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The macrochannel plate: A suitable detector for low energy γ-rays and high energy electromagnetic showers
500
Nuclear Instruments and Methods in Physics Research A273 (1988) 500-505 North-Holland, Amsterdam
THE MACROCHANNEL PLATE: A SUITABLE DETECTOR FOR LOW ENERGY -y-RAYS AND HIGH ENERGY ELECTROMAGNETIC SHOWERS A. DEL GUERRA, M. CONTI, G. GORINI, P. MAIANO and C. RIZZO Dipartimento di Fisica, University di Pisa, Piazza Torricelli 2, I-56100 Pisa, Italy INFN, Sezione di Pisa, Via Vecchia Livornese, I-56010 S. Piero a Grado (PI), Italy
V. PEREZ-MENDEZ
Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720, USA
We have developed a manufacturing process for arrays of high resistance lead glass tubing of various diameters, ranging from 500 gm to several mm, for the detection of -y-rays. We describe the applications of this type of device to positron emission tomography and high energy physics. 1. Introduction In our continuous effort to develop position sensitive detectors for positron emission tomography imaging we have implemented a high density projection chamber, which consists of a standard gas multiwire proportional chamber equipped with a high density, high Z honeycomb converter [1]. The converter is made as an array of high resistance lead glass tubing which acts as combined gamma ray converter and electron drift structure. Both the MWPC and the converter are in the same gas enclosure. A schematic drawing of such a device is presented in fig. 1. The lead glass matrices are treated in a H2 reduction process to form a resistive layer, with a typical uniformity along the tube of - 10%. A voltage difference applied between the ends of the tube gives the necessary electric field for drifting the electrons towards the avalanche region of the MWPC . Depending on the type of glass used, the resistivity can vary by several orders of magnitude. A resistivity in the range 10 8-1012 S2/ß is suitable for this application . The detailed description for the construction of the lead glass matrices can be found elsewhere [2,3]. As a most unusual spin-off of medical imaging, we proposed to use the same principle for a lead glass drift calorimeter for high energy physics [4]. In this latter case the lead glass tubing structure acts as a radiator for the electromagnetic particles; the energy of the particle is proportional to the energy deposited in the gas, which is measured by collecting the ionization electrons at the end of the drift channels . A standard two coordinate readout MWPC can be used to give the projection of the shower on the plane perpendicular to the tube axes, whereas the third coordinate (along the tubes) is given by the drift time . 0168-9002/88/$03 .50 C Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
conductive surface
" -VO
d
-t1V(x )= Vo d (continuous voltage divider)
0
electron detection region (MWPC'
Fig. 1. Schematic drawing of a high density electron drift structure equipped with a conventional MWPC, enclosed in the same gas atmosphere .
For PET imaging it is necessary to have a high surface to volume ratio to increase the probability for the low energy Compton- or photo-electrons (produced by the 511 keV photon interacting inside the converter walls) to reach the gas drift region within the tubes. The drift channel length cannot be too long, otherwise it produces a much too big parallax error. For the
A. Del Guerra et al. / The macrochannel plate Table 1 Physical properties of the type of glass and geometrical configurations of the "macrochannel plate" Type of glass
Hi-D Pacific
a)
71
Density (g/cm3 )
6.2
5.18
1.28
1.66
10 8 -10 9
-lo ll
PET
PET
(cm)
Resistivity after H z treatment Application
Tubing diameter inner/outer (mm) 0.5/0 .6 Drift length (cm) a)
b>
up to 2
b)
'va
Schott RS-520
PbO composition by weight (%) 79
Radiation length
501
electromagnetic calorimeter
0.5/0 .6 5/7 1
up to 40
This glass was originally used at Lawrence Berkeley Laboratory [2]. Among the various ID/OD configurations we have used, the 0.5/0.6 mm is the one with the highest surface to volume ratio.
- Vconu. Fig. 2. Schematic drawing of the HISPET prototype. calorimetry application, instead, the main requirement is to provide a dense and compact structure for the
confinement of a multi-GeV shower in a reasonable space, with a high granularity and a drift length as long
as allowed by the lateral diffusion of the electron in gas.
Fig. 3. The HISPET prototype under test at the Department of Physics of Pisa University together with the front-end electronics and the data taking system . II . GASEOUS DETECTORS
50 2
A. Del Guerra et al. / The macrochannel plate
Hence, different requirements dictate different geometries for the two applications, but the same principle and the same technical construction are involved and the same type of glass has been used. The tubing diameter varies from 500 ltm for PET to 5 mm for the high energy physics application, and the drift length scales approximately by the same ratio. By applying a high enough voltage and using the proper gas, we have shown that it is also possible to have multiplication of the electrons which are drifting within the tubes [5]. This device seems to us to be similar in design, technology and physical properties to a microchannel plate, but much rougher, bigger, thicker, and with larger holes. All in all, we decided to call it macrochannel plate [6]. In table 1 we show the physical properties of the glass used and its geometric characteristics for the two applications . The capillaries were originally drawn to the various sizes by a private firm [7], so were the tubes for the high energy physics application. Later on we decided to draw the capillaries ourselves from the tubes by using a commercial drawing machine [8]. This allows us to vary "ad libitum" the tubing diameter, of course maintaining the same inner/outer diameter ratio. In sections 2 and 3 of this paper we describe the current status of the applications of this type of device in PET and in electromagnetic calorimetry, respectively . 2. Use of the macrochannel plate for PET We have proposed [9] a 3-D high spatial resolution positron emission tomograph (HISPET), based on the use of this macrochannel plate device coupled to a MWPC . HISPET will consist of six modules arranged so as to form the lateral surface of a hexagonal prism with a solid angle coverage of - 27r sr . Each module of HISPET will have two MWPC each, with two 1 cm thick converter planes of lead-glass tubing (0 .5 and 0.6 mm ID and OD, respectively), with a measured efficiency of 6.5% per cm of converter for 511 keV y-rays . The expected performance of HISPET are a volume sensitivity of - 100000 c/s per 0.1 p.Ci/ml for a true to accidental coincidences ratio of 3 : 1. A spatial resolution of - 4 mm FWHM in x, y, and z has been obtained with data simulated by a fully 3-D Monte Carlo program and reconstructed by a 3-D filtered back-projection algorithm [10] . We have tested (111 a first prototype positron camera (fig . 2), which consists of two 50 x 50 cm2 MWPC approximately 80 cm apart, each one equipped with a 2 cm thick lead-glass converter plane. The planes were built in the early stage of our experimentation with the Hi-D glass (80% PbO by weight, density of 6.2 g/cm3 ); the tubes have inner and outer diameters of 1.33 and 1 .59 mm, respectively . Fast delay lines capacitively coupled to the cathode planes are used for measuring both
arbitrary units
LA
FWHM
.
Fig. 4. Spatial resolution histograms for a 22 Na point-like source at the center of the prototype: along the y-axis (top); along the z-axis (bottom) . Each channel corresponds to 8 mm. coordinates. A coincidence within the time resolution window between the anode signals from both MWPC defines the trigger, which is used as the common START of a multiple STOP time to digital converter. The signals from both ends of all four delay lines (2 for each MWPC) act as the eight independent STOPS for the
Fig. 5. Photograph of the calorimeter prototype consisting of three blocks of lead glass tubing for a total of 11 .2 radiation lengths.
503
A . Del Guerra et al. / The macrochannel plate
eight channels of the TDC. The coordinate position is then directly related to the time difference between the two corresponding STOPs. The data from the TDC are directly fed into a FASTBUS memory Lecroy 1892, which acts as a temporary storage, and then into a PDPIl-23 computer (fig. 3) . Very preliminary data [111 have been taken and analyzed with the 3-D reconstruction algorithm we have implemented. The spatial resolu22 tion obtained for a pointlike Na source at the center is presented in fig. 4. Along both the yof the prototype and z-direction the FWHM of the distribution is approximately one channel, which corresponds to 8 mm, and is consistent with the expected value for this prototype . The additional spikes in the histograms are mainly due to the poor statistics (less than one thousand events). 3. Electromagnetic application We have designed a drift collection calorimeter [12], which makes use of the macrochannel plate, with lead
glass tubes of 40 cm drift length and a total longitudinal dimension of --- 18 .6 radiation lengths (Xo) for a total physical length of 70 cm . The calorimeter is made by assembling 7 modules of 10 x 14 X 40 cm3; it has a very good sampling (Xo/6) ; furthermore, the tubing structure limits the track length fluctuations on the released energy. A detailed Monte Carlo study has been performed [13] to evaluate the shower containment and the energy resolution. An energy resolution a/E of 17%/FE has been obtained in the simulation for full containment of the shower at 1 atm. We have already reported preliminary results on smaller modules of 5 X 5 cm2 by up to 14 cm long [14] . Here we present the energy resolution measurements we have taken with a full drift length prototype (fig . 5), made of three blocks (10 X 14 cm2 X 40 cm drift length, each). The prototype covers 11 .2X o in the longitudinal direction (42 cm physical length). The data were obtained in late 1986 at the CERN-PS. Three scintillators defined a beam spot of 1 cm; their Time spectrum
Energy spectrum
output to MCA
gate signal
Fig. 6. Schematic drawing of the electronics setup for the test of the calorimeter prototype. Typical time and energy spectra are also shown. II . GASEOUS DETECTORS
504
A . Del Guerra et al. / The macrochannel plate 200
1
2
3
4
5
6
ENERGY(GeV) Fig. 7. Released energy, Edep, versus incident particle energy . A linear fit is overdrawn.
coincidence formed the trigger. The calorimeter could be moved up and down to shift the beam spot along its axis . We worked with a negative particle beam, and used a threshold CO, Cherenkov counter to select electrons. For each run we had both spectra for the released energy and the time distribution (fig . 6) . We will refer to a gas filling of Ar-CH4 (70-30) at 2 atm. Further measurements at atmospheric pressure are under elaboration . Particle momentum from 2 to 5 GeV/c was selected . We fitted the energy spectra as Gaussian distribu_o fE
Edep fGeV] 112 0.3
0.2
0.0
Fig. 8. Energy resolution (a/Edep)x El /2 as a function of the incident energy . A linear fit is overdrawn, which gives a constant value of 0.18+0.02.
tions, and no high energy tail was observed, probably due to the track length limitations of the tubes ; the good linear relationship between incident particle energy (E) and the energy released (Edep) within the calorimeter is shown in fig. 7. The energy resolution (a/Edep) is in agreement with the sampling calorimeter general law (- 1/F). A resolution a/Edep = (18 ± 2)%/ rE was obtained (fig . 8), which compares fairly well with a Monte Carlo simulation value of about 17/> E % at 2 atm for this prototype . We shifted the beam spot along the axes of the tubes, and recorded the time shift of the signal and the pulse height attenuation in the energy spectra; given the electron drift velocity in the gas, we are then able to obtain a value for the attenuation length within the tubes : data analysis will be completed in the next months. We are now assembling the 18 .6X, calorimeter, which will contain 97% of a 10 GeV e.m. shower [13] . A pad cathode will be used for the read-out of the charge from one end of the tubing structure; each pad will have its preamplifier - shaper - Flash ADC - 2kbyte Memory. We expect to test the calorimeter in spring 1988 at CERN . 3. Conclusions We have shown that the macrochannel plate is a suitable detector for PET imaging. Its technology is simple, reliable and not expensive. The 3-D tomograph we are building using this device will have excellent performance, in particular a spatial resolution of 4 mm in all three directions ; the experimental results we have obtained so far are very encouraging. On a larger scale such a device can be used as a drift collection calorimeter, expecially for low count rate experiments . In the search for proton decay large detection volumes are needed with a high granularity and a fair energy resolution . The proposed macrochannel plate seems to have the necessary energy and spatial resolution . The cost of the bulk glass material is negligible and the construction cost is also very reasonable expecially for large quantities. However, to make full use of the granularity of the device, the number of electronics readout channels may become excessively high . An optical readout system, as recently proposed by Charpak [15], would be ideal for this application .
References [11 M. Conti, A. Del Guerra, R. Habel, T. Mulera, V. PerezMendez and G. Schwartz, Nucl. Instr. and Meth . A255 (1987) 207.
A. Del Guerra et al. / The macrochannel plate [2] G.K. Lum, M.I . Green, V. Perez-Mendez and K.C . Tam IEEE Trans. Nucl . Sci. NS-27 (1980) 157. G. Schwartz, M. Cinti, M. Conti, A. Del Guerra, M. Di Fino, R. Habel, V. Perez-Mendez and L. Righini, INFN Sezione di Pisa Report, INFN/TC-85/6 (1985). [4] V. Perez-Mendez, A. Del Guerra, T. Mulera, H. Hirayama and W.R . Nelson, Nucl . Instr. and Meth. 217 (1983) 255. I. Fujieda, T.A . Mulera, V. Perez-Mendez and A. Del Guerra, IEEE Trans. Nucl . Sci. NS-33 (1986) 587. [6] In fact, Hamamatsu has provided us with a converter sample (dimensions of 5 x 5 cmz and thickness of 1 cm), made with the technology of the microchannel plates . The
sample is now under test. [7] Garner Glass Co ., Claremont, California 97111, USA. [8] Shimadzu Glass Drawing Machine, GDM 1B . [9] A. Del Guerra, G.K. Lum, V. Perez-Mendez and G. Schwartz, in : Positron Annihilation, eds. P.C . Jain, R.M . Singru and K.P . Gopinathan (World Scientific, Singapore, 1985) p. 810. [10] C. Rizzo, M. Conti and A. Del Guerra, Physica Medica 1 (1987) 19 . [111 A. Del Guerra, A. Bandettini, G. De Pascalis, M. Conti, P. Maiano, C. Rizzo and V. Perez-Mendez, Proc. Int. Symp . on Wire Chambers in Medical Imaging, Bruxelles, 28-30 June 1987, Nucl. Instr. and Meth . A269 (1988) 425. [12] T. Mulera, V. Perez-Mendez, H. Hirayama, W.R. Nelson,
50 5
R. Bellazzini, A. Del Guerra and M.M. Massai, IEEE Trans. Nucl . Sci. NS-31 (1984) 64. [13] H. Hirayama, W.R. Nelson, A. Del Guerra, T. Mulera and V. Perez-Mendez, Nucl . Instr. and Meth . 220 (1984) 327. [14] A. Del Guerra, M. Conti, G. Gorini, I. Fujieda, T.A . Mulera and V. Perez-Mendez, Nucl . Instr. and Meth . A257 (1987) 609. [15] G. Charpak, J.-P. Fabre, F. Sauli, M. Suzuki and W. Dominik, preprint CERN-EP/87-56,1987 .
Discussion T. Jones: Do you anticipate a problem with the scattering within the stack of glass tubes? What is the coincidence resolving time of your system? A. Del Guerra : Monte Carlo simulation suggests that the scattering within the stack of glass tubes is not much of a problem: less than 10% of the events will be detected in two detectors because of the scattering, and these events will be simply rejected. The coincidence resolving time is 100 ns. The drift time of the electrons in 1 cm of argon-methane (70-30) is in fact -100 ns.
II . GASEOUS DETECTORS
Nuclear Instruments and Methods in Physics Research A273 (1988) 500-505 North-Holland, Amsterdam
THE MACROCHANNEL PLATE: A SUITABLE DETECTOR FOR LOW ENERGY -y-RAYS AND HIGH ENERGY ELECTROMAGNETIC SHOWERS A. DEL GUERRA, M. CONTI, G. GORINI, P. MAIANO and C. RIZZO Dipartimento di Fisica, University di Pisa, Piazza Torricelli 2, I-56100 Pisa, Italy INFN, Sezione di Pisa, Via Vecchia Livornese, I-56010 S. Piero a Grado (PI), Italy
V. PEREZ-MENDEZ
Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720, USA
We have developed a manufacturing process for arrays of high resistance lead glass tubing of various diameters, ranging from 500 gm to several mm, for the detection of -y-rays. We describe the applications of this type of device to positron emission tomography and high energy physics. 1. Introduction In our continuous effort to develop position sensitive detectors for positron emission tomography imaging we have implemented a high density projection chamber, which consists of a standard gas multiwire proportional chamber equipped with a high density, high Z honeycomb converter [1]. The converter is made as an array of high resistance lead glass tubing which acts as combined gamma ray converter and electron drift structure. Both the MWPC and the converter are in the same gas enclosure. A schematic drawing of such a device is presented in fig. 1. The lead glass matrices are treated in a H2 reduction process to form a resistive layer, with a typical uniformity along the tube of - 10%. A voltage difference applied between the ends of the tube gives the necessary electric field for drifting the electrons towards the avalanche region of the MWPC . Depending on the type of glass used, the resistivity can vary by several orders of magnitude. A resistivity in the range 10 8-1012 S2/ß is suitable for this application . The detailed description for the construction of the lead glass matrices can be found elsewhere [2,3]. As a most unusual spin-off of medical imaging, we proposed to use the same principle for a lead glass drift calorimeter for high energy physics [4]. In this latter case the lead glass tubing structure acts as a radiator for the electromagnetic particles; the energy of the particle is proportional to the energy deposited in the gas, which is measured by collecting the ionization electrons at the end of the drift channels . A standard two coordinate readout MWPC can be used to give the projection of the shower on the plane perpendicular to the tube axes, whereas the third coordinate (along the tubes) is given by the drift time . 0168-9002/88/$03 .50 C Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
conductive surface
" -VO
d
-t1V(x )= Vo d (continuous voltage divider)
0
electron detection region (MWPC'
Fig. 1. Schematic drawing of a high density electron drift structure equipped with a conventional MWPC, enclosed in the same gas atmosphere .
For PET imaging it is necessary to have a high surface to volume ratio to increase the probability for the low energy Compton- or photo-electrons (produced by the 511 keV photon interacting inside the converter walls) to reach the gas drift region within the tubes. The drift channel length cannot be too long, otherwise it produces a much too big parallax error. For the
A. Del Guerra et al. / The macrochannel plate Table 1 Physical properties of the type of glass and geometrical configurations of the "macrochannel plate" Type of glass
Hi-D Pacific
a)
71
Density (g/cm3 )
6.2
5.18
1.28
1.66
10 8 -10 9
-lo ll
PET
PET
(cm)
Resistivity after H z treatment Application
Tubing diameter inner/outer (mm) 0.5/0 .6 Drift length (cm) a)
b>
up to 2
b)
'va
Schott RS-520
PbO composition by weight (%) 79
Radiation length
501
electromagnetic calorimeter
0.5/0 .6 5/7 1
up to 40
This glass was originally used at Lawrence Berkeley Laboratory [2]. Among the various ID/OD configurations we have used, the 0.5/0.6 mm is the one with the highest surface to volume ratio.
- Vconu. Fig. 2. Schematic drawing of the HISPET prototype. calorimetry application, instead, the main requirement is to provide a dense and compact structure for the
confinement of a multi-GeV shower in a reasonable space, with a high granularity and a drift length as long
as allowed by the lateral diffusion of the electron in gas.
Fig. 3. The HISPET prototype under test at the Department of Physics of Pisa University together with the front-end electronics and the data taking system . II . GASEOUS DETECTORS
50 2
A. Del Guerra et al. / The macrochannel plate
Hence, different requirements dictate different geometries for the two applications, but the same principle and the same technical construction are involved and the same type of glass has been used. The tubing diameter varies from 500 ltm for PET to 5 mm for the high energy physics application, and the drift length scales approximately by the same ratio. By applying a high enough voltage and using the proper gas, we have shown that it is also possible to have multiplication of the electrons which are drifting within the tubes [5]. This device seems to us to be similar in design, technology and physical properties to a microchannel plate, but much rougher, bigger, thicker, and with larger holes. All in all, we decided to call it macrochannel plate [6]. In table 1 we show the physical properties of the glass used and its geometric characteristics for the two applications . The capillaries were originally drawn to the various sizes by a private firm [7], so were the tubes for the high energy physics application. Later on we decided to draw the capillaries ourselves from the tubes by using a commercial drawing machine [8]. This allows us to vary "ad libitum" the tubing diameter, of course maintaining the same inner/outer diameter ratio. In sections 2 and 3 of this paper we describe the current status of the applications of this type of device in PET and in electromagnetic calorimetry, respectively . 2. Use of the macrochannel plate for PET We have proposed [9] a 3-D high spatial resolution positron emission tomograph (HISPET), based on the use of this macrochannel plate device coupled to a MWPC . HISPET will consist of six modules arranged so as to form the lateral surface of a hexagonal prism with a solid angle coverage of - 27r sr . Each module of HISPET will have two MWPC each, with two 1 cm thick converter planes of lead-glass tubing (0 .5 and 0.6 mm ID and OD, respectively), with a measured efficiency of 6.5% per cm of converter for 511 keV y-rays . The expected performance of HISPET are a volume sensitivity of - 100000 c/s per 0.1 p.Ci/ml for a true to accidental coincidences ratio of 3 : 1. A spatial resolution of - 4 mm FWHM in x, y, and z has been obtained with data simulated by a fully 3-D Monte Carlo program and reconstructed by a 3-D filtered back-projection algorithm [10] . We have tested (111 a first prototype positron camera (fig . 2), which consists of two 50 x 50 cm2 MWPC approximately 80 cm apart, each one equipped with a 2 cm thick lead-glass converter plane. The planes were built in the early stage of our experimentation with the Hi-D glass (80% PbO by weight, density of 6.2 g/cm3 ); the tubes have inner and outer diameters of 1.33 and 1 .59 mm, respectively . Fast delay lines capacitively coupled to the cathode planes are used for measuring both
arbitrary units
LA
FWHM
.
Fig. 4. Spatial resolution histograms for a 22 Na point-like source at the center of the prototype: along the y-axis (top); along the z-axis (bottom) . Each channel corresponds to 8 mm. coordinates. A coincidence within the time resolution window between the anode signals from both MWPC defines the trigger, which is used as the common START of a multiple STOP time to digital converter. The signals from both ends of all four delay lines (2 for each MWPC) act as the eight independent STOPS for the
Fig. 5. Photograph of the calorimeter prototype consisting of three blocks of lead glass tubing for a total of 11 .2 radiation lengths.
503
A . Del Guerra et al. / The macrochannel plate
eight channels of the TDC. The coordinate position is then directly related to the time difference between the two corresponding STOPs. The data from the TDC are directly fed into a FASTBUS memory Lecroy 1892, which acts as a temporary storage, and then into a PDPIl-23 computer (fig. 3) . Very preliminary data [111 have been taken and analyzed with the 3-D reconstruction algorithm we have implemented. The spatial resolu22 tion obtained for a pointlike Na source at the center is presented in fig. 4. Along both the yof the prototype and z-direction the FWHM of the distribution is approximately one channel, which corresponds to 8 mm, and is consistent with the expected value for this prototype . The additional spikes in the histograms are mainly due to the poor statistics (less than one thousand events). 3. Electromagnetic application We have designed a drift collection calorimeter [12], which makes use of the macrochannel plate, with lead
glass tubes of 40 cm drift length and a total longitudinal dimension of --- 18 .6 radiation lengths (Xo) for a total physical length of 70 cm . The calorimeter is made by assembling 7 modules of 10 x 14 X 40 cm3; it has a very good sampling (Xo/6) ; furthermore, the tubing structure limits the track length fluctuations on the released energy. A detailed Monte Carlo study has been performed [13] to evaluate the shower containment and the energy resolution. An energy resolution a/E of 17%/FE has been obtained in the simulation for full containment of the shower at 1 atm. We have already reported preliminary results on smaller modules of 5 X 5 cm2 by up to 14 cm long [14] . Here we present the energy resolution measurements we have taken with a full drift length prototype (fig . 5), made of three blocks (10 X 14 cm2 X 40 cm drift length, each). The prototype covers 11 .2X o in the longitudinal direction (42 cm physical length). The data were obtained in late 1986 at the CERN-PS. Three scintillators defined a beam spot of 1 cm; their Time spectrum
Energy spectrum
output to MCA
gate signal
Fig. 6. Schematic drawing of the electronics setup for the test of the calorimeter prototype. Typical time and energy spectra are also shown. II . GASEOUS DETECTORS
504
A . Del Guerra et al. / The macrochannel plate 200
1
2
3
4
5
6
ENERGY(GeV) Fig. 7. Released energy, Edep, versus incident particle energy . A linear fit is overdrawn.
coincidence formed the trigger. The calorimeter could be moved up and down to shift the beam spot along its axis . We worked with a negative particle beam, and used a threshold CO, Cherenkov counter to select electrons. For each run we had both spectra for the released energy and the time distribution (fig . 6) . We will refer to a gas filling of Ar-CH4 (70-30) at 2 atm. Further measurements at atmospheric pressure are under elaboration . Particle momentum from 2 to 5 GeV/c was selected . We fitted the energy spectra as Gaussian distribu_o fE
Edep fGeV] 112 0.3
0.2
0.0
Fig. 8. Energy resolution (a/Edep)x El /2 as a function of the incident energy . A linear fit is overdrawn, which gives a constant value of 0.18+0.02.
tions, and no high energy tail was observed, probably due to the track length limitations of the tubes ; the good linear relationship between incident particle energy (E) and the energy released (Edep) within the calorimeter is shown in fig. 7. The energy resolution (a/Edep) is in agreement with the sampling calorimeter general law (- 1/F). A resolution a/Edep = (18 ± 2)%/ rE was obtained (fig . 8), which compares fairly well with a Monte Carlo simulation value of about 17/> E % at 2 atm for this prototype . We shifted the beam spot along the axes of the tubes, and recorded the time shift of the signal and the pulse height attenuation in the energy spectra; given the electron drift velocity in the gas, we are then able to obtain a value for the attenuation length within the tubes : data analysis will be completed in the next months. We are now assembling the 18 .6X, calorimeter, which will contain 97% of a 10 GeV e.m. shower [13] . A pad cathode will be used for the read-out of the charge from one end of the tubing structure; each pad will have its preamplifier - shaper - Flash ADC - 2kbyte Memory. We expect to test the calorimeter in spring 1988 at CERN . 3. Conclusions We have shown that the macrochannel plate is a suitable detector for PET imaging. Its technology is simple, reliable and not expensive. The 3-D tomograph we are building using this device will have excellent performance, in particular a spatial resolution of 4 mm in all three directions ; the experimental results we have obtained so far are very encouraging. On a larger scale such a device can be used as a drift collection calorimeter, expecially for low count rate experiments . In the search for proton decay large detection volumes are needed with a high granularity and a fair energy resolution . The proposed macrochannel plate seems to have the necessary energy and spatial resolution . The cost of the bulk glass material is negligible and the construction cost is also very reasonable expecially for large quantities. However, to make full use of the granularity of the device, the number of electronics readout channels may become excessively high . An optical readout system, as recently proposed by Charpak [15], would be ideal for this application .
References [11 M. Conti, A. Del Guerra, R. Habel, T. Mulera, V. PerezMendez and G. Schwartz, Nucl. Instr. and Meth . A255 (1987) 207.
A. Del Guerra et al. / The macrochannel plate [2] G.K. Lum, M.I . Green, V. Perez-Mendez and K.C . Tam IEEE Trans. Nucl . Sci. NS-27 (1980) 157. G. Schwartz, M. Cinti, M. Conti, A. Del Guerra, M. Di Fino, R. Habel, V. Perez-Mendez and L. Righini, INFN Sezione di Pisa Report, INFN/TC-85/6 (1985). [4] V. Perez-Mendez, A. Del Guerra, T. Mulera, H. Hirayama and W.R . Nelson, Nucl . Instr. and Meth. 217 (1983) 255. I. Fujieda, T.A . Mulera, V. Perez-Mendez and A. Del Guerra, IEEE Trans. Nucl . Sci. NS-33 (1986) 587. [6] In fact, Hamamatsu has provided us with a converter sample (dimensions of 5 x 5 cmz and thickness of 1 cm), made with the technology of the microchannel plates . The
sample is now under test. [7] Garner Glass Co ., Claremont, California 97111, USA. [8] Shimadzu Glass Drawing Machine, GDM 1B . [9] A. Del Guerra, G.K. Lum, V. Perez-Mendez and G. Schwartz, in : Positron Annihilation, eds. P.C . Jain, R.M . Singru and K.P . Gopinathan (World Scientific, Singapore, 1985) p. 810. [10] C. Rizzo, M. Conti and A. Del Guerra, Physica Medica 1 (1987) 19 . [111 A. Del Guerra, A. Bandettini, G. De Pascalis, M. Conti, P. Maiano, C. Rizzo and V. Perez-Mendez, Proc. Int. Symp . on Wire Chambers in Medical Imaging, Bruxelles, 28-30 June 1987, Nucl. Instr. and Meth . A269 (1988) 425. [12] T. Mulera, V. Perez-Mendez, H. Hirayama, W.R. Nelson,
50 5
R. Bellazzini, A. Del Guerra and M.M. Massai, IEEE Trans. Nucl . Sci. NS-31 (1984) 64. [13] H. Hirayama, W.R. Nelson, A. Del Guerra, T. Mulera and V. Perez-Mendez, Nucl . Instr. and Meth . 220 (1984) 327. [14] A. Del Guerra, M. Conti, G. Gorini, I. Fujieda, T.A . Mulera and V. Perez-Mendez, Nucl . Instr. and Meth . A257 (1987) 609. [15] G. Charpak, J.-P. Fabre, F. Sauli, M. Suzuki and W. Dominik, preprint CERN-EP/87-56,1987 .
Discussion T. Jones: Do you anticipate a problem with the scattering within the stack of glass tubes? What is the coincidence resolving time of your system? A. Del Guerra : Monte Carlo simulation suggests that the scattering within the stack of glass tubes is not much of a problem: less than 10% of the events will be detected in two detectors because of the scattering, and these events will be simply rejected. The coincidence resolving time is 100 ns. The drift time of the electrons in 1 cm of argon-methane (70-30) is in fact -100 ns.
II . GASEOUS DETECTORS