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Performance of ultra-thin silicon detectors in a 5MeV antiproton beam
Nuclear Instruments and Methods in Physics Research A 478 (2002) 316–320
Performance of ultra-thin silicon detectors in a 5 MeV antiproton beam P. Riedlera,*, J. Rocheta,b, A. Rudgeb, M. Doserb, R. Landuab a
University of Zurich, CH-8057 Zurich, Switzerland . . b CERN, CH-1211 Geneva 23, Switzerland
Abstract The performance of ultra-thin silicon detectors (6; 55 and 67 mm) was tested in order to provide beam position information, intensity monitoring and a fast trigger signal for the ATHENA experiment. The aim of this experiment is to produce cold antihydrogen using a low energetic (5 MeV) bunched (E500 ns) antiproton beam from the CERN Antiproton Decelerator (AD). The detector has to be operated in vacuum at E10 K and inside a 3 T superconducting magnet. To avoid absorption in the low energetic beam it is imperative to use a thin detector, produced by special lapping and etching techniques. Several detectors were tested in one of the extraction lines of the AD. Additional tests were performed using a laser setup at 660 and 1060 nm to prove the functionality of the complete system. One detector was finally installed in the ATHENA experiment for the August 2000 run where currents of up to 1 A were observed in the detector with simple 50 O resistive readout. Results are presented from this detector showing beam profiles, intensities and time structure. r 2002 Elsevier Science B.V. All rights reserved. PACS: 29.40; 25.43; 24.80 Keywords: Silicon; Low energetic antiprotons; ATHENA
1. Introduction The ATHENA experiment attempts to produce cold antihydrogen atoms and to spectroscopically investigate the 1S–2S transition [1]. The spectroscopic comparison with hydrogen aims at testing fundamental CPT symmetry to a precision of E1015 : The experiment is situated at one of the extraction lines of the 1999 newly constructed CERN antiproton decelerator (AD). Every 140 s a spill of E107 5:3 MeV antiprotons is extracted *Corresponding author. Present address: CERN. E-mail address: [email protected] (P. Riedler).
into the experiment with a spill length of several hundred nanoseconds. In order to recombine the antiprotons with positrons from a 5 mCi 22 Na-source to antihydrogen atoms, the antiprotons have to be decelerated by passing through material and have to be trapped in a cylindrical trap, using combined electric and magnetic fields, where they are further cooled by interaction with cold electrons. To optimize and control the number of antiprotons entering the trap, the experiment requires an entrance detector fully functional at temperatures around 10 K; inside a 3 T superconducting magnet and in a vacuum of 108 mbar: In addition, the
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 8 1 7 - 4
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P. Riedler et al. / Nuclear Instruments and Methods in Physics Research A 478 (2002) 316–320
Table 1 Specifications of the diodes used in the tests and in the final ATHENA setup Detector
Vfd (V)
Thickness (mm)
BC67 BC55 S152-10 S152-13 S152-16 S152-17 S152-21
3.5 2.3 4 4.5 4.5 4.5 4.5
67 55 7.06 6.03 5.88 6.09 6.02
Fig. 1. Schematic sketch of the ATHENA experimental setup viewing the position of the beam counter at the entrance of the antiproton catching trap.
range of 5:3 MeV antiprotons in e.g. silicon is approximately 200 mm [2], thus a sufficiently thin entrance detector is required. Fig. 1 shows a schematic sketch of the experimental setup. The vacuum system of the experiment is directly connected to the accelerator system. A set of ultra-thin silicon detectors was chosen to be tested as possible beam counters for ATHENA due to their operability in vacuum, inside a strong magnetic field and at cryogenic temperatures.
2. Detectors and readout electronics A set of 5.88–7:06 mm thick p-in-n diodes produced by SINTEF (Norway) for the CHICSiexperiment [3] and two p-in-n diodes (55 and 67 mm) produced by MICRON (Great Britain) for the Crystal-Barrel experiment were used for the tests. The SINTEF diodes are produced from 300 mm wafers by special etching techniques [3] with a thickness uniformity of better than 70:3 mm in the active area. The guard ring region is not thinned leaving a 300 mm frame which facilitates handling. The diodes consist of a single 10 10 mm2 wide pad surrounded by multiple guard rings. The MICRON diodes are segmented with one circular center pad and four surrounding pads. Detailed informations on the diodes are
Fig. 2. Total leakage current measured as a function of the bias voltage at room temperature.
given in Table 1. The I–V characteristics of all diodes are shown in Fig. 2. The energy loss in silicon from 5:3 MeV antiprotons is estimated from experimental data [4] to be 11:4 keV=mm compared to 288 eV=mm of a minimum ionizing particle. Approximately 107 particles are traversing the silicon detector every spill, generating about 3:2 1010 electron– hole pairs per micrometer of silicon. Due to the high number of charges created per spill the output current can be observed directly. A 100 O protection resistor was incorporated into the detector assembly, and the output taken directly to the 50 O input of the oscilloscope. This simplifies the direct observation of the instantaneous beam current. The total antiproton charge was obtained by integrating the recorded
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P. Riedler et al. / Nuclear Instruments and Methods in Physics Research A 478 (2002) 316–320
oscilloscope data. The output can also be directly connected to an ADC.
3. Results 3.1. Results from the DEM beam line The first tests of the complete system were carried out using an array of thin diodes mounted in an aluminum slide-holder system inside the DEM test line of the AD ring in November 1999. The test diodes were installed at the end of the beam line inside the AD vacuum. This required careful preparation to minimize outgassing of all components. An array consisting of 5 SINTEF diodes and BC55 was installed. Fig. 3 shows a picture of this array. Each of the SINTEF diodes was mounted on a ceramic hybrid and wirebonded. The BC55 diode was already pre-mounted on a PCB ring. The complete readout chain was installed using a digital oscilloscope (LC334A) to record the data. Fig. 3 shows the first signal observed in the first diode in the array (S152-17). The trigger was provided by the AD derived from the timing signals of the kicker magnet. The beam was extracted into the DEM line 11 ms after the trigger signal. The tests were carried out in the start-up phase of the AD, when the beam intensity and emittance were far from the design values. The extraction timing was set to 400 ns by the AD. The measured signal length of 438 ns is in good agreement with this value. An accurate measurement of the baseline was obtained by fitting over 5 ms before the pulse and found to be 0:370:2 mV: The mean signal amplitude of 3:971:4 mV was corrected for the baseline shift. Taking into account the 100 O resistor in parallel and the 50 O input impedance of the scope this corresponded to a current of 118:2742:9 mA: Using the energy loss of 11:4 keV=mm for 5 MeV antiprotons in silicon this corresponded to a number of 849073086 antiprotons and is in agreement with the 104 extracted antiprotons estimated by the AD. After the first diode a 12 mm mylar foil was placed, followed by BC55, a 130 mm aluminum foil and S152-16, S152-13 and S152-21. Signals could
Fig. 3. Signal observed in diode 152-17 during the extraction of a 100 MeV=c antiproton beam into the DEM test line. The bias voltage applied is 20 V: The photograph shows the array of diodes in the aluminum slide holder.
also be observed in BC55, S52-16 and S152-13, leading to a stopping curve in silicon of approximately 206 mm: A more precise measurement of the stopping curve was not possible due to the limited beam time. 3.2. Results from the laser setup One thin diode (S152-10) was tested using a 660 and 1060 nm laser in order to prove the functionality of the full readout system. The laser pulse was generated using a pulse generator with 100 Hz repetition rate. The absorption length of 660 nm light in silicon is in the order of 3 mm and of 1060 nm light in the order of 1 mm: The measured signal height for the two wavelengths as a function of the input pulse of the laser is shown in Fig. 4. The linearity was tested up to an input pulse of 2:32 V: For the 660 nm laser this corresponds to a power of E1 mW: The output voltage was found to be a linear function of the input power and the functionality of the readout system could be shown independently from the beam test. 3.3. Results from the ATHENA beam counter The diode BC67 was installed as beam counter for the August 2000 run. The pþ -implant is
P. Riedler et al. / Nuclear Instruments and Methods in Physics Research A 478 (2002) 316–320
319
Fig. 4. Pulse height for two different laser wavelengths measured as a function of the laser input pulse using the complete readout system. The bias voltage on the diode is 30 V: The inserted graph shows the pulses registered on the 50 O input of the oscilloscope for the two wavelengths at 2:32 V input pulse.
segmented in five pads thus allowing to measure the beam profile. Four equally large pads (34:6 mm2 ) are arranged around a circular center pad (38:5 mm2 ). The entrance counter mounted on the trap vacuum system is shown in the photograph in Fig. 1. The connectors for the high voltage capture system of the trap are also visible. The functionality of the beam counter with high voltage pulses applied to the trap was tested by measuring the induced signal in the central pad. A linear behavior was observed up to the maximum applied voltage of 7 kV: The maximum induced signal at 7 kV was 125 mV: The high voltage pulse in the experiment is applied at least 200 ns after the beam extraction and thus does not influence the beam current measurement. This delay can be changed to optimize the trapping efficiency. Fig. 5 shows the integrated signal measured as a function of the bias voltage with and without degrader foil 1 m before the beam counter. The measurement was taken during the run of ATHENA with 3 T and at E10 K: The signal increases as the bias voltage is raised above the depletion voltage but starts to show a plateau at around 30– 40 V: This indicates that at lower bias voltages a large fraction of the charge pairs recombine, but are separated at higher voltages.
Fig. 5. Measurement of the integrated signal versus bias voltage with and without degrader foil in the beam. The image plots show beam profiles measured at 100 V: The gray rectangles correspond to the five pads of BC67, white corresponds to zero integrated signal and black to 1:5 106 V s:
The measurement with and without degrading foil was in good agreement with the MonteCarlo simulations which predict a stronger beam blow-up and thus less antiprotons focussed on the beam counter with degrading foil. With the foil all five pads registered about the same amount of
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P. Riedler et al. / Nuclear Instruments and Methods in Physics Research A 478 (2002) 316–320
signal. Without foil the beam was well focussed on the center pad (E35% of total signal) with a higher total intensity, see also Fig. 5. After optimization of the beam steering, the BC67 diode provided the trigger signal for the catching trap and an independent measurement of the beam intensity. The number of antiprotons measured with the beam counter was in good agreement with the intensity measurements provided by the AD and with the results from activation measurements.
4. Conclusion A beam detection system for low energetic antiprotons has been developed for the ATHENA experiment using ultra-thin silicon detectors. A simple 50 O readout system was installed to register currents of up to 1 A generated in the diodes. The full functionality of this system was shown independently in a testbeam and using a laser setup. In the August 2000 run of the ATHENA experiment the beam counter was installed and was fully working. The beam counter provided the trigger signal for the antiproton catching trap and was used to determine the
optimum degrader thickness of the experiment and the capture efficiency of the trap. It was also used for beam steering to optimize the number of captured antiprotons.
Acknowledgements We would like to acknowledge the ATHENA collaboration for their help and support. Thanks are due to S. Maury and the AD team for their support during the beam test and the start-up phase. We would also like to express our gratitude to the group of J. Bosser for offering us the possibility to carry out tests in the DEM line. We are especially thankful to W. Dunnweber . for generously providing BC67 and BC55 to us.
References [1] ATHENA Collaboration, CERN SPSC Report 013, 2000. [2] C. Amsler, et al., Proceedings of the Hydrogen II Workshop 2000, Castiglione de Pescaia, Italy. [3] L. Evensen, et al., IEEE Trans. Nucl. Sci. NS-44 (1997) 629. [4] R. Medenwaldt, et al., Nucl. Instr. and Meth. B 58 (1991) 1.
Performance of ultra-thin silicon detectors in a 5 MeV antiproton beam P. Riedlera,*, J. Rocheta,b, A. Rudgeb, M. Doserb, R. Landuab a
University of Zurich, CH-8057 Zurich, Switzerland . . b CERN, CH-1211 Geneva 23, Switzerland
Abstract The performance of ultra-thin silicon detectors (6; 55 and 67 mm) was tested in order to provide beam position information, intensity monitoring and a fast trigger signal for the ATHENA experiment. The aim of this experiment is to produce cold antihydrogen using a low energetic (5 MeV) bunched (E500 ns) antiproton beam from the CERN Antiproton Decelerator (AD). The detector has to be operated in vacuum at E10 K and inside a 3 T superconducting magnet. To avoid absorption in the low energetic beam it is imperative to use a thin detector, produced by special lapping and etching techniques. Several detectors were tested in one of the extraction lines of the AD. Additional tests were performed using a laser setup at 660 and 1060 nm to prove the functionality of the complete system. One detector was finally installed in the ATHENA experiment for the August 2000 run where currents of up to 1 A were observed in the detector with simple 50 O resistive readout. Results are presented from this detector showing beam profiles, intensities and time structure. r 2002 Elsevier Science B.V. All rights reserved. PACS: 29.40; 25.43; 24.80 Keywords: Silicon; Low energetic antiprotons; ATHENA
1. Introduction The ATHENA experiment attempts to produce cold antihydrogen atoms and to spectroscopically investigate the 1S–2S transition [1]. The spectroscopic comparison with hydrogen aims at testing fundamental CPT symmetry to a precision of E1015 : The experiment is situated at one of the extraction lines of the 1999 newly constructed CERN antiproton decelerator (AD). Every 140 s a spill of E107 5:3 MeV antiprotons is extracted *Corresponding author. Present address: CERN. E-mail address: [email protected] (P. Riedler).
into the experiment with a spill length of several hundred nanoseconds. In order to recombine the antiprotons with positrons from a 5 mCi 22 Na-source to antihydrogen atoms, the antiprotons have to be decelerated by passing through material and have to be trapped in a cylindrical trap, using combined electric and magnetic fields, where they are further cooled by interaction with cold electrons. To optimize and control the number of antiprotons entering the trap, the experiment requires an entrance detector fully functional at temperatures around 10 K; inside a 3 T superconducting magnet and in a vacuum of 108 mbar: In addition, the
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 8 1 7 - 4
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P. Riedler et al. / Nuclear Instruments and Methods in Physics Research A 478 (2002) 316–320
Table 1 Specifications of the diodes used in the tests and in the final ATHENA setup Detector
Vfd (V)
Thickness (mm)
BC67 BC55 S152-10 S152-13 S152-16 S152-17 S152-21
3.5 2.3 4 4.5 4.5 4.5 4.5
67 55 7.06 6.03 5.88 6.09 6.02
Fig. 1. Schematic sketch of the ATHENA experimental setup viewing the position of the beam counter at the entrance of the antiproton catching trap.
range of 5:3 MeV antiprotons in e.g. silicon is approximately 200 mm [2], thus a sufficiently thin entrance detector is required. Fig. 1 shows a schematic sketch of the experimental setup. The vacuum system of the experiment is directly connected to the accelerator system. A set of ultra-thin silicon detectors was chosen to be tested as possible beam counters for ATHENA due to their operability in vacuum, inside a strong magnetic field and at cryogenic temperatures.
2. Detectors and readout electronics A set of 5.88–7:06 mm thick p-in-n diodes produced by SINTEF (Norway) for the CHICSiexperiment [3] and two p-in-n diodes (55 and 67 mm) produced by MICRON (Great Britain) for the Crystal-Barrel experiment were used for the tests. The SINTEF diodes are produced from 300 mm wafers by special etching techniques [3] with a thickness uniformity of better than 70:3 mm in the active area. The guard ring region is not thinned leaving a 300 mm frame which facilitates handling. The diodes consist of a single 10 10 mm2 wide pad surrounded by multiple guard rings. The MICRON diodes are segmented with one circular center pad and four surrounding pads. Detailed informations on the diodes are
Fig. 2. Total leakage current measured as a function of the bias voltage at room temperature.
given in Table 1. The I–V characteristics of all diodes are shown in Fig. 2. The energy loss in silicon from 5:3 MeV antiprotons is estimated from experimental data [4] to be 11:4 keV=mm compared to 288 eV=mm of a minimum ionizing particle. Approximately 107 particles are traversing the silicon detector every spill, generating about 3:2 1010 electron– hole pairs per micrometer of silicon. Due to the high number of charges created per spill the output current can be observed directly. A 100 O protection resistor was incorporated into the detector assembly, and the output taken directly to the 50 O input of the oscilloscope. This simplifies the direct observation of the instantaneous beam current. The total antiproton charge was obtained by integrating the recorded
318
P. Riedler et al. / Nuclear Instruments and Methods in Physics Research A 478 (2002) 316–320
oscilloscope data. The output can also be directly connected to an ADC.
3. Results 3.1. Results from the DEM beam line The first tests of the complete system were carried out using an array of thin diodes mounted in an aluminum slide-holder system inside the DEM test line of the AD ring in November 1999. The test diodes were installed at the end of the beam line inside the AD vacuum. This required careful preparation to minimize outgassing of all components. An array consisting of 5 SINTEF diodes and BC55 was installed. Fig. 3 shows a picture of this array. Each of the SINTEF diodes was mounted on a ceramic hybrid and wirebonded. The BC55 diode was already pre-mounted on a PCB ring. The complete readout chain was installed using a digital oscilloscope (LC334A) to record the data. Fig. 3 shows the first signal observed in the first diode in the array (S152-17). The trigger was provided by the AD derived from the timing signals of the kicker magnet. The beam was extracted into the DEM line 11 ms after the trigger signal. The tests were carried out in the start-up phase of the AD, when the beam intensity and emittance were far from the design values. The extraction timing was set to 400 ns by the AD. The measured signal length of 438 ns is in good agreement with this value. An accurate measurement of the baseline was obtained by fitting over 5 ms before the pulse and found to be 0:370:2 mV: The mean signal amplitude of 3:971:4 mV was corrected for the baseline shift. Taking into account the 100 O resistor in parallel and the 50 O input impedance of the scope this corresponded to a current of 118:2742:9 mA: Using the energy loss of 11:4 keV=mm for 5 MeV antiprotons in silicon this corresponded to a number of 849073086 antiprotons and is in agreement with the 104 extracted antiprotons estimated by the AD. After the first diode a 12 mm mylar foil was placed, followed by BC55, a 130 mm aluminum foil and S152-16, S152-13 and S152-21. Signals could
Fig. 3. Signal observed in diode 152-17 during the extraction of a 100 MeV=c antiproton beam into the DEM test line. The bias voltage applied is 20 V: The photograph shows the array of diodes in the aluminum slide holder.
also be observed in BC55, S52-16 and S152-13, leading to a stopping curve in silicon of approximately 206 mm: A more precise measurement of the stopping curve was not possible due to the limited beam time. 3.2. Results from the laser setup One thin diode (S152-10) was tested using a 660 and 1060 nm laser in order to prove the functionality of the full readout system. The laser pulse was generated using a pulse generator with 100 Hz repetition rate. The absorption length of 660 nm light in silicon is in the order of 3 mm and of 1060 nm light in the order of 1 mm: The measured signal height for the two wavelengths as a function of the input pulse of the laser is shown in Fig. 4. The linearity was tested up to an input pulse of 2:32 V: For the 660 nm laser this corresponds to a power of E1 mW: The output voltage was found to be a linear function of the input power and the functionality of the readout system could be shown independently from the beam test. 3.3. Results from the ATHENA beam counter The diode BC67 was installed as beam counter for the August 2000 run. The pþ -implant is
P. Riedler et al. / Nuclear Instruments and Methods in Physics Research A 478 (2002) 316–320
319
Fig. 4. Pulse height for two different laser wavelengths measured as a function of the laser input pulse using the complete readout system. The bias voltage on the diode is 30 V: The inserted graph shows the pulses registered on the 50 O input of the oscilloscope for the two wavelengths at 2:32 V input pulse.
segmented in five pads thus allowing to measure the beam profile. Four equally large pads (34:6 mm2 ) are arranged around a circular center pad (38:5 mm2 ). The entrance counter mounted on the trap vacuum system is shown in the photograph in Fig. 1. The connectors for the high voltage capture system of the trap are also visible. The functionality of the beam counter with high voltage pulses applied to the trap was tested by measuring the induced signal in the central pad. A linear behavior was observed up to the maximum applied voltage of 7 kV: The maximum induced signal at 7 kV was 125 mV: The high voltage pulse in the experiment is applied at least 200 ns after the beam extraction and thus does not influence the beam current measurement. This delay can be changed to optimize the trapping efficiency. Fig. 5 shows the integrated signal measured as a function of the bias voltage with and without degrader foil 1 m before the beam counter. The measurement was taken during the run of ATHENA with 3 T and at E10 K: The signal increases as the bias voltage is raised above the depletion voltage but starts to show a plateau at around 30– 40 V: This indicates that at lower bias voltages a large fraction of the charge pairs recombine, but are separated at higher voltages.
Fig. 5. Measurement of the integrated signal versus bias voltage with and without degrader foil in the beam. The image plots show beam profiles measured at 100 V: The gray rectangles correspond to the five pads of BC67, white corresponds to zero integrated signal and black to 1:5 106 V s:
The measurement with and without degrading foil was in good agreement with the MonteCarlo simulations which predict a stronger beam blow-up and thus less antiprotons focussed on the beam counter with degrading foil. With the foil all five pads registered about the same amount of
320
P. Riedler et al. / Nuclear Instruments and Methods in Physics Research A 478 (2002) 316–320
signal. Without foil the beam was well focussed on the center pad (E35% of total signal) with a higher total intensity, see also Fig. 5. After optimization of the beam steering, the BC67 diode provided the trigger signal for the catching trap and an independent measurement of the beam intensity. The number of antiprotons measured with the beam counter was in good agreement with the intensity measurements provided by the AD and with the results from activation measurements.
4. Conclusion A beam detection system for low energetic antiprotons has been developed for the ATHENA experiment using ultra-thin silicon detectors. A simple 50 O readout system was installed to register currents of up to 1 A generated in the diodes. The full functionality of this system was shown independently in a testbeam and using a laser setup. In the August 2000 run of the ATHENA experiment the beam counter was installed and was fully working. The beam counter provided the trigger signal for the antiproton catching trap and was used to determine the
optimum degrader thickness of the experiment and the capture efficiency of the trap. It was also used for beam steering to optimize the number of captured antiprotons.
Acknowledgements We would like to acknowledge the ATHENA collaboration for their help and support. Thanks are due to S. Maury and the AD team for their support during the beam test and the start-up phase. We would also like to express our gratitude to the group of J. Bosser for offering us the possibility to carry out tests in the DEM line. We are especially thankful to W. Dunnweber . for generously providing BC67 and BC55 to us.
References [1] ATHENA Collaboration, CERN SPSC Report 013, 2000. [2] C. Amsler, et al., Proceedings of the Hydrogen II Workshop 2000, Castiglione de Pescaia, Italy. [3] L. Evensen, et al., IEEE Trans. Nucl. Sci. NS-44 (1997) 629. [4] R. Medenwaldt, et al., Nucl. Instr. and Meth. B 58 (1991) 1.