Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
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First tests with Timepix2 and heavy ions Stuart George a,b ,∗, Daniel Turecek b,c , Scott Wheeler d , Satoshi Kodaira e , Lawrence Pinsky b a
Department of Health and Human Performance, University of Houston, Houston, TX 77004, USA Department of Physics, University of Houston, Houston, TX 77004, USA Advacam SRO, Prague, Czech Republic d NASA Johnson Space Center, Houston, TX 77058, USA e National Institute of Radiological Sciences, Inage Ward, Chiba, Japan b c
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Keywords: Hybrid pixel detector Space radiation instrumentation Heavy ion instrumentation Timepix Medipix
ABSTRACT We investigate the response of the new Timepix2 chip to heavy ions. Understanding this response is important to understand the performance and capabilities of this chip in the space environment, where it is a possible replacement for the existing Timepix chip which is already widely used in space. We carry out measurements on Timepix2 using a variety of heavy ion beams. We find that the Timepix2 generally performs as expected for a variety of measurements. We confirm that the Timepix2 front end can measure up to at least 2.2 MeV in silicon per pixel. We also confirm that improvements in the pixel interaction with the shutter allow for measurements of individual tracks in fields substantially more intense than those measurable by Timepix. We conclude that the Timepix2 is a very promising candidate for use in space radiation measurements.
1. Introduction Hybrid pixel detectors offer a number of significant advantages for use in spaceflight radiation measurements. These include low mass, size and low power consumption resulting in radiation measurement systems with a significantly smaller resource footprint than competing solutions. For these reasons the Timepix detector [1–3], produced by the Medipix 2 collaboration at CERN has been widely adopted for use in space by a number of organizations including NASA and ESA. At time of writing NASA is flying 15 Timepix assemblies on the International Space Station as part of a number of different instruments, some used for scientific measurements, others for monitoring the radiation exposure of NASA astronauts [4]. Other Timepix based NASA systems include the BIRD instrument on Orion ETF-1 [5], the HERA instrument which forms part of the Orion radiation system and is expected to fly on Orion Exploration Mission 1 onwards [6], the planned flights of 3 Timepix detectors as part of the Bio-Sentinel system [7] and the planned flight of a Timepix based system to the surface of the moon as part of the NASA Commercial Lunar Payload Services program. An example radiation monitor on ISS is shown in Fig. 1. The Timepix2 detector consists of a pixellated silicon sensor of 256 × 256 pixels with 55 μm pitch and arbitrary thickness attached to the Timepix2 ASIC [8]. The Timepix2 ASIC is produced by the Medipix Collaboration at CERN and is a successor chip to the original Timepix ASIC. Each semiconductor sensor pixel is individually bump bonded to the corresponding pixel in the Timepix2 ASIC. Similarly
Fig. 1. Timepix based radiation environment monitor on the international space station.
to the original counter the Timepix2 ASIC contains pulse processing electronics for each pixel consisting of a preamplifier, shaping circuit, threshold discriminator and digital counter. In this way Timepix 2 can measure the so called Time Over Threshold (TOT) or the discharge time of the preamplifier which is proportional to the input charge. Timepix2 operates with a frame based read out. This means that in effect it behaves like a digital camera, with pixels counts only being registered when a digital ‘shutter’ is held open. This functionality also allows a measurement of the Time Of Arrival (TOA) relative to the closing of the shutter, by counting from the time the pixel preamplifier goes high to the end of the frame. A schematic showing the TOT and TOA measurement schemes is shown in Fig. 2.
∗ Corresponding author at: Department of Health and Human Performance, University of Houston, Houston, TX 77004, USA. E-mail address:
[email protected] (S. George).
https://doi.org/10.1016/j.nima.2019.162725 Received 1 April 2019; Received in revised form 4 September 2019; Accepted 6 September 2019 Available online xxxx 0168-9002/© 2019 Published by Elsevier B.V.
Please cite this article as: S. George, D. Turecek, S. Wheeler et al., First tests with Timepix2 and heavy ions, Nuclear Inst. and Methods in Physics Research, A (2019) 162725, https://doi.org/10.1016/j.nima.2019.162725.
S. George, D. Turecek, S. Wheeler et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 2. Timepix2 pixel logic for measurement of TOT and TOA. Fig. 3. Timepix2 setup at HIMAC.
Much of the functionality described above will be recognizable to anyone familiar with the Timepix ASIC. However, Timepix2 features several important upgrades compared to its predecessor, these include:
3. Front end testing
• Upgraded front end with ‘‘adaptive gain’’ for measurement of charges up to 950 kilo electrons per pixel and no ‘‘volcano effect’’ [9]. • Suppression of ‘‘ghost tracks’’ where the frame and TOT measurement overlap. • Simultaneous per pixel Time over Threshold (energy) and Time of Arrival measurement, with a 28 bit configurable counter that can be split between the two.
Of key interest for any application measuring heavy ions is the front end performance up to high (several MeV) levels of per pixel energy deposition. The marquee feature of the Timepix2 is the new adaptive gain mode where the gain of the charge sensitive amplifier depends on the input charge. This provides quasi-linear pixel response up to 750 ke – and monotonic response to 950 ke – or 2.7 MeV and 3.4 MeV per pixel respectively. Fig. 4 shows tracks created by Si28 ions with the adaptive gain turned on and off. The panels in the bottom left show an individual cluster and an ensemble cluster made by summing over all measured clusters aligned by their center of mass. Also shown are profiles through the track long and short axes, which confirm that the maximum measured TOT is about 1.9× the non adaptive gain, which is reasonably consistent with simulations of device performance carried out by the chip designers (2.1×). Finally of interest is the Cluster Size distribution which shows that the number of pixels in the track is considerably larger with the adaptive gain disabled. This is reflected in the track images which show a larger ‘brim’ or ‘skirt’ region (the large area of low counting pixels that surrounds the high counting core). This region is known to be formed through the dynamic interaction of the Timepix shaping circuit with the small amount of charge induced in these pixels by the main charge cloud. It is presumably different with the adaptive gain enabled due to the different rise time of the preamplifier. When operated in hole measurement mode the Timepix suffers from a saturation effect referred to as the ‘volcano effect’ when exposed to high input charges over 2 MeV. In this situation the measured counts become weakly inversely proportional to the deposited charge [9]. This causes a characteristic caldera like shape in the tracks, especially at low incidence angles. Fig. 5 shows comparative measurements of a Timepix and Timepix2 with 290 MeV Fe56 ions at oblique incidence. While no volcano effect is visible in this case, the saturation of the Timepix2 front end is still clearly visible. Simulations performed with Geant4 indicate that the energy deposition in the center pixels for these tracks should be of the order of tens of MeV. For this set of measurements Timepix and Timepix2 particle tracks are very similar because they operate with very similar silicon sensors at the same bias voltage. This means that calibrated Timepix average tracks can be directly compared to uncalibrated Timepix2 average tracks, and that the average Timepix energy deposition as a function of track position can be used as a surrogate for the energy input to a Timepix2 pixel at the same track position in order to generate an approximate calibration curve. This is shown in Fig. 6 for measurements of N14 ions at 430 MeV∕A for different incident track polar angles. This graph is approximately linear with an inflection point at approximately 900 keV which may be due to a feature of the Timepix2 front end, or to the high energy calibration procedure used for the Timepix, which
In turn these features offer several interesting applications for the space environment, for example: 𝑑𝐸 • Improved ion dosimetry, 𝑑𝑋 measurement and particle ID due to new front end. • Overlapping track ID and separation using simultaneous TOT and TOA. • Easy identification of particle showers and fragmentation events using TOA data. • Measurement in very high rate radiation fields, for example extra vehicular measurements of particle belts. • Simple multi layer tracking with TOA data.
2. Materials and methods A prototype read out system produced by Advacam s.r.o. [10] was used to read out a Timepix2 with a 500 μm thick p-in-n silicon sensor which was operated at 100 V bias voltage, just over full depletion. Each pixel in the Timepix2 contains four threshold adjustment bits, which need to be set in order for the device to have a uniform energy threshold. This threshold equalization was performed following the standard procedure outlined by Tlustos [11]. The detector counters were operated in dual TOT/TOA mode with 14 bits used of the counter for the TOT Data and 14 bits for the TOA data. Data taken by Timepix2 were also compared to a reference Timepix detector also with a 500 μm silicon p-in-n sensor operated at 100 V. The reference Timepix was calibrated through a threshold equalization, photon energy calibration and a high energy calibration with monoenergetic protons which provides the capability to measure deposited energies in a single pixel up to approximately 2 MeV [12,13]. At HIMAC the Timepix2 was exposed to four ions of increasing stopping power to exercise the front end. For each heavy ion the incidence angle and configuration settings of the Timepix2 were systematically varied. Tested heavy ions were He4 at 100 MeV∕A, N14 at 400 MeV∕A, Si28 at 350 MeV∕A, and Fe56 at 290 MeV∕A. A picture of the test setup at HIMAC is shown in Fig. 3. 2
Please cite this article as: S. George, D. Turecek, S. Wheeler et al., First tests with Timepix2 and heavy ions, Nuclear Inst. and Methods in Physics Research, A (2019) 162725, https://doi.org/10.1016/j.nima.2019.162725.
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Fig. 4. Comparison of Timepix2 clusters with Adaptive Gain (AG) enabled and disabled.
Fig. 5. Comparison of Timepix and Timepix2 clusters Fe56 @ 290 MeV∕A, the Timepix2 does not exhibit a volcano effect.
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Please cite this article as: S. George, D. Turecek, S. Wheeler et al., First tests with Timepix2 and heavy ions, Nuclear Inst. and Methods in Physics Research, A (2019) 162725, https://doi.org/10.1016/j.nima.2019.162725.
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Fig. 7. Comparison of TOT signal length and total ADC counts in a track for 10 ms and a 100 μs frame lengths.
Fig. 6. Timepix2 ADC counts calibrated to Timepix measured energy for different track polar angles. Note that the Timepix data is shown in terms of calibrated energy (in keV) and the Timepix2 data in ADC counts. It is a convenient coincidence that 1 Timepix2 ADC count appears to correspond with approximately 1 keV of energy deposition.
results. The Timepix2 front end was confirmed to measure up to at least 2.2 MeV/611 ke− per pixel. The capability of the Timepix2 to measure at much higher rates than the Timepix due to new pixel logic was also tested and confirmed. This functionality has a number of exciting applications for measuring in environments where rates are too high for a Timepix such as radiation belts, or in medical ion therapy beams. These tests confirm that the Timepix2 is an exceptionally promising candidate for use measuring space radiation.
in the case of the particular tested assembly, applies an additional calibration correction from 900 keV up. Regardless of any other interpretation of the graph it clearly shows that Timepix2 responds monotonically up to at least 2.2 MeV per pixel or around 650 ke−. Testing beyond this limit is not possible with this method as 2.2 MeV is the maximum measurable per pixel energy with a Timepix detector.
Acknowledgments We gratefully acknowledge the support of several members of the CERN Microelectronics section, especially Lukas Tlustos and Jerome Alozy, without whom the fast turnaround required to meet our beamtime dates at HIMAC would not have been possible. This work was performed through KBR on the Human Human Health and Performance Contract (NNJ15HK11B) for NASA.
4. Suppression of ghost tracks One notable limitation of the Timepix is that the minimum shutter time for the measurement of charged particles is around 10 ms. This is because the signal length of the highest counting pixels in a heavy ion track can be as long as 1 ms and so start to approach similar magnitude to the frame length. When this happens the beginning and end of the frame will commonly intersect the TOT signal, resulting in some tracks that are missing lower charge pixels, or tracks which appear ‘decapitated’ where all the pixels over a certain value count the same. Collectively these phenomena are referred to as ghost tracks. This essentially limited the Timepix to measurements of particle fluences less than about 104 ∕cm2 s if the ability to distinguish individual tracks is desired. The Timepix2 contains additional logic to deal with ghost tracks. Hits that arrive while the shutter is closed are ignored and the TOT continues to count until the end of the pulse for signals that start while the shutter is open. In principal this functionality should completely suppress ghost tracks and make measurements of single tracks possible down to the pulse pile up level. To confirm this functionality measurements were taken with a Timepix2 of a 350 MeV∕A Si28 beam at both 10 ms and 100 μs as shown in Fig. 7. In both cases the spectrum of total track ADC counts remained the same indicating that no pulses are being truncated by the shutter. Further confirmation of this functionality is also shown by plotting the maximum measured TOT signal spectrum for each track. In the case of a 100 μs frame these TOT times are longer than the frame length, confirming that this logic works as designed.
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Please cite this article as: S. George, D. Turecek, S. Wheeler et al., First tests with Timepix2 and heavy ions, Nuclear Inst. and Methods in Physics Research, A (2019) 162725, https://doi.org/10.1016/j.nima.2019.162725.
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