- Email: [email protected]
Operation of sealed microstrip gas chambers at the ILL
Nuclear Instruments and Methods in Physics Research A 471 (2001) 60–68
Operation of sealed microstrip gas chambers at the ILL J.F. Clergeaua, P. Converta, D. Feltina, H.E. Fischerb, B. Guerarda,*, T. Hansena, G. Manzina, A. Oeda, P. Palleaua b
a Institut Laue Langevin, BP156 38042 Grenoble Cedex 9, France Laboratoire LURE, Bat. 209d, B.P. 34, 91898 Orsay Cedex, France
Abstract Microstrip Gas Counters (MSGCs) were introduced at the ILL as a response to the problem of fabricating the large area neutron detector of the D20 neutron powder diffractometer. This banana-like detector consists of 48 MSGCs, each comprising 32 counting cells. It was in operation during 18 months before being stopped due to the progressive deterioration of the anode strips. In order to increase its lifetime, significant modifications were introduced in the recently assembled new version. Another instrument, D4C, was recently equipped with a modular detector made of nine MSGCs, each of them in an individual gas vessel. Besides the unidimensional individual readout MSGC of D20 and D4C, the ILL has developed bidimensional MSGCs with a charge division readout. All these detectors employ sealed vessels containing a gas mixture at a pressure which can be as high as 15 bar, necessitating very clean conditions. This paper describes the experience acquired at the ILL in the fabrication and operation of these detectors. r 2001 Elsevier Science B.V. All rights reserved.
1. Introduction A Microstrip Gas Chamber (MSGC) has several advantages, compared with a Multiwire Proportional Chamber: precision and mechanical stability of the strips; reproducibility of the fabrication; better counting rate and position resolution. But it has also some disadvantages: its maximum amplification gain is significantly lower, and it is not resistant to sparks induced by highly ionizing particles. Furthermore, charging up of the surface and electrochemical reaction of the glass under high electric field must be avoided through a careful choice of the substrate. These restrictions severely limit the use of MSGC in HEP, and
*Corresponding author. E-mail address: [email protected] (B. Guerard).
solutions using a pre-amplification stage have been proposed [1]. In neutron instrumentation, the detection conditions are simpler for the following reasons: counting rates are moderate, there is no constraint on the thickness or the density of the substrate, and the primary charge produced by the interaction of a neutron in the gas is of the order of 2 104 electrons, a factor of 100 higher than with minimum ionizing particles.
2. Fabrication outlines The main difficulty of fabricating neutron gas detectors rests in the fact that, due to the cost of 3 He used as the neutron converter, it is not possible to flush the gas through the detector, and, as it is well known, the long term stability of
0168-9002/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 9 1 6 - 0
J.F. Clergeau et al. / Nuclear Instruments and Methods in Physics Research A 471 (2001) 60–68
any sealed gas detector can only be obtained with a very low outgassing of its components. 2.1. Gas choice In addition to 3He, CF4 is added as the stopping and quenching gas. The nuclear interaction of a thermal neutron with an atom of 3He releases 764 keV of energy, shared by two ionizing particles, a proton and a triton, of different track lengths. This difference induces an error of localization which, to a good approximation, is given by 0.8Rp, where Rp is the proton range [2]. A typical pulse height distribution measured with an Am–Be source is seen in Fig. 1. The wall effect, corresponding to tracks interacting with the wall of the detector, as well as the contribution of gamma rays can be seen. The usual amplification gain is 50. Increasing the CF4 pressure has several advantages: (1) The localization error due to the shift between the neutron interaction position and the centre of the charges is reduced. (2) The number of events suffering from the wall effect is minimized. (3) By concentrating the primary charge cloud in a smaller volume, the dead time of the detector is reduced. The drift velocity of He–CF4 mixtures increases with the CF4 concentration, contributing also to the dead time reduction.
Fig. 1. Pulse height spectrum measured with a MSGC (the bidim80), and an Am–Be neutron source.
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CF4 is used for its short attenuation length, and the high velocity of electrons, but this gas is known to contain freon impurities, which are electronegative and corrosive. Only gas of very high purity is used.1 2.2. System purity In order to decrease the outgassing of the detector during its operation, internal parts are made of ceramic, glass, and metal, and the detector is heated under ultra high vacuum pumping for several days. Fig. 2 shows the outgassing curve of a detector over 25 days. The gas quality is important not only for the collection of the primary charge, but also to prevent oxidation of the chromium anode strips. As discussed in Section 3, this effect has been identified as a possible source of destruction of earlier MSGC detectors. 2.3. Substrate Among the different parameters influencing the stability of an MSGC, the material of the substrate has been identified as one of the most important one. When using an ionic conductive glass like the Desag D263, or the Corning 7740, we observed an increase in the gain after several months of operation, and the change was independent of the cumulated radiation dose. Parallel to this effect, the detector could not sustain a counting rate of more than a few kHz/cm2, as, by comparison, the original detector was not limited with the maximum beam intensity available on our beam line. In all cases, the resistance between anode and cathode Rac was increased by several orders of magnitude, inducing charging of the substrate at even low counting rates. The increase of gain can be explained by the sharpening of the electric field consecutive to the higher value of Rac : This aging is not correlated to a high irradiation dose, but results from an electrolytic decomposition of the glass: under polarization, Na+ ions drift towards the cathode where they accumulate underneath. The same phenomena could be 1 See for example the 99.999% purity gas from Air Liquid, used for micro-electronics applications.
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made at the ILL use a virtual cathode. The limited counting rate they can achieve is sufficient for most applications. The S8900 glass plate2 is polished3 and controlled under grazing light before fabrication of the MSGC.4 As an additional control for series of more than 10 plates we test the glass melt by first fabricating a single MSGC with this melt. 2.4. Metallization
Fig. 2. Variation of the residual pressure of a detector during the outgassing procedure. The detector is heated to 1501C, and pumped with a turbo-molecular pump. The first step at 1301C is applied during 24 h. The pressure drop after 25 days corresponds to the end of the heating.
reproduced by heating the detector at 1001C for several days with the high voltage switched on. This accelerated aging procedure is described in Ref. [3]. There is no such effect with the electron conductive glass Schott S8900: Rac remains constant under high voltage during several months of operation, and tests performed with the accelerated aging procedure showed that the glass is electrically stable over several years of operation. Similar results concerning the stability have been obtained with semiconductive layers deposited onto the surface of the MSGC plate. Unfortunately, the maximum gain we could achieve with this MSGC was lower than that without the layer, and the quality of the layers was not reproducible [4]. Actually, at the ILL, only detectors made with the Schott S8900 glass have given satisfaction concerning the long term stability, and all MSGC are made with this substrate. This glass has also the special property that makes it possible to fabricate MSGCs with anode strips on one side of the glass, and cathode strips on the other side. This configuration, the so-called ‘‘virtual cathode’’ [5], provides a signal with the same amplitude, but of opposite sign, on the anode and on the cathode. Furthermore, amplification gain greater than 105 has been measured. All the 2D MSGC detectors
Chromium has excellent mechanical properties and adheres very well to the glass substrate. The time spreading of the signals induced by the electrical resistivity of chromium is not a limitation for neutrons, regarding the time development of the signals they produce in gas detectors. This resistivity has in fact been turned into an advantage for charge division readout detectors by engraving a resistive line directly on the substrate. Another very important point to consider is the quality of the electrical contacts which must be gold plated to avoid possible sparks.
3. The multi-MSGC D20 detector The D20 instrument, was the first large area detector at the ILL to make use of microstrip technology. A complete description of this detector can be found in Ref. [6]. The functional parameters of this detector are the most demanding at the ILL: 1536 readout channels, each of the cells 2.57 mm wide (corresponding to an angle of 0.11), sustaining a counting rate of 50 kHz/ channel; a very good counting uniformity over the whole length of the detector (no dead space, nor dead channels), and a counting stability better than 0.1% over several days. A 128 channels MWPC prototype was first fabricated, but small uncertainties in the position of the sensitive wires, and of their mechanical tension, resulted in insufficient uniformity and stability. Therefore, the concept of the Microstrip Gas Counter was applied [7]. At the beginning of 1997 the D20 2
Schott Glass, Duryea, Pennsylvania, USA. Guinchard, Yverdon-les-Bains, Switzerland. 4 IMT, Greifensee, Switzerland. 3
J.F. Clergeau et al. / Nuclear Instruments and Methods in Physics Research A 471 (2001) 60–68
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Photo 1. Mounting of the D20 detector.
detector started its first phase of operation. It worked for 18 months without interruption, and produced scientific results of high quality. Photo 1 shows the detector during its mounting phase. Unfortunately, the detector had to be stopped following a serious degradation of its performances due to a progressive destruction of the anode strips. The fact that sparks started to occur only after about one year of operation indicates that they were not the origin of the problem, but a consequence of it. However, they probably contributed to the propagation of the degradation. An analysis of the electrodes was performed by Auger electron spectroscopy.5 This analysis showed that the chromium metal of the anodes was partially replaced by chromium-oxide. It resulted in a reduction of the effective width of the anodes, which subsequently induced destructive sparks. In Photo 2, we can see that the chromium-oxide is localized on the edges of the anode strips where the electric field is the highest. Before dismounting the D20 detector, we irradiated one of the plates locally with the primary neutron beam for several days. The integrated intensity was equivalent to the dose received for several months in normal conditions. We could not observe any specific degradation of the plate related to this test. We applied the accelerated
Photo 2. Details of an anode after opening of the D20 detector. The majority of the anodes showed a similar aspect. There destruction is due to the progressive oxidation of chromium.
aging procedure to three fresh plates identical to the MSGC installed in D20. We did not observe any visible change on the anodes, and the anode– cathode resistance remained the same. It follows that the origin of the anode destruction had nothing to do with irradiation, and was not present during the accelerated aging procedure. This observation suggests that the oxidation of the anode strips was initiated by the impurities present in the gas, or on the surface of the MSGC. Several actions were performed in order to improve the lifetime of the detector. The most significant are the following: *
5
LETI/dpt optronique/CEA-Grenoble.
We suspected that the cleaning procedure of the MSGC didn’t remove completely residues
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coming from the engraving procedure. It has been reinforced with a bath of deionized water at 601C for 30 min. During the assembling of the detector, the electrical control of the anode continuity was performed with a machine containing a gold needle scanning continuously on the surface of the MSGC. In some cases, there were traces of gold attached to the surface, and even small scratches. This machine was replaced by a motionless system connected to a PC, which allows the simultaneous measurement of the resistivity of all the anodes of an MSGC. The gas tightness is provided by two O-rings. The internal one was not gas-tight. The two new O-rings were commissioned. For reason of ease of mounting, the MSGC anodes were connected together in two groups of 25. This configuration creates a large electrical capacitance, and exposes the whole detector to the effect of high energy sparks. To reduce the energy of these sparks and their propagation, we changed the circuit of the anodes by individually filtered connections. This modification required to change the number of MSGC from 50 to 48. The cathode contacts were improved with a gold plating. During the detector outgassing procedure (1501C for 2 weeks), a mass spectrometer has been used to control the content of the residual gases, and in particular the water vapour. The resistance of semi-conductive substrates varies with temperature. Although we used an air-conditioning system, we observed gain variation which resulted in counting variation of about 0.1%. This problem was overcome by decreasing the value of the resistor in the HV noise filter. One of the MSGC plates was sparking at the nominal voltage, and we had to lower its amplification gain in order to get rid of this dangerous situation. The new detector is monitored continuously in real time with a multiplexed analysing system which measures the pulse height spectrum of every MSGC.
The optimal pressure of the detection gas was determined as follows: (1) it should not exceed 4 bars to avoid safety annual inspections, (2) the 3He pressure should be as high as possible to get the maximum of efficiency and (3) the CF4 pressure should be at above the value required to prevent double counting events. This last point is explained as follows: An anti-coincidence logical electronic system is used to select the channel N with the maximum of amplitude. After the channel N has detected a pulse, channels N+1 and N1 are disabled by the electronics. In order to ensure that a single event will not result in more than one active channel, it is necessary that signals on channels N2 and N+2 are bellow the discrimination threshold. This condition is achieved when the projection of the track produced after interaction of a neutron is shorter than twice the cathode width. We measured the correlation between the analogue signals of two neighbouring channels as shown in Fig. 3, and the result is shown in Fig. 4. At 1.8 bar, the points are distributed along a line oriented at 451, indicating that the two signals are sharing the whole avalanche charge, and that the projection of the track is shorter than one cathode width. With the anti-coincidence electronics, 1.2 bar of CF4 was found to be sufficient to prevent multi-hit events. To complete the gas mixture 2.8 bar of 3He were added. The anode–cathode voltage was fixed at 750 V in the middle of the 200 V efficiency plateau. Due to the large width of the cathodes as compared to the
Fig. 3. Principle of the correlation measurement.
J.F. Clergeau et al. / Nuclear Instruments and Methods in Physics Research A 471 (2001) 60–68
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Fig. 4. Correlation measurement, ADC2 versus ADC1, with 1 bar of CF4 (left), and 1.8 bar of CF4 (right).
anodes, the drift field depends mainly on the cathode voltage. Two effects may lower the amount of charge collected: attachment of electrons in the gas, and ballistic effects due to the shaping time of the charge amplifiers. Fig. 5 shows the variation of the pulse height on varying the drift electric field. From this figure, the cathode voltage was fixed at 800 V. The D20 detector, which was at the origin of the development of micro-pattern detectors, is again in the users’ hands since September 2000.
Fig. 5. Charge signal measured on the anode as a function of the drift electric field.
4. The modular D4C detector D4C is a liquid and amorphous materials neutron diffraction instrument. It is made of nine 1D MSGCs, 64 channels each, included in an individual gas vessel containing 15 bar of gas (see Photo 3). Each module is mounted with a metallic joint Helicoflex.6 The modular configuration of D4C allows a quick repair in case of failure, and guarantees the continuous availability of the instrument. This detector, described with more details in Ref. [8], started its operation in August 2000. Photo 3. Assembling of the nine MSGC modules of the D4C detector.
5. Bidim80 two-dimensional detectors The first version of this series of detectors is described in Ref. [9]. Unlike the D20 and D4C 1D 6
Carbone Lorraine, Cefilac, St Etienne, France.
detectors which use MSGC with anode and cathode on the upper side of the plate, a virtual cathode layout is used for the last generation of 2D detectors. The second difference is that D20
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where hx and hy are calibration constants, and ADC[h1; h2] are the signal pulse height measured with charge amplifiers on the anode, and ADC[v1; v2] on the cathode, after conversion by the ADCs. The acquisition system consists of a multiparameter card (FAST) in a PC. Fig. 6 shows the position resolution measured with a bidim80 at two different pressure of CF4. 5.2. Backgammon layout
Photo 4. Internal view of a bidim80 detector. The base is made of a round stainless steel flange with feed-through connectors. The detector is closed with an aluminum cover, directly in contact with the knife gasket which is tooled on the flange.
and D4C are using an individual readout, as the bidim80 uses a charge division readout. With this detector, the second coordinate is measured either by using the signals from a pattern of cathodes engraved on the opposite face of the substrate, and perpendicular to the anodes, or with a backgammon geometry parallel to the anodes. Five detectors have been installed at the ILL, the first one in 1996 (see Photo 4).
A further development of those detectors use the so-called backgammon structure [10]; the cathodes engraved on the rear side of the MSGC are parallel to the anodes. They are cut diagonally along their length in order to obtain two triangular semicathodes. The triangles of each cathode pattern are connected together by a resistive line of 5 kO (Fig. 7). With this technique it is possible to obtain a two-dimensional information from the detector, by correctly combining the four cathode signals. The anodes provide the electrical field focalization needed to fully exploit the backgammon structure properties. The results obtained with the backgammon are similar to those obtained with the orthogonal structure in terms of quality image. It would be possible to make a segmentation of the detector to
5.1. Orthogonal MSGC layout The MSGC plate is made of a 12.7 12.7 cm2 Schott S8900 glass, 0.5 mm thick. The sensitive area is 80 80 mm2. The front side structure, made ( chromium layer, consists of a pattern of a 1500 A of anodes, 10 mm wide with a pitch of 1 mm, interconnected together by a resistive line of 5 kO. On the rear side, the cathodes, 900 mm wide, are orthogonal to the anode structure and also interconnected together by a resistive line of 5 kO. The position is calculated on-line by these formulas X ¼ ½ADCh1=ðADCh1 þ ADCh2Þhx ; ð1Þ Y ¼ ½ADCv1=ðADCv1 þ ADCv2Þhy ;
Fig. 6. Position resolution measured by charge division with a bidim80 detector. Intrinsic values (2.5 mm at 1 bar of CF4, and 1.4 mm at 2 bar of CF4) are obtained at high signal/noise ratio, where the electronic noise does not contribute.
J.F. Clergeau et al. / Nuclear Instruments and Methods in Physics Research A 471 (2001) 60–68
Fig. 7. The two sides of the backgammon MSGC are represented in a superimposed schematic view; anodes on left, and cathode on right. The horizontal position is measured by electronic charge division, and the vertical position by geometric charge division.
improve the position resolution and the counting rate, but this advantage has not yet been exploited at the ILL. 5.2.1. Backgammon readout electrode with a GEM pre-amplification stage The backgammon structure has also been used to readout the avalanche signal generated by a preamplification stage. The GEM used in this study is a Kapton foil, metal-clad on both sides with a regular pattern of holes realized at CERN by means of conventional printed circuit technology; a more detailed description of this structure can be found in Ref. [11]. The GEM has been assembled over a backgammon MSGC, at a distance of 8 mm from the substrate, and at about 20 mm from the detector entrance window. The MSGC plate has been mounted upside-down, in order to have the cathodes structure facing the lower GEM electrode; the purpose of this mounting was to verify if it is possible to use the GEM for the amplification and the MSGC just as a collector electrode, preserving the 2D properties of the backgammon structure. The measurements have been done in a gas mixture of 0.2 bar of 3He and 0.3 bar of CF4. Applying a difference of potential of 430 V between the two GEM planes and a voltage of
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1.5 kV on the cathodes (the anodes being left floating), we obtained an amplification gain which was sufficient to detect neutrons with full efficiency. The position measurement in the direction parallel to the resistive line gave similar results to the standard bidim80 MSGC, but, due to the absence of the focussing effect of the anodes, the signals were not sensitive to the other direction. At higher CF4 pressure, it was not possible to maintain the same amplification gain without sparks. As the gain limitation probably depends on the charge density in each hole, it follows that the gain limitation is directly correlated to the size of the primary charge cloud, and thus to the stopping gas pressure. Good position resolution with GEM neutron detectors may require several steps of amplification. In the near future, we will continue this study by using a GEM 2D localization double side layout, precluding the need for a complicated 2D readout electrode.
6. Conclusion Fast and accurate detectors are developed at the ILL to fully benefit from the neutron flux of the high power reactor. The large area D20 bananalike detector was the first detector to make use of Micro Strip Gas Chamber, but also the most complex one, and in that sense, represents an ideal tool for observation of this technology. A significant effort has been made to improve the lifetime of sealed MSGC detectors, and the experience acquired over 12 years is now converging towards stable fabrication requirements. New conditions of detection imposed by the future spallation sources will reinforce the interest for these detectors.
References [1] F. Sauli, Nucl. Instr. and Meth. A 386 (1997) 531. [2] J. Fischer, et al., in: P. Convert, J.B. Forsyth (Eds.), Proceedings of the Workshop on the positionsensitive detection of thermal neutrons, ILL, Grenoble, France, 11–12 October 1982, Academic Press, London, p. 129.
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[3] G. Cicognani, et al., Proceedings of the International Workshop on Micro-Strip Gas Chamber, Lyon, November 30–December 2, 1995. [4] G. Cicognani, et al., Nucl. Instr. and Meth. A 392 (1997) 115. [5] M. Capeans, et al., CERN-PPE/97-61. [6] P. Convert, et al., Phys. B 241–243 (1998) 195.
[7] [8] [9] [10]
A. Oed, Nucl. Instr. and Meth. A 263 (1988) 351. H.E. Fischer, et al., Phys. B 276–278 (2000) 93–94. N. Velletaz, et al., Nucl. Instr. and Meth. A 392 (1997) 73. B. Guerard, Proceedings of the Workshop on Neutron Detectors for Spallation Sources, Brookhaven National Laboratory 24–26, 1998. [11] R. Bouclier, et al., Nucl. Instr. and Meth. A 396 (1997) 50.
Operation of sealed microstrip gas chambers at the ILL J.F. Clergeaua, P. Converta, D. Feltina, H.E. Fischerb, B. Guerarda,*, T. Hansena, G. Manzina, A. Oeda, P. Palleaua b
a Institut Laue Langevin, BP156 38042 Grenoble Cedex 9, France Laboratoire LURE, Bat. 209d, B.P. 34, 91898 Orsay Cedex, France
Abstract Microstrip Gas Counters (MSGCs) were introduced at the ILL as a response to the problem of fabricating the large area neutron detector of the D20 neutron powder diffractometer. This banana-like detector consists of 48 MSGCs, each comprising 32 counting cells. It was in operation during 18 months before being stopped due to the progressive deterioration of the anode strips. In order to increase its lifetime, significant modifications were introduced in the recently assembled new version. Another instrument, D4C, was recently equipped with a modular detector made of nine MSGCs, each of them in an individual gas vessel. Besides the unidimensional individual readout MSGC of D20 and D4C, the ILL has developed bidimensional MSGCs with a charge division readout. All these detectors employ sealed vessels containing a gas mixture at a pressure which can be as high as 15 bar, necessitating very clean conditions. This paper describes the experience acquired at the ILL in the fabrication and operation of these detectors. r 2001 Elsevier Science B.V. All rights reserved.
1. Introduction A Microstrip Gas Chamber (MSGC) has several advantages, compared with a Multiwire Proportional Chamber: precision and mechanical stability of the strips; reproducibility of the fabrication; better counting rate and position resolution. But it has also some disadvantages: its maximum amplification gain is significantly lower, and it is not resistant to sparks induced by highly ionizing particles. Furthermore, charging up of the surface and electrochemical reaction of the glass under high electric field must be avoided through a careful choice of the substrate. These restrictions severely limit the use of MSGC in HEP, and
*Corresponding author. E-mail address: [email protected] (B. Guerard).
solutions using a pre-amplification stage have been proposed [1]. In neutron instrumentation, the detection conditions are simpler for the following reasons: counting rates are moderate, there is no constraint on the thickness or the density of the substrate, and the primary charge produced by the interaction of a neutron in the gas is of the order of 2 104 electrons, a factor of 100 higher than with minimum ionizing particles.
2. Fabrication outlines The main difficulty of fabricating neutron gas detectors rests in the fact that, due to the cost of 3 He used as the neutron converter, it is not possible to flush the gas through the detector, and, as it is well known, the long term stability of
0168-9002/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 9 1 6 - 0
J.F. Clergeau et al. / Nuclear Instruments and Methods in Physics Research A 471 (2001) 60–68
any sealed gas detector can only be obtained with a very low outgassing of its components. 2.1. Gas choice In addition to 3He, CF4 is added as the stopping and quenching gas. The nuclear interaction of a thermal neutron with an atom of 3He releases 764 keV of energy, shared by two ionizing particles, a proton and a triton, of different track lengths. This difference induces an error of localization which, to a good approximation, is given by 0.8Rp, where Rp is the proton range [2]. A typical pulse height distribution measured with an Am–Be source is seen in Fig. 1. The wall effect, corresponding to tracks interacting with the wall of the detector, as well as the contribution of gamma rays can be seen. The usual amplification gain is 50. Increasing the CF4 pressure has several advantages: (1) The localization error due to the shift between the neutron interaction position and the centre of the charges is reduced. (2) The number of events suffering from the wall effect is minimized. (3) By concentrating the primary charge cloud in a smaller volume, the dead time of the detector is reduced. The drift velocity of He–CF4 mixtures increases with the CF4 concentration, contributing also to the dead time reduction.
Fig. 1. Pulse height spectrum measured with a MSGC (the bidim80), and an Am–Be neutron source.
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CF4 is used for its short attenuation length, and the high velocity of electrons, but this gas is known to contain freon impurities, which are electronegative and corrosive. Only gas of very high purity is used.1 2.2. System purity In order to decrease the outgassing of the detector during its operation, internal parts are made of ceramic, glass, and metal, and the detector is heated under ultra high vacuum pumping for several days. Fig. 2 shows the outgassing curve of a detector over 25 days. The gas quality is important not only for the collection of the primary charge, but also to prevent oxidation of the chromium anode strips. As discussed in Section 3, this effect has been identified as a possible source of destruction of earlier MSGC detectors. 2.3. Substrate Among the different parameters influencing the stability of an MSGC, the material of the substrate has been identified as one of the most important one. When using an ionic conductive glass like the Desag D263, or the Corning 7740, we observed an increase in the gain after several months of operation, and the change was independent of the cumulated radiation dose. Parallel to this effect, the detector could not sustain a counting rate of more than a few kHz/cm2, as, by comparison, the original detector was not limited with the maximum beam intensity available on our beam line. In all cases, the resistance between anode and cathode Rac was increased by several orders of magnitude, inducing charging of the substrate at even low counting rates. The increase of gain can be explained by the sharpening of the electric field consecutive to the higher value of Rac : This aging is not correlated to a high irradiation dose, but results from an electrolytic decomposition of the glass: under polarization, Na+ ions drift towards the cathode where they accumulate underneath. The same phenomena could be 1 See for example the 99.999% purity gas from Air Liquid, used for micro-electronics applications.
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made at the ILL use a virtual cathode. The limited counting rate they can achieve is sufficient for most applications. The S8900 glass plate2 is polished3 and controlled under grazing light before fabrication of the MSGC.4 As an additional control for series of more than 10 plates we test the glass melt by first fabricating a single MSGC with this melt. 2.4. Metallization
Fig. 2. Variation of the residual pressure of a detector during the outgassing procedure. The detector is heated to 1501C, and pumped with a turbo-molecular pump. The first step at 1301C is applied during 24 h. The pressure drop after 25 days corresponds to the end of the heating.
reproduced by heating the detector at 1001C for several days with the high voltage switched on. This accelerated aging procedure is described in Ref. [3]. There is no such effect with the electron conductive glass Schott S8900: Rac remains constant under high voltage during several months of operation, and tests performed with the accelerated aging procedure showed that the glass is electrically stable over several years of operation. Similar results concerning the stability have been obtained with semiconductive layers deposited onto the surface of the MSGC plate. Unfortunately, the maximum gain we could achieve with this MSGC was lower than that without the layer, and the quality of the layers was not reproducible [4]. Actually, at the ILL, only detectors made with the Schott S8900 glass have given satisfaction concerning the long term stability, and all MSGC are made with this substrate. This glass has also the special property that makes it possible to fabricate MSGCs with anode strips on one side of the glass, and cathode strips on the other side. This configuration, the so-called ‘‘virtual cathode’’ [5], provides a signal with the same amplitude, but of opposite sign, on the anode and on the cathode. Furthermore, amplification gain greater than 105 has been measured. All the 2D MSGC detectors
Chromium has excellent mechanical properties and adheres very well to the glass substrate. The time spreading of the signals induced by the electrical resistivity of chromium is not a limitation for neutrons, regarding the time development of the signals they produce in gas detectors. This resistivity has in fact been turned into an advantage for charge division readout detectors by engraving a resistive line directly on the substrate. Another very important point to consider is the quality of the electrical contacts which must be gold plated to avoid possible sparks.
3. The multi-MSGC D20 detector The D20 instrument, was the first large area detector at the ILL to make use of microstrip technology. A complete description of this detector can be found in Ref. [6]. The functional parameters of this detector are the most demanding at the ILL: 1536 readout channels, each of the cells 2.57 mm wide (corresponding to an angle of 0.11), sustaining a counting rate of 50 kHz/ channel; a very good counting uniformity over the whole length of the detector (no dead space, nor dead channels), and a counting stability better than 0.1% over several days. A 128 channels MWPC prototype was first fabricated, but small uncertainties in the position of the sensitive wires, and of their mechanical tension, resulted in insufficient uniformity and stability. Therefore, the concept of the Microstrip Gas Counter was applied [7]. At the beginning of 1997 the D20 2
Schott Glass, Duryea, Pennsylvania, USA. Guinchard, Yverdon-les-Bains, Switzerland. 4 IMT, Greifensee, Switzerland. 3
J.F. Clergeau et al. / Nuclear Instruments and Methods in Physics Research A 471 (2001) 60–68
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Photo 1. Mounting of the D20 detector.
detector started its first phase of operation. It worked for 18 months without interruption, and produced scientific results of high quality. Photo 1 shows the detector during its mounting phase. Unfortunately, the detector had to be stopped following a serious degradation of its performances due to a progressive destruction of the anode strips. The fact that sparks started to occur only after about one year of operation indicates that they were not the origin of the problem, but a consequence of it. However, they probably contributed to the propagation of the degradation. An analysis of the electrodes was performed by Auger electron spectroscopy.5 This analysis showed that the chromium metal of the anodes was partially replaced by chromium-oxide. It resulted in a reduction of the effective width of the anodes, which subsequently induced destructive sparks. In Photo 2, we can see that the chromium-oxide is localized on the edges of the anode strips where the electric field is the highest. Before dismounting the D20 detector, we irradiated one of the plates locally with the primary neutron beam for several days. The integrated intensity was equivalent to the dose received for several months in normal conditions. We could not observe any specific degradation of the plate related to this test. We applied the accelerated
Photo 2. Details of an anode after opening of the D20 detector. The majority of the anodes showed a similar aspect. There destruction is due to the progressive oxidation of chromium.
aging procedure to three fresh plates identical to the MSGC installed in D20. We did not observe any visible change on the anodes, and the anode– cathode resistance remained the same. It follows that the origin of the anode destruction had nothing to do with irradiation, and was not present during the accelerated aging procedure. This observation suggests that the oxidation of the anode strips was initiated by the impurities present in the gas, or on the surface of the MSGC. Several actions were performed in order to improve the lifetime of the detector. The most significant are the following: *
5
LETI/dpt optronique/CEA-Grenoble.
We suspected that the cleaning procedure of the MSGC didn’t remove completely residues
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coming from the engraving procedure. It has been reinforced with a bath of deionized water at 601C for 30 min. During the assembling of the detector, the electrical control of the anode continuity was performed with a machine containing a gold needle scanning continuously on the surface of the MSGC. In some cases, there were traces of gold attached to the surface, and even small scratches. This machine was replaced by a motionless system connected to a PC, which allows the simultaneous measurement of the resistivity of all the anodes of an MSGC. The gas tightness is provided by two O-rings. The internal one was not gas-tight. The two new O-rings were commissioned. For reason of ease of mounting, the MSGC anodes were connected together in two groups of 25. This configuration creates a large electrical capacitance, and exposes the whole detector to the effect of high energy sparks. To reduce the energy of these sparks and their propagation, we changed the circuit of the anodes by individually filtered connections. This modification required to change the number of MSGC from 50 to 48. The cathode contacts were improved with a gold plating. During the detector outgassing procedure (1501C for 2 weeks), a mass spectrometer has been used to control the content of the residual gases, and in particular the water vapour. The resistance of semi-conductive substrates varies with temperature. Although we used an air-conditioning system, we observed gain variation which resulted in counting variation of about 0.1%. This problem was overcome by decreasing the value of the resistor in the HV noise filter. One of the MSGC plates was sparking at the nominal voltage, and we had to lower its amplification gain in order to get rid of this dangerous situation. The new detector is monitored continuously in real time with a multiplexed analysing system which measures the pulse height spectrum of every MSGC.
The optimal pressure of the detection gas was determined as follows: (1) it should not exceed 4 bars to avoid safety annual inspections, (2) the 3He pressure should be as high as possible to get the maximum of efficiency and (3) the CF4 pressure should be at above the value required to prevent double counting events. This last point is explained as follows: An anti-coincidence logical electronic system is used to select the channel N with the maximum of amplitude. After the channel N has detected a pulse, channels N+1 and N1 are disabled by the electronics. In order to ensure that a single event will not result in more than one active channel, it is necessary that signals on channels N2 and N+2 are bellow the discrimination threshold. This condition is achieved when the projection of the track produced after interaction of a neutron is shorter than twice the cathode width. We measured the correlation between the analogue signals of two neighbouring channels as shown in Fig. 3, and the result is shown in Fig. 4. At 1.8 bar, the points are distributed along a line oriented at 451, indicating that the two signals are sharing the whole avalanche charge, and that the projection of the track is shorter than one cathode width. With the anti-coincidence electronics, 1.2 bar of CF4 was found to be sufficient to prevent multi-hit events. To complete the gas mixture 2.8 bar of 3He were added. The anode–cathode voltage was fixed at 750 V in the middle of the 200 V efficiency plateau. Due to the large width of the cathodes as compared to the
Fig. 3. Principle of the correlation measurement.
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Fig. 4. Correlation measurement, ADC2 versus ADC1, with 1 bar of CF4 (left), and 1.8 bar of CF4 (right).
anodes, the drift field depends mainly on the cathode voltage. Two effects may lower the amount of charge collected: attachment of electrons in the gas, and ballistic effects due to the shaping time of the charge amplifiers. Fig. 5 shows the variation of the pulse height on varying the drift electric field. From this figure, the cathode voltage was fixed at 800 V. The D20 detector, which was at the origin of the development of micro-pattern detectors, is again in the users’ hands since September 2000.
Fig. 5. Charge signal measured on the anode as a function of the drift electric field.
4. The modular D4C detector D4C is a liquid and amorphous materials neutron diffraction instrument. It is made of nine 1D MSGCs, 64 channels each, included in an individual gas vessel containing 15 bar of gas (see Photo 3). Each module is mounted with a metallic joint Helicoflex.6 The modular configuration of D4C allows a quick repair in case of failure, and guarantees the continuous availability of the instrument. This detector, described with more details in Ref. [8], started its operation in August 2000. Photo 3. Assembling of the nine MSGC modules of the D4C detector.
5. Bidim80 two-dimensional detectors The first version of this series of detectors is described in Ref. [9]. Unlike the D20 and D4C 1D 6
Carbone Lorraine, Cefilac, St Etienne, France.
detectors which use MSGC with anode and cathode on the upper side of the plate, a virtual cathode layout is used for the last generation of 2D detectors. The second difference is that D20
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where hx and hy are calibration constants, and ADC[h1; h2] are the signal pulse height measured with charge amplifiers on the anode, and ADC[v1; v2] on the cathode, after conversion by the ADCs. The acquisition system consists of a multiparameter card (FAST) in a PC. Fig. 6 shows the position resolution measured with a bidim80 at two different pressure of CF4. 5.2. Backgammon layout
Photo 4. Internal view of a bidim80 detector. The base is made of a round stainless steel flange with feed-through connectors. The detector is closed with an aluminum cover, directly in contact with the knife gasket which is tooled on the flange.
and D4C are using an individual readout, as the bidim80 uses a charge division readout. With this detector, the second coordinate is measured either by using the signals from a pattern of cathodes engraved on the opposite face of the substrate, and perpendicular to the anodes, or with a backgammon geometry parallel to the anodes. Five detectors have been installed at the ILL, the first one in 1996 (see Photo 4).
A further development of those detectors use the so-called backgammon structure [10]; the cathodes engraved on the rear side of the MSGC are parallel to the anodes. They are cut diagonally along their length in order to obtain two triangular semicathodes. The triangles of each cathode pattern are connected together by a resistive line of 5 kO (Fig. 7). With this technique it is possible to obtain a two-dimensional information from the detector, by correctly combining the four cathode signals. The anodes provide the electrical field focalization needed to fully exploit the backgammon structure properties. The results obtained with the backgammon are similar to those obtained with the orthogonal structure in terms of quality image. It would be possible to make a segmentation of the detector to
5.1. Orthogonal MSGC layout The MSGC plate is made of a 12.7 12.7 cm2 Schott S8900 glass, 0.5 mm thick. The sensitive area is 80 80 mm2. The front side structure, made ( chromium layer, consists of a pattern of a 1500 A of anodes, 10 mm wide with a pitch of 1 mm, interconnected together by a resistive line of 5 kO. On the rear side, the cathodes, 900 mm wide, are orthogonal to the anode structure and also interconnected together by a resistive line of 5 kO. The position is calculated on-line by these formulas X ¼ ½ADCh1=ðADCh1 þ ADCh2Þhx ; ð1Þ Y ¼ ½ADCv1=ðADCv1 þ ADCv2Þhy ;
Fig. 6. Position resolution measured by charge division with a bidim80 detector. Intrinsic values (2.5 mm at 1 bar of CF4, and 1.4 mm at 2 bar of CF4) are obtained at high signal/noise ratio, where the electronic noise does not contribute.
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Fig. 7. The two sides of the backgammon MSGC are represented in a superimposed schematic view; anodes on left, and cathode on right. The horizontal position is measured by electronic charge division, and the vertical position by geometric charge division.
improve the position resolution and the counting rate, but this advantage has not yet been exploited at the ILL. 5.2.1. Backgammon readout electrode with a GEM pre-amplification stage The backgammon structure has also been used to readout the avalanche signal generated by a preamplification stage. The GEM used in this study is a Kapton foil, metal-clad on both sides with a regular pattern of holes realized at CERN by means of conventional printed circuit technology; a more detailed description of this structure can be found in Ref. [11]. The GEM has been assembled over a backgammon MSGC, at a distance of 8 mm from the substrate, and at about 20 mm from the detector entrance window. The MSGC plate has been mounted upside-down, in order to have the cathodes structure facing the lower GEM electrode; the purpose of this mounting was to verify if it is possible to use the GEM for the amplification and the MSGC just as a collector electrode, preserving the 2D properties of the backgammon structure. The measurements have been done in a gas mixture of 0.2 bar of 3He and 0.3 bar of CF4. Applying a difference of potential of 430 V between the two GEM planes and a voltage of
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1.5 kV on the cathodes (the anodes being left floating), we obtained an amplification gain which was sufficient to detect neutrons with full efficiency. The position measurement in the direction parallel to the resistive line gave similar results to the standard bidim80 MSGC, but, due to the absence of the focussing effect of the anodes, the signals were not sensitive to the other direction. At higher CF4 pressure, it was not possible to maintain the same amplification gain without sparks. As the gain limitation probably depends on the charge density in each hole, it follows that the gain limitation is directly correlated to the size of the primary charge cloud, and thus to the stopping gas pressure. Good position resolution with GEM neutron detectors may require several steps of amplification. In the near future, we will continue this study by using a GEM 2D localization double side layout, precluding the need for a complicated 2D readout electrode.
6. Conclusion Fast and accurate detectors are developed at the ILL to fully benefit from the neutron flux of the high power reactor. The large area D20 bananalike detector was the first detector to make use of Micro Strip Gas Chamber, but also the most complex one, and in that sense, represents an ideal tool for observation of this technology. A significant effort has been made to improve the lifetime of sealed MSGC detectors, and the experience acquired over 12 years is now converging towards stable fabrication requirements. New conditions of detection imposed by the future spallation sources will reinforce the interest for these detectors.
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[3] G. Cicognani, et al., Proceedings of the International Workshop on Micro-Strip Gas Chamber, Lyon, November 30–December 2, 1995. [4] G. Cicognani, et al., Nucl. Instr. and Meth. A 392 (1997) 115. [5] M. Capeans, et al., CERN-PPE/97-61. [6] P. Convert, et al., Phys. B 241–243 (1998) 195.
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A. Oed, Nucl. Instr. and Meth. A 263 (1988) 351. H.E. Fischer, et al., Phys. B 276–278 (2000) 93–94. N. Velletaz, et al., Nucl. Instr. and Meth. A 392 (1997) 73. B. Guerard, Proceedings of the Workshop on Neutron Detectors for Spallation Sources, Brookhaven National Laboratory 24–26, 1998. [11] R. Bouclier, et al., Nucl. Instr. and Meth. A 396 (1997) 50.