
Act& filter
5 kHzcutoff
I .
7
Computed exabyte
4rtive tiltm
Fig. 4. Scheme of the TPC electric circuit, front-end
7. Operations
with the UV PMs
The 12 PMs for the detection of the LXe scintillation light worked at cold temperature. Under those conditions there was a strong reduction of the dark current. On the contrary the gain stability was reached only after 5-6 hr of continuous operation. Moreover, even a short interruption of the voltage supply could modify the PMs working conditions. The working points were established tagging the collinear 511 keV y-rays emitted by a “Na source. The y direction was along the chamber axis. One y ray was detected by a 4X4 inch cylindrical NaI(TI) detector and
PM30
0
.
and acquisition
the other by the LXe chamber. The 511 y ray of the NaI was defined by mean of a window discriminator. The threshold for the LXe PMs was -40 mV. We counted the number of coincidences between NaI and TPC as a function of the voltage applied to the PMs. An example of the resulting curves is plotted in Fig. 5. The plateau value defines the right working point. The working voltages ranged from - 1350 V to - 1700 V.
8. Results We measured the charge Q, extracted from the cathode and the charge Q, arriving to the anode after the drift and the time interval T,, T, and T, which are respectively the cathode-to-grid, the grid-to-grid and the grid-to-anode drift times. The relationship among these quantities and the electron lifetime 7 is
QC
T, 1 - exp( - T,/T)~~~
b :
K=+exp c
.
0 i II,I,,,,,,,,,,,,,,I,,,, 1100 1150 1200
I ,,,,I,,,.,.,,,: 1250
(
TD+ Tc 7
>’
PM1
which can be approximated,
loo0
system.
R _ Q, _ T, 1 - exp(- Ld
.
0 0
:
’
0
electronics
1300
1350
1400
1450
1500
Hv (Volts)
Fig. 5. Plateau curve for two different PMs based on the 511 keV y-y tagging method.
if T,, T, e T, as
( _r> (
7
where T = T, +(T, + T,)/2. haviour of R as a function of field of 10 V/cm. Each point measurements and the error
(2) The Fig. 6 shows the bethe run-time for an electric is the average of about 10 bars reflect both the in-
Acknowledgments
!
0.6 0.4
-
X9Y = 0.53
0.2 t
t
ot”“““‘i”“““‘I”” 0
10
20
30
40
50
Days
Fig. 6. Ratio of the charge arriving to the anode over the charge extracted from the cathode vs time. The electric drift field was 10 V/cm.
strumental and the statistical errors. The hypothesis that R was constant during the run gives a ,$/d.o.f. = 3.7/7. The average value is R = 0.986kO.032, corresponding to r = 15.2 ms. The uncertainty introduced by the error in T is 9%. The evaluation of the error on 7 requires some care since the x2 versus T plot is highly asymmetric when R is compatible with the unity within the errors. In our case T is even consistent with being infinity. On the left side of the minimum however the x2 curve behaves more regularly so that in genera1 one can determine a lower limit on P. We get r 2 2.7 ms
(95% C.L.).
9. Conclusions A 64 I LXe TPC was for the first time brought to an experimental floor and worked for physics, showing that this technique is ready for applications. There are several points we would like to stress: 1) we liquefied thrice in the TPC with the purification technique described above and we always obtained an electron lifetime some milliseconds long. One of those results has been reported in Ref. [18]; 2) although we feel that the operation can be speeded up, the liquefaction was a rather lengthly procedure (5 days); 3) the liquid purity was stable through the whole period of two months without any need for recirculation; 4) thermal stability was obtained and kept under control without any problem; 5) the operations with the UV PMs at cryogenic temperature was reliable.
The authors wish to thank S. Marigo. M. Negrato, L. Ziomi (Legnaro), D. Maniero, G. Greggio, G. Salvato. M. Turcato and P. Zatti (Padova) for their skilful technical assistance. Many design problems would not have been solved without the help of C. Fanin (Padova). The authors are also very grateful to S. Ventura (Padova) for his help during the data taking at the PSI. The assistance of P Gheno at the PSI. is acknowledged. Finally, the authors thank the I.N.F.N. National Laboratory in Legnaro and the Paul Scherrer Institute (Villigen, Switzerland) for their hospitality and support.
References G. Boca et al., Preprint Universita di Roma “La Sapienza” no. 733 (1990); G. Carugno et al., Proc. Int. Conf. on Liquid Radiation Detectors. Waseda University, Tokyo, 1992, ed. T. Doke. r21 T. Doke, Portugal. Phys. I2 (1981) 9. 131 T. Doke, Nucl. Instr. and Meth. 196 (1982) 87. 141 P. Benetti at al., Nucl. Instr. and Meth. A 329 (1993) 361, 151 M. Ichige, E. Aprile, T. Doke, K. Hasuike. K. Itoh, J. Kikuchi and K. Masuda. Nucl. Instr. and Meth. A 333 (1993) 355. R. [61 G. Carugno, S. Cerdonio. E. Conti, A. Meneguzzo. Onofrio, G. Bressi and D. Zanello, Nucl. Instr. and Meth. A 335 (1993) 338. VI P. Cennini et al., Nucl. Instr. and Meth. A 345 (1994) 230. 181 G. Carugno, B. Dainese. F. Pietropaolo and F. Ptohos. Nucl. Instr. and Meth. A 292 (1990) 580. [91 Cefilac, Paris, France. r101 MACOR, a machinable ceramic made by Coming, U.S.A. [Ill Mod. 9531 QB, Thorn Emi, London. U.K. 1121 Ceramaseal, New Lebanon, New York, U.S.A. [I31 Spectron Laser System, Derby, U.K. [I41 A.G. Methewson, CERN-&R-VA 74-10 and 74-30. [151 Messer Grieheim GmbH, Dusseldorf, Germany. I161 Saes Getter S.p.A., Milano. Italy. [I71 E. Conti, A. T. Meneguzzo, R. Onofrio, Cl. Carogno. S. Cerdonio, G. Bressi and D. Zanello, Nucl. Instr. and Meth. A 356 (1995) 286. 1181 G. Carugno, S. Cerdonio, E. Conti. A. Meneguzzo and R. Onofrio. in: Gamma-ray detector physics and applications, ed. E. Aprile. Proc. SPIE 2305 (1994) p. 63.