- Email: [email protected]
CVD diamond sensors for charged particle detection
Diamond and Related Materials 10 Ž2001. 1778᎐1782
CVD diamond sensors for charged particle detection RD42 collaboration
M. Krammer a,U , W. Adama , E. Berdermann b, P. Bergonzo c , G. Bertuccio d, F. Bogani e, E. Borchi f , A. Brambilla c , M. Bruzzi f , C. Colledani g , J. Conway h , P. D’Angelo i , W. Dabrowski j, P. Delpierre h , A. Deneuville l , W. Dulinski g , B. van Eijk m , A. Fallou k , F. Fizzotti n , F. Foulonc , M. Friedl a , K.K. Gan o, E. Gheeraert l , G. Hallewell k , S. Han o, F. Hartjes m , J. Hrubec a , D. Husson g , H. Kagan o, D. Kaniao, J. Kaplon p, R. Kass o, T. Koeth h , A. Logiudice n , R. Lu n , L. MacLynne h , C. Manfredotti n , D. Meier p, M. Mishina r, L. Moroni i , A. Oh q , L.S. Pan o, M. Pernickaa , A. Peitz h , L. Perera h , S. Pirollo f , M. Procario s , J.L. Riester g , S. Roe p, L. Rousseau c , A. Rudge p, J. Russ s , S. Salai , M. Sampietro d, S. Schnetzer h , S. Sciortino f , H. Stelzer b, R. Stone h , B. Suter s , R.J. Tapper t , R. Tesarek h , W. Trischuk u , D. Tromsonc , E. Vittone n , A.M. Walsh h , R. Wedenig a , P. Weilhammer p, M. Wetstein h , C. White v, W. Zeuner q , M. Zoeller o a
¨ Institut fur Akademie der Wissenschaften, Vienna, Austria ¨ Hochenergiephysik der Osterreichischen, b GSI, Darmstadt, Germany c LETI (CEA-Technologies A¨ ancees) DEINr SPE᎐CEA Saclay, Gif-Sur-Y¨ ette, France d Polytechnico Milano, Milan, Italy e LENS, Florence, Italy f Uni¨ ersity of Florence, Florence, Italy g LEPSI, IN2P3r CNRS-ULP, Strasbourg, France h Rutgers Uni¨ ersity, Piscataway, NJ, USA i INFN, Milan, Italy j Faculty of Physics and Nuclear Techniques, UMM, Cracow, Poland k CPPM, Marseille, France l LEPES, Grenoble, France m NIKHEF, Amsterdam, Netherlands n Uni¨ ersity of Torino, Turin, Italy o The Ohio State Uni¨ ersity, Columbus, OH, USA p CERN, Gene¨ a, Switzerland q II Institut. fur ¨ Experimental Physik, Hamburg, Germany r FNAL, Bata¨ ia, Illinois USA s Carnegie-Mellon Uni¨ ersity, Pittsburgh, PA, USA t Bristol Uni¨ ersity, Bristol, UK u Uni¨ ersity of Toronto, Toronto, ON, Canada v Illinois Institute of Technology, Chicago, IL, USA
Abstract CVD diamond material was used to build position-sensitive detectors for single-charged particles to be employed in high-intensity physics experiments. To obtain position information, metal contacts shaped as strips or pixels are applied to the
U
Corresponding author. E-mail address: [email protected] ŽM. Krammer.. 0925-9635r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 4 4 6 - 0
M. Krammer et al. r Diamond and Related Materials 10 (2001) 1778᎐1782
1779
detector surface for one- or two-dimensional coordinate measurement. Strip detectors 2 = 4 cm2 in size with a strip distance of 50 m were tested. Pixel detectors of various pixel sizes were bump bonded to electronics chips and investigated. A key issue for the use of these sensors in high intensity experiments is the radiation hardness. Several irradiation experiments were carried out with pions, protons and neutrons exceeding a fluence of 10 15 particlesrcm 2. The paper presents an overview of the results obtained with strip and pixel detectors in high-energy test beams and summarises the irradiation studies. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Diamond sensor; Particle detector; Pixel detector; Radiation damage
1. Introduction Experiments in the field of elementary particle physics study high-energy collisions of particles, e.g. electrons or protons. The reaction products are also particles, which have to be detected, electronically recorded and analysed for their mass, momentum, energy, etc. A broad range of different detector materials and techniques have been developed and are applied according to their specific properties. Development in elementary particle physics is involved with the investigation of increasingly difficult to detect particles. To prove their existence and to investigate rare events, particle accelerators with very high intensities are needed. One of the machines presently under construction is the large hadron collider ŽLHC. at CERN. In the LHC, protons will collide with protons at a centre-of-mass energy of 2 = 7 TeV. Every 25 ns a bucket of approximately 1.1= 10 11 protons will collide head-on with another bucket. An intense flux of secondary particles will emerge from the collision region. Therefore, the detectors operating in this region will have to withstand very high radiation levels. Over the expected lifetime of the experiment Žtypically 10 years of operation. a fluence of 1.8= 10 15 charged hadrons and 3 = 10 14 neutronsrcm 2 for detector elements located at a radius of approximately 4.3 cm can be estimated. It is the goal of the RD42 collaboration ŽCERN, Geneva, Switzerland. at CERN to develop detectors based on CVD diamond capable of detecting and measuring the position of traversing charged particles. These CVD diamond detectors are expected to survive in such a harsh radiation environment.
2. Properties of CVD diamond sensors The CVD diamond used by RD42 is industrially produced ŽDeBeers, Ascot, UK.. The wafers, with a diameter of 10 cm, are poly-crystalline with a thickness in the range of 400 m to several mm. The material is lapped and polished to the final thickness and cut to the appropriate size. To achieve good ohmic contacts, electrodes are applied on both surfaces of the detector
and a voltage is connected to the electrodes to create an electric field. The metallization consists of chromium covered by a thin layer of gold, or titanium covered by tungsten. Charged particles traversing the detector lose energy and produce electron᎐hole pairs. Along their tracks, high-energy particles create, on average, approximately 11 000 electron᎐hole pairs in a 300-m-thick diamond. These electrons and holes separate due to the applied electric field and drift towards the electrodes. The drift of these charge carriers induces a signal measured by the connected low-noise electronics. The signal depends on the electron and hole mobilities, the electric field and the lifetimes of the charge carriers. The product of lifetime, electric field and mobility defines the local charge-collection distance, the distance electrons and holes move before being trapped. Due to the inhomogeneity of the material, the charge-collection distance varies through the bulk of the material, increasing from the nucleation to the growth side. Over the past number of years, great progress was made by the industry in improving the charge-collection distance. The average charge-collection distances measured for the CVD diamond samples described in this paper range from 150 to 200 m, corresponding to a mean charge of 5400᎐7200 e collected for the passage of high-energy particles.
3. Strip detectors To build strip detectors, we deposit metal strips of various width and distance on the growth side and a uniform electrode on the nucleation side of the CVD diamond. Every strip electrode is connected by wire bonding to the input of an electronic channel realised in a VLSI chip. A schematic diagram of a strip detector is shown in Fig. 1. The charge-sensitive amplifier for each channel integrates the current induced by the moving charges. The position of the traversing particle is deduced from the position of the channel measuring a signal. Several detectors of up to 2 = 4 cm2 in surface area and a pattern of strips with a width of 10᎐40 m and a
1780
M. Krammer et al. r Diamond and Related Materials 10 (2001) 1778᎐1782
Fig. 1. Schematic drawing of a diamond strip detector, including the electronics readout.
distance of 50 m have been built. These strip detectors were connected to two types of electronics: to the VLSI electronic chip VA2 w1x with an integration time of approximately 2 s, which is considered as too slow for our application; and to the fast and radiation-hard chip SCT128HC w2x with an integration time of 25 ns, which is compatible with the time structure of the LHC. To study their performance, the strip detectors have been tested in a high-energy pion beam Ž100 GeVrc. at CERN. The strip detectors were mounted in a precise beam telescope consisting of four x᎐y planes of silicon detectors, two in front of and two behind the diamond strip sensor. The accuracy of the position measurement of the beam telescope in the plane of the test detector was approximately 2 m. For a general investigation of the properties of CVD diamond sensors and the development of strip detectors, VA2 electronics were used. In beam tests of strip detectors equipped with VA2 chips, a signalrnoise ratio of up to 50:1 for the most probable value Ž72:1 for the mean value. and a position resolution of approximately 12 m have been achieved w3x. Using electronics with fast shaping time leads to much higher electronic noise, reducing the signalrnoise ratio of the detector. A typical result obtained with a strip detector equipped with SCT128HC chips is shown in Fig. 2. The figure shows the pulse height distribution of a detector with a thickness of 432 m, strips of 25 m in width and 50 m in distance. The applied electric field was 1 Vrm. The signal distribution is separated from zero, and using the calibration results we calculate a most probable signalrnoise ratio of 7.2:1 Ž10:1 for the mean value.. The difference of the measured track position and the telescope prediction is shown in Fig. 3 Žresidual distribution.. A Gaussian function with a standard deviation is fitted to this residual distribution, yielding a spatial resolution of 16.5 m. This result satisfies the requirements of LHC
Fig. 2. Pulse height distribution of a diamond strip detector connected to SCT128HC electronics in a high-energy test beam.
experiments, where a position resolution of approximately 20 m w4x is necessary.
4. Pixel detectors In a pixel detector, the electrodes on the growth side are segmented into small elements called pixels. An advantage of such a device is that unambiguous twodimensional position information is retrieved if each pixel is read individually by an electronic channel. An obvious drawback is the number of electronic channels and electrical connections necessary. The solution described here is the use of electronic chips which have the electronics channels geometrically arranged in a pattern identical to the pixel detector. These chips are placed face-to-face to the detector pixel side and each detector pixel is electrically connected to one electronic channel using a bump-bonding technique. We have constructed pixel detectors with pixels of 125 = 125 and 50 = 433.4 m2 connected to radiationhard electronics operating at the LHC frequency of 40
Fig. 3. The residual distribution of a diamond strip detector with a strip distance of 50 m ŽSCT128HC electronics ..
M. Krammer et al. r Diamond and Related Materials 10 (2001) 1778᎐1782
1781
Fig. 5. Residual distribution in both coordinates for a pixel detector Ž125 = 125 m2 . in a high-energy test beam. The top plot shows the x- and the bottom plot the y-coordinate. Fig. 4. CVD diamond pixel detector prepared for bump bonding. The picture shows the indium balls on top of the electrodes.
MHz. Fig. 4 shows a diamond pixel detector with indium balls on top of the electrodes prepared for bump bonding. Test beam measurements performed with such detectors have shown that a bump-bonding efficiency of close to 100% can be achieved w5x. Fig. 5 shows the residual distribution of a pixel detector Ž125 = 125 m2 . in a 200-GeVrc pion beam. The position resolution Žrms. is calculated to be approximately 36 m in both coordinates. This resolution corresponds precisely to the theoretically predicted resolution for a detector with a 125-m pixel size and non-analogue data readout.
effect of pions on CVD diamond, a series of irradiations was performed at the Paul Scherrer Institut ŽSwitzerland. using pions with a momentum of 300 MeVrc. A maximum fluence of 1 = 10 15 pionsrcm2 was reached. Fig. 6 compares the pulse height distributions of a detector before and after an irradiation of 5.4= 10 14 pionsrcm2 and after 1 = 10 15 pionsrcm2 . The shape of the distributions after irradiation is narrower than before and there are fewer entries with high values. The pedestal entries are shown intentionally to exclude a shift between the different distributions. Fig. 7 shows the development of the most
5. Irradiation studies To test the radiation resistance of CVD diamond, many detector samples were exposed to different types of particles. The measurements with photons and electrons, up to 10 and 100 Mrad, respectively, are reported in w6x and do not show a significant effect on the detector signals. More important for our application are the exposures to pions, protons and neutrons, as these particles will dominate the spectrum at LHC. The most abundant particle type in the region close to the LHC collision point is the pion. To study the
Fig. 6. Pulse height distribution of a detector before irradiation Žsolid line. and after exposure to 5.4= 10 14 Ždotted line. and to 1 = 10 15 pionsrcm2 Ždashed line..
1782
M. Krammer et al. r Diamond and Related Materials 10 (2001) 1778᎐1782
Fig. 7. Most probable detector signal as a function of the pion fluence normalised to the value before irradiation.
probable signal as a function of the pion fluence. The most probable signal is reduced by approximately 40% at the highest fluence. A similar narrowing of the distribution and a decrease in signal were observed with protons w7x Žfluence up to 5 = 10 15 protonsrcm2 . and neutrons w8x Žfluence up to 1 = 10 15 neutronsrcm 2 .. The highest fluence levels reached in these irradiation studies exceed the expected fluences in the LHC experiments. To test the performance of an irradiated CVD diamond as a position detector, a strip detector was exposed to 1 = 10 15 protons and measured in a test beam before and after the exposure. The residual distributions before and after the irradiation are shown in Fig. 8. The measured spatial resolution improves significantly, from 11.6 m before irradiation to approximately 9.4 m after irradiation. This is possibly connected to a more homogeneous diamond material after irradiation, as suggested by the narrower pulse-height distribution.
6. Conclusions Over the past years, the RD42 collaboration has made excellent progress in the development of CVD diamond detectors. Strip and pixel detectors have been built and equipped with electronics compatible with the time structure and radiation environment at LHC. Test beam measurements have shown that the signal-tonoise ratio and the position resolution achieved for
Fig. 8. Distribution of residuals before Žsolid line. and after irradiation with 1 = 10 15 protonsrcm2 Ždashed line.. The detector has a strip distance of 50 m and was connected to VA2 electronics.
these detectors fulfil the requirements for tracking detectors in LHC experiments. Extensive irradiation studies have proven the survivability of diamond detectors over the expected running time of LHC. At present, the application of diamond detectors in LHC and other experiments, e.g. the CDF experiment at Fermilab ŽUSA., is under investigation. References w1x Integrated Detectors & Electronics ŽIDE AS., The VA Circuits Catalogue 95r96, Hovik, Norway, 1995. w2x F. Anghinolfi, W. Dabrowski, E. Delagnes et al., SCTA ᎏ a radiation hard BiCMOS analogue readout ASIC for the ATLAS semiconductor tracker, IEEE Trans. Nucl. Sci. 44 Ž1997. 298᎐302. w3x W. Adam, C. Bauer, E. Berdermann et al., Recent results with CVD diamond trackers, Nucl. Phys. B 78 ŽProc. Suppl.. Ž1999. 329᎐334. w4x CMS, The Tracker Project, Technical Design Report CERNrLHCC 98-6 Ž1998.. w5x R. Wedenig, W. Adam, C. Bauer et al., CVD diamond pixel detectors for LHC experiments, Nucl. Phys. B 78 ŽProc. Suppl.. Ž1999. 497᎐504. w6x C. Bauer, I. Baumann, C. Colledani et al., Radiation hardness studies of CVD diamond detectors, Nucl. Instrum. Methods A 367 Ž1995. 207᎐211. w7x W. Adam, E. Berdermann, P. Bergonzo et al., Pulse height distribution and radiation tolerance of CVD diamond detectors, Nucl. Instrum. Methods A 447 Ž2000. 244᎐250. w8x S. Schnetzer, W. Adam, C. Bauer et al., Tracking with CVD diamond radiation sensors at high luminosity colliders, IEEE Trans. Nucl. Sci. 46 Ž3. Ž1999..
CVD diamond sensors for charged particle detection RD42 collaboration
M. Krammer a,U , W. Adama , E. Berdermann b, P. Bergonzo c , G. Bertuccio d, F. Bogani e, E. Borchi f , A. Brambilla c , M. Bruzzi f , C. Colledani g , J. Conway h , P. D’Angelo i , W. Dabrowski j, P. Delpierre h , A. Deneuville l , W. Dulinski g , B. van Eijk m , A. Fallou k , F. Fizzotti n , F. Foulonc , M. Friedl a , K.K. Gan o, E. Gheeraert l , G. Hallewell k , S. Han o, F. Hartjes m , J. Hrubec a , D. Husson g , H. Kagan o, D. Kaniao, J. Kaplon p, R. Kass o, T. Koeth h , A. Logiudice n , R. Lu n , L. MacLynne h , C. Manfredotti n , D. Meier p, M. Mishina r, L. Moroni i , A. Oh q , L.S. Pan o, M. Pernickaa , A. Peitz h , L. Perera h , S. Pirollo f , M. Procario s , J.L. Riester g , S. Roe p, L. Rousseau c , A. Rudge p, J. Russ s , S. Salai , M. Sampietro d, S. Schnetzer h , S. Sciortino f , H. Stelzer b, R. Stone h , B. Suter s , R.J. Tapper t , R. Tesarek h , W. Trischuk u , D. Tromsonc , E. Vittone n , A.M. Walsh h , R. Wedenig a , P. Weilhammer p, M. Wetstein h , C. White v, W. Zeuner q , M. Zoeller o a
¨ Institut fur Akademie der Wissenschaften, Vienna, Austria ¨ Hochenergiephysik der Osterreichischen, b GSI, Darmstadt, Germany c LETI (CEA-Technologies A¨ ancees) DEINr SPE᎐CEA Saclay, Gif-Sur-Y¨ ette, France d Polytechnico Milano, Milan, Italy e LENS, Florence, Italy f Uni¨ ersity of Florence, Florence, Italy g LEPSI, IN2P3r CNRS-ULP, Strasbourg, France h Rutgers Uni¨ ersity, Piscataway, NJ, USA i INFN, Milan, Italy j Faculty of Physics and Nuclear Techniques, UMM, Cracow, Poland k CPPM, Marseille, France l LEPES, Grenoble, France m NIKHEF, Amsterdam, Netherlands n Uni¨ ersity of Torino, Turin, Italy o The Ohio State Uni¨ ersity, Columbus, OH, USA p CERN, Gene¨ a, Switzerland q II Institut. fur ¨ Experimental Physik, Hamburg, Germany r FNAL, Bata¨ ia, Illinois USA s Carnegie-Mellon Uni¨ ersity, Pittsburgh, PA, USA t Bristol Uni¨ ersity, Bristol, UK u Uni¨ ersity of Toronto, Toronto, ON, Canada v Illinois Institute of Technology, Chicago, IL, USA
Abstract CVD diamond material was used to build position-sensitive detectors for single-charged particles to be employed in high-intensity physics experiments. To obtain position information, metal contacts shaped as strips or pixels are applied to the
U
Corresponding author. E-mail address: [email protected] ŽM. Krammer.. 0925-9635r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 4 4 6 - 0
M. Krammer et al. r Diamond and Related Materials 10 (2001) 1778᎐1782
1779
detector surface for one- or two-dimensional coordinate measurement. Strip detectors 2 = 4 cm2 in size with a strip distance of 50 m were tested. Pixel detectors of various pixel sizes were bump bonded to electronics chips and investigated. A key issue for the use of these sensors in high intensity experiments is the radiation hardness. Several irradiation experiments were carried out with pions, protons and neutrons exceeding a fluence of 10 15 particlesrcm 2. The paper presents an overview of the results obtained with strip and pixel detectors in high-energy test beams and summarises the irradiation studies. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Diamond sensor; Particle detector; Pixel detector; Radiation damage
1. Introduction Experiments in the field of elementary particle physics study high-energy collisions of particles, e.g. electrons or protons. The reaction products are also particles, which have to be detected, electronically recorded and analysed for their mass, momentum, energy, etc. A broad range of different detector materials and techniques have been developed and are applied according to their specific properties. Development in elementary particle physics is involved with the investigation of increasingly difficult to detect particles. To prove their existence and to investigate rare events, particle accelerators with very high intensities are needed. One of the machines presently under construction is the large hadron collider ŽLHC. at CERN. In the LHC, protons will collide with protons at a centre-of-mass energy of 2 = 7 TeV. Every 25 ns a bucket of approximately 1.1= 10 11 protons will collide head-on with another bucket. An intense flux of secondary particles will emerge from the collision region. Therefore, the detectors operating in this region will have to withstand very high radiation levels. Over the expected lifetime of the experiment Žtypically 10 years of operation. a fluence of 1.8= 10 15 charged hadrons and 3 = 10 14 neutronsrcm 2 for detector elements located at a radius of approximately 4.3 cm can be estimated. It is the goal of the RD42 collaboration ŽCERN, Geneva, Switzerland. at CERN to develop detectors based on CVD diamond capable of detecting and measuring the position of traversing charged particles. These CVD diamond detectors are expected to survive in such a harsh radiation environment.
2. Properties of CVD diamond sensors The CVD diamond used by RD42 is industrially produced ŽDeBeers, Ascot, UK.. The wafers, with a diameter of 10 cm, are poly-crystalline with a thickness in the range of 400 m to several mm. The material is lapped and polished to the final thickness and cut to the appropriate size. To achieve good ohmic contacts, electrodes are applied on both surfaces of the detector
and a voltage is connected to the electrodes to create an electric field. The metallization consists of chromium covered by a thin layer of gold, or titanium covered by tungsten. Charged particles traversing the detector lose energy and produce electron᎐hole pairs. Along their tracks, high-energy particles create, on average, approximately 11 000 electron᎐hole pairs in a 300-m-thick diamond. These electrons and holes separate due to the applied electric field and drift towards the electrodes. The drift of these charge carriers induces a signal measured by the connected low-noise electronics. The signal depends on the electron and hole mobilities, the electric field and the lifetimes of the charge carriers. The product of lifetime, electric field and mobility defines the local charge-collection distance, the distance electrons and holes move before being trapped. Due to the inhomogeneity of the material, the charge-collection distance varies through the bulk of the material, increasing from the nucleation to the growth side. Over the past number of years, great progress was made by the industry in improving the charge-collection distance. The average charge-collection distances measured for the CVD diamond samples described in this paper range from 150 to 200 m, corresponding to a mean charge of 5400᎐7200 e collected for the passage of high-energy particles.
3. Strip detectors To build strip detectors, we deposit metal strips of various width and distance on the growth side and a uniform electrode on the nucleation side of the CVD diamond. Every strip electrode is connected by wire bonding to the input of an electronic channel realised in a VLSI chip. A schematic diagram of a strip detector is shown in Fig. 1. The charge-sensitive amplifier for each channel integrates the current induced by the moving charges. The position of the traversing particle is deduced from the position of the channel measuring a signal. Several detectors of up to 2 = 4 cm2 in surface area and a pattern of strips with a width of 10᎐40 m and a
1780
M. Krammer et al. r Diamond and Related Materials 10 (2001) 1778᎐1782
Fig. 1. Schematic drawing of a diamond strip detector, including the electronics readout.
distance of 50 m have been built. These strip detectors were connected to two types of electronics: to the VLSI electronic chip VA2 w1x with an integration time of approximately 2 s, which is considered as too slow for our application; and to the fast and radiation-hard chip SCT128HC w2x with an integration time of 25 ns, which is compatible with the time structure of the LHC. To study their performance, the strip detectors have been tested in a high-energy pion beam Ž100 GeVrc. at CERN. The strip detectors were mounted in a precise beam telescope consisting of four x᎐y planes of silicon detectors, two in front of and two behind the diamond strip sensor. The accuracy of the position measurement of the beam telescope in the plane of the test detector was approximately 2 m. For a general investigation of the properties of CVD diamond sensors and the development of strip detectors, VA2 electronics were used. In beam tests of strip detectors equipped with VA2 chips, a signalrnoise ratio of up to 50:1 for the most probable value Ž72:1 for the mean value. and a position resolution of approximately 12 m have been achieved w3x. Using electronics with fast shaping time leads to much higher electronic noise, reducing the signalrnoise ratio of the detector. A typical result obtained with a strip detector equipped with SCT128HC chips is shown in Fig. 2. The figure shows the pulse height distribution of a detector with a thickness of 432 m, strips of 25 m in width and 50 m in distance. The applied electric field was 1 Vrm. The signal distribution is separated from zero, and using the calibration results we calculate a most probable signalrnoise ratio of 7.2:1 Ž10:1 for the mean value.. The difference of the measured track position and the telescope prediction is shown in Fig. 3 Žresidual distribution.. A Gaussian function with a standard deviation is fitted to this residual distribution, yielding a spatial resolution of 16.5 m. This result satisfies the requirements of LHC
Fig. 2. Pulse height distribution of a diamond strip detector connected to SCT128HC electronics in a high-energy test beam.
experiments, where a position resolution of approximately 20 m w4x is necessary.
4. Pixel detectors In a pixel detector, the electrodes on the growth side are segmented into small elements called pixels. An advantage of such a device is that unambiguous twodimensional position information is retrieved if each pixel is read individually by an electronic channel. An obvious drawback is the number of electronic channels and electrical connections necessary. The solution described here is the use of electronic chips which have the electronics channels geometrically arranged in a pattern identical to the pixel detector. These chips are placed face-to-face to the detector pixel side and each detector pixel is electrically connected to one electronic channel using a bump-bonding technique. We have constructed pixel detectors with pixels of 125 = 125 and 50 = 433.4 m2 connected to radiationhard electronics operating at the LHC frequency of 40
Fig. 3. The residual distribution of a diamond strip detector with a strip distance of 50 m ŽSCT128HC electronics ..
M. Krammer et al. r Diamond and Related Materials 10 (2001) 1778᎐1782
1781
Fig. 5. Residual distribution in both coordinates for a pixel detector Ž125 = 125 m2 . in a high-energy test beam. The top plot shows the x- and the bottom plot the y-coordinate. Fig. 4. CVD diamond pixel detector prepared for bump bonding. The picture shows the indium balls on top of the electrodes.
MHz. Fig. 4 shows a diamond pixel detector with indium balls on top of the electrodes prepared for bump bonding. Test beam measurements performed with such detectors have shown that a bump-bonding efficiency of close to 100% can be achieved w5x. Fig. 5 shows the residual distribution of a pixel detector Ž125 = 125 m2 . in a 200-GeVrc pion beam. The position resolution Žrms. is calculated to be approximately 36 m in both coordinates. This resolution corresponds precisely to the theoretically predicted resolution for a detector with a 125-m pixel size and non-analogue data readout.
effect of pions on CVD diamond, a series of irradiations was performed at the Paul Scherrer Institut ŽSwitzerland. using pions with a momentum of 300 MeVrc. A maximum fluence of 1 = 10 15 pionsrcm2 was reached. Fig. 6 compares the pulse height distributions of a detector before and after an irradiation of 5.4= 10 14 pionsrcm2 and after 1 = 10 15 pionsrcm2 . The shape of the distributions after irradiation is narrower than before and there are fewer entries with high values. The pedestal entries are shown intentionally to exclude a shift between the different distributions. Fig. 7 shows the development of the most
5. Irradiation studies To test the radiation resistance of CVD diamond, many detector samples were exposed to different types of particles. The measurements with photons and electrons, up to 10 and 100 Mrad, respectively, are reported in w6x and do not show a significant effect on the detector signals. More important for our application are the exposures to pions, protons and neutrons, as these particles will dominate the spectrum at LHC. The most abundant particle type in the region close to the LHC collision point is the pion. To study the
Fig. 6. Pulse height distribution of a detector before irradiation Žsolid line. and after exposure to 5.4= 10 14 Ždotted line. and to 1 = 10 15 pionsrcm2 Ždashed line..
1782
M. Krammer et al. r Diamond and Related Materials 10 (2001) 1778᎐1782
Fig. 7. Most probable detector signal as a function of the pion fluence normalised to the value before irradiation.
probable signal as a function of the pion fluence. The most probable signal is reduced by approximately 40% at the highest fluence. A similar narrowing of the distribution and a decrease in signal were observed with protons w7x Žfluence up to 5 = 10 15 protonsrcm2 . and neutrons w8x Žfluence up to 1 = 10 15 neutronsrcm 2 .. The highest fluence levels reached in these irradiation studies exceed the expected fluences in the LHC experiments. To test the performance of an irradiated CVD diamond as a position detector, a strip detector was exposed to 1 = 10 15 protons and measured in a test beam before and after the exposure. The residual distributions before and after the irradiation are shown in Fig. 8. The measured spatial resolution improves significantly, from 11.6 m before irradiation to approximately 9.4 m after irradiation. This is possibly connected to a more homogeneous diamond material after irradiation, as suggested by the narrower pulse-height distribution.
6. Conclusions Over the past years, the RD42 collaboration has made excellent progress in the development of CVD diamond detectors. Strip and pixel detectors have been built and equipped with electronics compatible with the time structure and radiation environment at LHC. Test beam measurements have shown that the signal-tonoise ratio and the position resolution achieved for
Fig. 8. Distribution of residuals before Žsolid line. and after irradiation with 1 = 10 15 protonsrcm2 Ždashed line.. The detector has a strip distance of 50 m and was connected to VA2 electronics.
these detectors fulfil the requirements for tracking detectors in LHC experiments. Extensive irradiation studies have proven the survivability of diamond detectors over the expected running time of LHC. At present, the application of diamond detectors in LHC and other experiments, e.g. the CDF experiment at Fermilab ŽUSA., is under investigation. References w1x Integrated Detectors & Electronics ŽIDE AS., The VA Circuits Catalogue 95r96, Hovik, Norway, 1995. w2x F. Anghinolfi, W. Dabrowski, E. Delagnes et al., SCTA ᎏ a radiation hard BiCMOS analogue readout ASIC for the ATLAS semiconductor tracker, IEEE Trans. Nucl. Sci. 44 Ž1997. 298᎐302. w3x W. Adam, C. Bauer, E. Berdermann et al., Recent results with CVD diamond trackers, Nucl. Phys. B 78 ŽProc. Suppl.. Ž1999. 329᎐334. w4x CMS, The Tracker Project, Technical Design Report CERNrLHCC 98-6 Ž1998.. w5x R. Wedenig, W. Adam, C. Bauer et al., CVD diamond pixel detectors for LHC experiments, Nucl. Phys. B 78 ŽProc. Suppl.. Ž1999. 497᎐504. w6x C. Bauer, I. Baumann, C. Colledani et al., Radiation hardness studies of CVD diamond detectors, Nucl. Instrum. Methods A 367 Ž1995. 207᎐211. w7x W. Adam, E. Berdermann, P. Bergonzo et al., Pulse height distribution and radiation tolerance of CVD diamond detectors, Nucl. Instrum. Methods A 447 Ž2000. 244᎐250. w8x S. Schnetzer, W. Adam, C. Bauer et al., Tracking with CVD diamond radiation sensors at high luminosity colliders, IEEE Trans. Nucl. Sci. 46 Ž3. Ž1999..