- Email: [email protected]
Large-area low-pressure microstrip gas chambers for thermal neutron imaging
Nuclear Instruments and Methods in Physics Research A 409 (1998) 56—62
Large-area low-pressure microstrip gas chambers for thermal neutron imaging B. Gebauer!,*, Ch. Schulz!, Th. Wilpert!, S.F. Biagi" ! Hahn-Meitner-Institut, Glienicker Str. 100, D-14109 Berlin, Germany " Department of Physics, University of Liverpool, P.O.Box 147, Liverpool L69 3Bx, UK
Abstract Thermal neutron imaging gas detectors using counting gas mixtures with gaseous neutron convertors (e.g. 3He) suffer from inherent limitations in position and time-of-flight resolutions, resulting from the long ranges of the released secondary ions in the counting gas and from parallax errors due to the necessary depth of the detector gas volume of '1 cm. In order to overcome these limitations a novel detector generation is presently being constructed, utilizing composite neutron convertor foils (157Gd or 6Li overcoated with CsI) in combination with arrays of large-area microstrip and microdot gas chambers (MSGCs and MDOTs), which are operated in low-pressure, two-stage amplification mode and delivering unsurpassed gains. In this paper the optimization of MSGCs for low-pressure operation with 157Gd convertors is discussed. The MSGCs are of technologically advanced, robust design, using 3 mm thick synthetic quartz substrates and two closely spaced electrode planes for true two-dimensional position readout. These metal planes are separated by a &3 lm thick SiO /DLC double-layer deposited by means of plasma-enhanced CVD. The same 2 fabrication technology is presently prepared for production of large-area MDOTs. The work shall be conducted in collaboration with teams from research institutes and industry. ( 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Conventional thermal neutron imaging gas detectors (for neutron wavelength j&1.8A_ ), employing counting gas mixtures of quench gases with the gaseous neutron convertor 3He, deliver very limited position and time-of-flight resolutions due to the detector depth and to the long ranges of the protons and tritons, emitted after neutron capture
* Corresponding author. Fax: #49 30 8062 2293; e-mail: [email protected].
into random opposite directions (n # 3He Pp (573 keV) # t(191 keV)). In mixtures of 3He with CF and C H — the quench gases of highest stop4 3 8 ping power in use in neutron detectors — the common centroids of the secondary electron distributions, released along the proton and triton traces, fill spheres with diameters of &0.7 R and &0.8 1 R , respectively, around a capture point of many 1 neutrons; thus, the FWHM of the spatial resolution is restricted to these values. R , the projected pro1 ton range, is 0.43 and 0.45 bar cm in CF and C H , 4 3 8 respectively. The absorption length for thermal neutrons is 7.59 bar cm in 3He. Thus, overpressure
0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved PII S 0 1 6 8 - 9 0 0 2 ( 9 7 ) 0 1 2 3 4 - 5
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and detector depths '1 cm are necessary for achieving detection efficiencies '50%. Nevertheless, for large-area detectors of 0.5 m ] 0.5 m size, spatial resolutions remain restricted to & 5 mm FWHM due to the usable overpressures. In addition, due to the detector depth, at inclined incidence parallax errors occur, and electron collection times of several 100 ns cause time-of-flight resolutions and count rate limits which are inadequate for the next generation of pulsed neutron sources.
2. The novel detector concept In order to overcome these limitations, a novel concept (cf. Ref. [1]) for large-area imaging detectors for thermal and sub-thermal neutrons is being realized with the potential to combine — sensitive areas of 0.5—0.8 metres squared, — position resolutions of &0.1—0.3 mm FWHM, — neutron time-of-flight resolutions (10 ns, — a burst count rate capability '106~7 events/s. Resolution and count rate limits are governed by the readout technique chosen; for the sake of simplicity, for the first detector generation of 0.5 m ] 0.5 m active size, optimized delay line (DL) readout techniques are used [1], whereas for later versions array readout techniques shall be developed. The detector concept comprises: — a central thin composite convertor foil (157Gd or 6Li, overcoated with CsI, evaporated on both sides of a thin support foil); — two four-fold segmented detector planes on either side of the convertor, comprising either microstrip or microdot gas chambers (MSGCs or MDOTs), operated in low-pressure (LP), two-stage avalanche multiplication mode. This paper is, however, restricted to the optimization of LP MSGCs, operated in a two-stage avalanche multiplication mode, for use with 157Gd/CsI convertors by means of model calculations (cf. also Ref. [1]); LP operation of MSGCs and MDOTs was successfully demonstrated experimentally in Refs. [2—4]. Composite Gd/CsI and Li/CsI neutron convertors were used at first with LP multiwire chambers in Ref. [5]. After neutron capture, in 157Gd conversion electrons (29—182 keV) are released with 87.3$2.5%
57
efficiency, as can be inferred from data for /!5Gd [6] and for 155Gd [7]. These energetic electrons emit, in a secondary electron (SE) cascade, well localized clusters of eV electrons from the convertor surfaces into the two adjacent LP gas chambers. SE emission is enhanced by overcoating the 157Gd surfaces with &100 nm of CsI. However, presently neither the average SE number ((10) nor the SE distribution in the emitted clusters are well known; thus, in order to maximize detection efficiency, the MSGCs were optimized with respect to single electron response. For thermal neutrons in 157Gd the absorption length is 1.3 lm in comparison with a mean effective conversion electron attenuation length as function of convertor thickness (averaged over the conversion electron spectrum) of 11.6$0.3 lm; thus, for optimized convertor thicknesses conversion electron escape efficiencies from the 157Gd bulk into the SE emitting CsI surface layers of &60% for thermal and 70—80% for sub-thermal neutrons can be achieved, respectively [1]. In the upper part of Fig. 1 a cross-section detail of the employed true two-dimensional MSGC design is depicted which is optimized for low-pressure operation: on a 3 mm thick insulating Herasil II quartz plate, which was chosen since most technical glasses contain neutron absorbing boron, a second coordinate pad (SCP) plane (metal 1) for position encoding perpendicular to the microstrip (MS) plane (metal 2) is deposited. Both metal planes are sputter-evaporated Au layers of &1 lm thickness (0.03 ) cm) structured by a lift-off process [8]. They are separated by a &3 lm thick insulating SiO 2 like layer and a &100 nm thick resistive a-SiCN : H layer, which is similar to amorphous doped diamond-like carbon (DLC). Both layers are deposited by plasma enhanced chemical vapour deposition (PECVD) in the same reactor in a clean room [9]. The resistivity of the SiO layer is &1017 ) cm, 2 whereas the surface resistivity of the n-doped DLC layer is &1014 )/h. Coating of MSGCs with DLC layers was investigated first in Ref. [10]. In addition to eliminating charging up, the conductivity of the DLC layer is sufficiently high to collect the electric field lines and to make the surface field uniform (cf. Fig. 2a and Fig. 2b); this effect of a resistive surface layer was
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Fig. 1. Schematic (cross section) of the MSGC design (upper part) and detail of the equipotential lines (lower part) with DLC undercoating.
demonstrated first in Ref. [11]. As can be seen quantitatively from Fig. 2c, the field strength component E parallel to the surface above the MS x plate remains constant up to surface resistivities of the order of 1016 )/h. In comparison to uncoated insulating substrates, the rise of E at the edges x of the MS electrodes is drastically reduced, and thus the operating voltage limits, caused by surface
Fig. 2. Comparison of equipotential lines (a) without and (b) with DLC undercoating; (c) electric field strength component parallel to the MS surface in the anode—cathode gap for various surface resistivities R . S
flashover due to field emission from the cathode edges or UV photon triggered secondary avalanches (by feedback from the anode edges), are increased. Due to the smoothed field strengths at
B. Gebauer et al. /Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 56—62
the electrode’s edges the breakdown voltage through the insulating SiO layer is also raised. 2 Due to the field shaping by the DLC layer, the static potential at the underlying SCP plane is totally shielded from the gas volume and can thus be chosen independently. This effect allows to employ the adopted MSGC cross section, following Refs. [12,13]. In contrast to second coordinate signal induction on the backplane, which is strongly attenuated due to the unfavourable ratio of the anode—cathode edge-to-edge distance and the distance to the backplane, this design is true twodimensional because of the negligible distance to the SCP plane and since the high resistivity of the DLC layer makes it totally transparent to transient voltages in the relevant frequency range [14]. For the LP two-stage multiplication mode the improved uniformity of the field in the gas volume close above the MS plate (cf. Fig. 1 and Fig. 2b) is of utmost importance for achieving high gains. SEs released from the convertor surfaces undergo subsequently — parallel plate avalanche preamplification of gain G in the constant field regions of depth 4 mm, 1 commencing at the convertor surfaces and ranging up to 0.5 mm above the MS plates, and — MS amplification of additional gain G in the 2 strong alternating fields close to the MS plate, generated by the potential differences between the MS electrodes. Using the counting gas isobutane at a pressure of p&20 hPa, for safe operating conditions (see below) mean free paths for ionization j " 1 390—450 lm and Sj T" 34—47 lm are predicted 2 by model calculations for the G and G regions, 1 2 respectively, the latter value being averaged over all trajectories (see the lower part of Fig. 1). The contribution of all trajectories is nearly the same for all start positions, since diffusion in the 4 mm deep G region broadens the avalanche head to 1 &1.2 mm FWHM, due to the high reduced field strength E/p reached in LP operation; thus electrons are also scattered close to the cathode edges from where they follow trajectories of maximum gain. The minimum j at 20 hPa isobutane is 16 lm 2 [15], corresponding to the ionization cross-section maximum at much higher electron temperatures than reachable.
59
Thus, for optimizing the gain G , i.e. the average 2 number of multiplication steps along the inhomogeneous field trajectories, at safe field strengths and corresponding moderate electron temperatures, two conditions must be fulfilled: (1) j and thus the field strengths must be as equal and 2 constant as possible along all trajectories, and (2) the anode—cathode edge-to-edge distance d , AC which also governs the depth of the inhomogeneous field region, must be sufficiently large in view of the long attainable j . Therefore, 2 a large pitch of 635 lm was chosen, taking also into account a DL tap spacing of 1.27 mm and using one only capacitively coupled ‘interpolating’ strip between readout nodes (cf. Ref. [1]). Due to diffusion broadening this pitch delivers negligible differential nonlinearity with DL readout. Thus, optimization delivered d "255 lm and MS anode and cathAC ode widths of 25 and 100 lm, respectively. The SCP widths and positions were varied as discussed below.
3. Model calculations results Extensive model calculations were performed using the codes MAXWELL (2D and 3D electric field simulator) [16], MSGCSIM (simulating gas avalanche multiplication and induced charges) [17] and SPICE (for modelling signal propagation in the coupled MS and SCP networks and the respective DLs) [18]; thus electrode shapes, gains, induced signal currents and electrical networks were optimized. In the MSGCSIM Monte Carlo calculations single-electron avalanches were calculated for statistically distributed starting points at the convertor surfaces including diffusion in the avalanche trajectories. In order to demonstrate the achievable amplification factors, in Fig. 3 gains G ,G and total 1 2 two-stage gains G"G ]G are shown for anode 1 2 voltages º " 350—500 V, º "!800 V at the A D drift cathode (i.e. the convertor) and º "0 V at C the MS cathode. Average total electron numbers in the avalanche qN are shown; the geometry-dependent collected charges on the individual MS electrodes are smaller by factors &0.5—0.7 for the integrated charges (if the absolute values of both
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B. Gebauer et al. /Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 56—62
Fig. 3. Gains G ,G and G"G ]G in the parallel plate, 1 2 1 2 microstrip and in the combined amplification stages, respectively.
Fig. 5. Anode and SCP signals for º "400 V and d " A A~SCP 50 lm.
Fig. 4. Electron number distribution q for º "500 V with A Polya fit P(q).
polarities are added up) and shown below for the fast current pulse components. For º "500 V in A Fig. 4 the distribution of electron numbers q in the avalanches, calculated for 105 events, are depicted in comparison with a Polya distribution P(q) fit (cf. e.g. Ref. [19]); the simulated data are fitted for H"0.35, showing that, in spite of the very high average avalanche charge, the operating voltage º "500 V is below the instable region (H(0) A which can be correlated with negative space charge. This result was to be expected since in the LP case avalanches are much more widened by diffusion than at normal pressures and space charge is
accordingly reduced. It is also consistent with the very high gains achieved in the LP operation of MDOTs [4]. In this case the avalanches are spread over a number of MDOT cells. In Fig. 4 secondary avalanches released, e.g., from the CsI SE emitters were not taken into account. As depicted in Fig. 3, in spite of the optimization of G , G remains 20—30 times higher than G ; 2 1 2 however, this factor is decreasing for higher G . For 2 constant voltages º and º , G is strongly depenD C 1 dent on anode voltage º , since the constant field A is also raised with º . A Compared to parallel plate amplification, the higher field strengths in the second amplification step increase the avalanche speed, thus reducing the width of the fast current signal component. For º "400 V current signals collected on the MS A anodes and SCP electrodes are shown in Fig. 5 for a superposition of 1000 events. For the superposition the pulse width at FWHM is &5 ns, for the individual pulses it is 4—5 ns. The average signal charges integrated over 10 ns are 2.1]106 and 1.3]106 charge units for the anode and SCP
B. Gebauer et al. /Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 56—62
Fig. 6. Detail of the MS and SCP layout. For improved signal induction the SCPs (black) are split into two strips running parallel to the anodes at edge-to-edge distances d . Thus, A~SCP anode—SCP capacitances are reduced, too. The pads are interconnected perpendicular to the MS electrodes and separated by return current strips.
61
strongly dependent on the SCP positions. For d (55 lm the collected charges decrease A~SCP strongly for both the anodes and the SCP electrodes, and signals become bipolar, whereas the cathode signals rise. This is related to the mean free paths for ionization j &42 lm at this voltage: for 2 d 'j the charge carriers are predominantly A~SCP 2 generated between anodes and the SCP electrodes, and thus the SCP signals have opposite polarity to the negative anode signals, and signal induction on the cathodes is shielded. The weighting fields of these three adjacent electrodes are strongly correlated (cf. Refs. [20,21]) and must be evaluated separately for each SCP position. Anode and SCP signals become smaller for both smaller and wider SCP electrodes than 50 lm.
4. Summary and outlook
Fig. 7. Dependence of average signal charge, accumulated within 10 ns on the anodes, cathodes and SCP electrodes, on d . A~SCP
signals, respectively. These short and high signals, obtained at very moderate voltages well below sparking limits, are very well suited for DL readout [1]. For the signals shown in Fig. 5 the anode-SCP edge-to-edge distances d and the SCP widths A~SCP are 50 lm, respectively (cf. Fig. 6). In Fig. 7 the average signal charges obtained by integration over 10 ns on the three electrodes are depicted, normalized to the total number of electrons in the avalanche. Within the very short integration time a very high fraction of charge is collected, e.g. &33% on the anodes and &22% on the SCP electrodes. However, the induced charges are
For detectors using 157Gd/CsI neutron convertors an optimized MSGC design was developed delivering — according to model calculations — very high and short signals in a two-stage low-pressure operation mode; it is well suited for single-electron imaging and thus also for high-resolution thermal neutron imaging. With this design large-area MSGCs are presently built on 3 mm thick robust quartz substrates; the fabrication technology using CVD layers is developed in a collaboration. It will also be refined to built large-area MDOTs.
References [1] B. Gebauer et al., Nucl. Instr. and Meth. A 392 (1997) 68. [2] A. Breskin et al., Nucl. Instr. and Meth. A 345 (1994) 205. [3] D.F. Anderson et al., Nucl. Instr. and Meth. A 346 (1994) 102. [4] A. Breskin et al., Nucl. Instr. and Meth. A 394 (1997) 21. [5] V. Dangendorf et al., Nucl. Instr. and Meth. A 350 (1994) 503. [6] C.K. Hargrove et al., Nucl. Instr. and Meth. A 357 (1995) 157. [7] A. Ba¨cklin et al., Nucl. Phys. A 380 (1982) 189. [8] IMT Masken und Teilungen AG, CH-8606 Greifensee, Switzerland. [9] A. Weber et al., Fraunhofer-Institut fu¨r Schicht- und Oberfla¨chentechnik, D-38108 Braunschweig, Germany.
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[10] R. Bouclier et al., Nucl. Instr. and Meth. A 369 (1996) 328. [11] J.J. Florent et al., Nucl. Instr. and Meth. A 329 (1993) 125. [12] F. Angelini et al., Nucl. Instr. and Meth. A 323 (1992) 229. [13] F. Angelini et al., Nucl. Instr. and Meth. A 336 (1993) 106. [14] G. Battistoni et al., Nucl. Instr. and Meth. 202 (1982) 459. [15] S.F. Biagi, Nucl. Instr. and Meth. A 310 (1991) 133.
[16] MAXWELL, Ansoft Corp., Pittsburgh, PA 15219-1119, USA. [17] S.F. Biagi, MSGCSIM, publication in preparation. [18] SPICE, c/o EECS/ERL Industrial Support Office, U.C. Berkeley, Berkeley, CA 94720, USA. [19] J. Va’vra, Nucl. Instr. and Meth. A 371 (1996) 33. [20] A.H. Walenta, Nucl. Instr. and Meth. 151 (1978) 461. [21] E. Gatti et al., Nucl. Instr. and Meth. 193 (1982) 651.
Large-area low-pressure microstrip gas chambers for thermal neutron imaging B. Gebauer!,*, Ch. Schulz!, Th. Wilpert!, S.F. Biagi" ! Hahn-Meitner-Institut, Glienicker Str. 100, D-14109 Berlin, Germany " Department of Physics, University of Liverpool, P.O.Box 147, Liverpool L69 3Bx, UK
Abstract Thermal neutron imaging gas detectors using counting gas mixtures with gaseous neutron convertors (e.g. 3He) suffer from inherent limitations in position and time-of-flight resolutions, resulting from the long ranges of the released secondary ions in the counting gas and from parallax errors due to the necessary depth of the detector gas volume of '1 cm. In order to overcome these limitations a novel detector generation is presently being constructed, utilizing composite neutron convertor foils (157Gd or 6Li overcoated with CsI) in combination with arrays of large-area microstrip and microdot gas chambers (MSGCs and MDOTs), which are operated in low-pressure, two-stage amplification mode and delivering unsurpassed gains. In this paper the optimization of MSGCs for low-pressure operation with 157Gd convertors is discussed. The MSGCs are of technologically advanced, robust design, using 3 mm thick synthetic quartz substrates and two closely spaced electrode planes for true two-dimensional position readout. These metal planes are separated by a &3 lm thick SiO /DLC double-layer deposited by means of plasma-enhanced CVD. The same 2 fabrication technology is presently prepared for production of large-area MDOTs. The work shall be conducted in collaboration with teams from research institutes and industry. ( 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Conventional thermal neutron imaging gas detectors (for neutron wavelength j&1.8A_ ), employing counting gas mixtures of quench gases with the gaseous neutron convertor 3He, deliver very limited position and time-of-flight resolutions due to the detector depth and to the long ranges of the protons and tritons, emitted after neutron capture
* Corresponding author. Fax: #49 30 8062 2293; e-mail: [email protected].
into random opposite directions (n # 3He Pp (573 keV) # t(191 keV)). In mixtures of 3He with CF and C H — the quench gases of highest stop4 3 8 ping power in use in neutron detectors — the common centroids of the secondary electron distributions, released along the proton and triton traces, fill spheres with diameters of &0.7 R and &0.8 1 R , respectively, around a capture point of many 1 neutrons; thus, the FWHM of the spatial resolution is restricted to these values. R , the projected pro1 ton range, is 0.43 and 0.45 bar cm in CF and C H , 4 3 8 respectively. The absorption length for thermal neutrons is 7.59 bar cm in 3He. Thus, overpressure
0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved PII S 0 1 6 8 - 9 0 0 2 ( 9 7 ) 0 1 2 3 4 - 5
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and detector depths '1 cm are necessary for achieving detection efficiencies '50%. Nevertheless, for large-area detectors of 0.5 m ] 0.5 m size, spatial resolutions remain restricted to & 5 mm FWHM due to the usable overpressures. In addition, due to the detector depth, at inclined incidence parallax errors occur, and electron collection times of several 100 ns cause time-of-flight resolutions and count rate limits which are inadequate for the next generation of pulsed neutron sources.
2. The novel detector concept In order to overcome these limitations, a novel concept (cf. Ref. [1]) for large-area imaging detectors for thermal and sub-thermal neutrons is being realized with the potential to combine — sensitive areas of 0.5—0.8 metres squared, — position resolutions of &0.1—0.3 mm FWHM, — neutron time-of-flight resolutions (10 ns, — a burst count rate capability '106~7 events/s. Resolution and count rate limits are governed by the readout technique chosen; for the sake of simplicity, for the first detector generation of 0.5 m ] 0.5 m active size, optimized delay line (DL) readout techniques are used [1], whereas for later versions array readout techniques shall be developed. The detector concept comprises: — a central thin composite convertor foil (157Gd or 6Li, overcoated with CsI, evaporated on both sides of a thin support foil); — two four-fold segmented detector planes on either side of the convertor, comprising either microstrip or microdot gas chambers (MSGCs or MDOTs), operated in low-pressure (LP), two-stage avalanche multiplication mode. This paper is, however, restricted to the optimization of LP MSGCs, operated in a two-stage avalanche multiplication mode, for use with 157Gd/CsI convertors by means of model calculations (cf. also Ref. [1]); LP operation of MSGCs and MDOTs was successfully demonstrated experimentally in Refs. [2—4]. Composite Gd/CsI and Li/CsI neutron convertors were used at first with LP multiwire chambers in Ref. [5]. After neutron capture, in 157Gd conversion electrons (29—182 keV) are released with 87.3$2.5%
57
efficiency, as can be inferred from data for /!5Gd [6] and for 155Gd [7]. These energetic electrons emit, in a secondary electron (SE) cascade, well localized clusters of eV electrons from the convertor surfaces into the two adjacent LP gas chambers. SE emission is enhanced by overcoating the 157Gd surfaces with &100 nm of CsI. However, presently neither the average SE number ((10) nor the SE distribution in the emitted clusters are well known; thus, in order to maximize detection efficiency, the MSGCs were optimized with respect to single electron response. For thermal neutrons in 157Gd the absorption length is 1.3 lm in comparison with a mean effective conversion electron attenuation length as function of convertor thickness (averaged over the conversion electron spectrum) of 11.6$0.3 lm; thus, for optimized convertor thicknesses conversion electron escape efficiencies from the 157Gd bulk into the SE emitting CsI surface layers of &60% for thermal and 70—80% for sub-thermal neutrons can be achieved, respectively [1]. In the upper part of Fig. 1 a cross-section detail of the employed true two-dimensional MSGC design is depicted which is optimized for low-pressure operation: on a 3 mm thick insulating Herasil II quartz plate, which was chosen since most technical glasses contain neutron absorbing boron, a second coordinate pad (SCP) plane (metal 1) for position encoding perpendicular to the microstrip (MS) plane (metal 2) is deposited. Both metal planes are sputter-evaporated Au layers of &1 lm thickness (0.03 ) cm) structured by a lift-off process [8]. They are separated by a &3 lm thick insulating SiO 2 like layer and a &100 nm thick resistive a-SiCN : H layer, which is similar to amorphous doped diamond-like carbon (DLC). Both layers are deposited by plasma enhanced chemical vapour deposition (PECVD) in the same reactor in a clean room [9]. The resistivity of the SiO layer is &1017 ) cm, 2 whereas the surface resistivity of the n-doped DLC layer is &1014 )/h. Coating of MSGCs with DLC layers was investigated first in Ref. [10]. In addition to eliminating charging up, the conductivity of the DLC layer is sufficiently high to collect the electric field lines and to make the surface field uniform (cf. Fig. 2a and Fig. 2b); this effect of a resistive surface layer was
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Fig. 1. Schematic (cross section) of the MSGC design (upper part) and detail of the equipotential lines (lower part) with DLC undercoating.
demonstrated first in Ref. [11]. As can be seen quantitatively from Fig. 2c, the field strength component E parallel to the surface above the MS x plate remains constant up to surface resistivities of the order of 1016 )/h. In comparison to uncoated insulating substrates, the rise of E at the edges x of the MS electrodes is drastically reduced, and thus the operating voltage limits, caused by surface
Fig. 2. Comparison of equipotential lines (a) without and (b) with DLC undercoating; (c) electric field strength component parallel to the MS surface in the anode—cathode gap for various surface resistivities R . S
flashover due to field emission from the cathode edges or UV photon triggered secondary avalanches (by feedback from the anode edges), are increased. Due to the smoothed field strengths at
B. Gebauer et al. /Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 56—62
the electrode’s edges the breakdown voltage through the insulating SiO layer is also raised. 2 Due to the field shaping by the DLC layer, the static potential at the underlying SCP plane is totally shielded from the gas volume and can thus be chosen independently. This effect allows to employ the adopted MSGC cross section, following Refs. [12,13]. In contrast to second coordinate signal induction on the backplane, which is strongly attenuated due to the unfavourable ratio of the anode—cathode edge-to-edge distance and the distance to the backplane, this design is true twodimensional because of the negligible distance to the SCP plane and since the high resistivity of the DLC layer makes it totally transparent to transient voltages in the relevant frequency range [14]. For the LP two-stage multiplication mode the improved uniformity of the field in the gas volume close above the MS plate (cf. Fig. 1 and Fig. 2b) is of utmost importance for achieving high gains. SEs released from the convertor surfaces undergo subsequently — parallel plate avalanche preamplification of gain G in the constant field regions of depth 4 mm, 1 commencing at the convertor surfaces and ranging up to 0.5 mm above the MS plates, and — MS amplification of additional gain G in the 2 strong alternating fields close to the MS plate, generated by the potential differences between the MS electrodes. Using the counting gas isobutane at a pressure of p&20 hPa, for safe operating conditions (see below) mean free paths for ionization j " 1 390—450 lm and Sj T" 34—47 lm are predicted 2 by model calculations for the G and G regions, 1 2 respectively, the latter value being averaged over all trajectories (see the lower part of Fig. 1). The contribution of all trajectories is nearly the same for all start positions, since diffusion in the 4 mm deep G region broadens the avalanche head to 1 &1.2 mm FWHM, due to the high reduced field strength E/p reached in LP operation; thus electrons are also scattered close to the cathode edges from where they follow trajectories of maximum gain. The minimum j at 20 hPa isobutane is 16 lm 2 [15], corresponding to the ionization cross-section maximum at much higher electron temperatures than reachable.
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Thus, for optimizing the gain G , i.e. the average 2 number of multiplication steps along the inhomogeneous field trajectories, at safe field strengths and corresponding moderate electron temperatures, two conditions must be fulfilled: (1) j and thus the field strengths must be as equal and 2 constant as possible along all trajectories, and (2) the anode—cathode edge-to-edge distance d , AC which also governs the depth of the inhomogeneous field region, must be sufficiently large in view of the long attainable j . Therefore, 2 a large pitch of 635 lm was chosen, taking also into account a DL tap spacing of 1.27 mm and using one only capacitively coupled ‘interpolating’ strip between readout nodes (cf. Ref. [1]). Due to diffusion broadening this pitch delivers negligible differential nonlinearity with DL readout. Thus, optimization delivered d "255 lm and MS anode and cathAC ode widths of 25 and 100 lm, respectively. The SCP widths and positions were varied as discussed below.
3. Model calculations results Extensive model calculations were performed using the codes MAXWELL (2D and 3D electric field simulator) [16], MSGCSIM (simulating gas avalanche multiplication and induced charges) [17] and SPICE (for modelling signal propagation in the coupled MS and SCP networks and the respective DLs) [18]; thus electrode shapes, gains, induced signal currents and electrical networks were optimized. In the MSGCSIM Monte Carlo calculations single-electron avalanches were calculated for statistically distributed starting points at the convertor surfaces including diffusion in the avalanche trajectories. In order to demonstrate the achievable amplification factors, in Fig. 3 gains G ,G and total 1 2 two-stage gains G"G ]G are shown for anode 1 2 voltages º " 350—500 V, º "!800 V at the A D drift cathode (i.e. the convertor) and º "0 V at C the MS cathode. Average total electron numbers in the avalanche qN are shown; the geometry-dependent collected charges on the individual MS electrodes are smaller by factors &0.5—0.7 for the integrated charges (if the absolute values of both
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Fig. 3. Gains G ,G and G"G ]G in the parallel plate, 1 2 1 2 microstrip and in the combined amplification stages, respectively.
Fig. 5. Anode and SCP signals for º "400 V and d " A A~SCP 50 lm.
Fig. 4. Electron number distribution q for º "500 V with A Polya fit P(q).
polarities are added up) and shown below for the fast current pulse components. For º "500 V in A Fig. 4 the distribution of electron numbers q in the avalanches, calculated for 105 events, are depicted in comparison with a Polya distribution P(q) fit (cf. e.g. Ref. [19]); the simulated data are fitted for H"0.35, showing that, in spite of the very high average avalanche charge, the operating voltage º "500 V is below the instable region (H(0) A which can be correlated with negative space charge. This result was to be expected since in the LP case avalanches are much more widened by diffusion than at normal pressures and space charge is
accordingly reduced. It is also consistent with the very high gains achieved in the LP operation of MDOTs [4]. In this case the avalanches are spread over a number of MDOT cells. In Fig. 4 secondary avalanches released, e.g., from the CsI SE emitters were not taken into account. As depicted in Fig. 3, in spite of the optimization of G , G remains 20—30 times higher than G ; 2 1 2 however, this factor is decreasing for higher G . For 2 constant voltages º and º , G is strongly depenD C 1 dent on anode voltage º , since the constant field A is also raised with º . A Compared to parallel plate amplification, the higher field strengths in the second amplification step increase the avalanche speed, thus reducing the width of the fast current signal component. For º "400 V current signals collected on the MS A anodes and SCP electrodes are shown in Fig. 5 for a superposition of 1000 events. For the superposition the pulse width at FWHM is &5 ns, for the individual pulses it is 4—5 ns. The average signal charges integrated over 10 ns are 2.1]106 and 1.3]106 charge units for the anode and SCP
B. Gebauer et al. /Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 56—62
Fig. 6. Detail of the MS and SCP layout. For improved signal induction the SCPs (black) are split into two strips running parallel to the anodes at edge-to-edge distances d . Thus, A~SCP anode—SCP capacitances are reduced, too. The pads are interconnected perpendicular to the MS electrodes and separated by return current strips.
61
strongly dependent on the SCP positions. For d (55 lm the collected charges decrease A~SCP strongly for both the anodes and the SCP electrodes, and signals become bipolar, whereas the cathode signals rise. This is related to the mean free paths for ionization j &42 lm at this voltage: for 2 d 'j the charge carriers are predominantly A~SCP 2 generated between anodes and the SCP electrodes, and thus the SCP signals have opposite polarity to the negative anode signals, and signal induction on the cathodes is shielded. The weighting fields of these three adjacent electrodes are strongly correlated (cf. Refs. [20,21]) and must be evaluated separately for each SCP position. Anode and SCP signals become smaller for both smaller and wider SCP electrodes than 50 lm.
4. Summary and outlook
Fig. 7. Dependence of average signal charge, accumulated within 10 ns on the anodes, cathodes and SCP electrodes, on d . A~SCP
signals, respectively. These short and high signals, obtained at very moderate voltages well below sparking limits, are very well suited for DL readout [1]. For the signals shown in Fig. 5 the anode-SCP edge-to-edge distances d and the SCP widths A~SCP are 50 lm, respectively (cf. Fig. 6). In Fig. 7 the average signal charges obtained by integration over 10 ns on the three electrodes are depicted, normalized to the total number of electrons in the avalanche. Within the very short integration time a very high fraction of charge is collected, e.g. &33% on the anodes and &22% on the SCP electrodes. However, the induced charges are
For detectors using 157Gd/CsI neutron convertors an optimized MSGC design was developed delivering — according to model calculations — very high and short signals in a two-stage low-pressure operation mode; it is well suited for single-electron imaging and thus also for high-resolution thermal neutron imaging. With this design large-area MSGCs are presently built on 3 mm thick robust quartz substrates; the fabrication technology using CVD layers is developed in a collaboration. It will also be refined to built large-area MDOTs.
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I. TRACKING (GAS)
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