Prototype CMOS SSPM solar particle dosimeter with

Prototype CMOS SSPM solar particle dosimeter with tissue-equivalent sensor C.J.Stapels* a, E.B Johnson a, S.Mukhopadhyay a, E.C. Chapman a, J.F.Christian a Eric Benton b a RMD Inc., 44 Hunt St Watertown, MA 02472 b Oklahoma State University Department of Physics; 145 Physical Sciences; Stillwater, OK 74078 ABSTRACT A dosimeter-on-a-chip (DoseChip) comprised of a tissue-equivalent scintillator coupled to a solid-state photomultiplier (SSPM) built using CMOS (complementary metal-oxide semiconductor) technology represents an ideal technology for a space-worthy, real-time solar-particle monitor for astronauts. It provides a tissue-equivalent response to the relevant energies and types of radiation for Low-Earth Orbit (LEO) and interplanetary space flight to the moon or Mars. The DoseChip will complement the existing Crew Passive Dosimeters by providing real-time dosimetry and as an alarming monitor for solar particle events (SPEs).[1] A prototype of the DoseChip, comprised of a 3 x 3 x 3 mm3 cube of BC-430 plastic scintillator coupled to a 2000-pixel SSPM, has successfully demonstrated response to protons at the NASA Space Radiation Laboratory (NSRL) at Brookhaven National Laboratory and at the HIMAC facility in Japan. The dynamic range of the dose has been verified over four orders of magnitude for particles with LET ranging from 0.2 keV/μm to 200 keV/μm, which includes 1-GeV protons to 420-MeV/n Fe nuclei. To exploit the benefits of the CMOS SSPM, we have developed our first autonomous prototype using the DoseChip. An analog circuit is used to process the signals from the SSPM, and an on-board microprocessor is used to digitize and store the pulse height information. Power is distributed over the device from a single dual voltage supply through various regulators and boost converters to appropriate supply voltages to each of the components. Keywords: SSPM, avalanche photodiode, GPD, Space dosimeter, solar proton detector, plastic scintillator

1. INTRODUCTION Accurate, real-time monitoring of human dose caused by space radiation has been limited by existing technologies that are either too bulky or require too much power. The use of a solid-state photodetector coupled to a plastic scintillation material provides a tissue-equivalent response within a small form-factor with a small power budget. Such real-time information can allow astronauts to take protective measures in the event of a sudden increase in the dose rate due to space weather. The Space Studies Board of the National Research Council recommends that real-time dose rate monitoring at an astronaut’s location is necessary to ensure crew safety against potentially hazardous SPEs [2]. At present, the NASA Space Radiation and Analysis Group (SRAG) at Johnson Space Center uses a combination of thermoluminescence detector (TLD) and CR-39 plastic nuclear track detectors (PNTD) to measure the total absorbed dose and dose equivalent received by each crew member during missions aboard the International Space Station (ISS) and the Space Shuttles. The complexity of human space-flight outside of LEO requires reliable, fault-tolerant instrumentation capable of providing real-time dose readings during a mission, which is not feasible with existing TLD technology, especially during extravehicular activity (EVA). This work describes a prototype active, lightweight, and compact Dosimeter-on-a-Chip (DoseChip) for personal crew dosimetry comprised of a tissue-equivalent plastic scintillator coupled to a solid-state photomultiplier (SSPM) being tested for measuring solar particle events (SPE). The SSPM consists of an array of Geiger photodiodes (GPDs) with supporting electronics built using complementary metal-oxide semiconductor (CMOS) structures. The ubiquitous nature of CMOS technology provides a standardized development platform, and the ability to integrate the supporting electronics into a miniature, lightweight, low-power design. Solar Physics and Space Weather Instrumentation III, edited by Silvano Fineschi, Judy A. Fennelly, Proc. of SPIE Vol. 7438, 743812 · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.825500

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1.1 Space Environment For space flight outside of LEO, the dosimeter must be sensitive to the ambient radiation fields. Galactic cosmic rays (GCR) consist of 85% protons, 12% helium ions, 1% heavy ions from Li to U, and 2% electrons. The GCR spectrum peaks near 1 GeV. SPE can be separated into two types of events: prompt and gradual [2, 3]. Prompt events are dominated by electrons with a total fluence between 107 and 108 cm-2 that subtend a narrow angular width of 30 - 45º. Gradual events are dominated by protons with a total fluence that can exceed 109 cm-2 and are spread over a wide angle from 60 to 180º. The flux from gradual SPE has an exponential growth that will peak in minutes to hours and could last for hours to as long as days [3]. The Martian environment is predicted to have fluxes up to 2 particles cm-2s-1 [4]. The total fluence from gamma-rays and electrons is at least an order of magnitude less than from charged particles; the dosimeter is designed to be most sensitive to the most abundant components of the ambient charged particle spectrum outside of LEO. 1.2 Existing Technology Although accurate measurement of radiation dose is a critical need in space flight and has been studied for many decades, existing methods have limitations. Existing instrumentation can be divided into passive and active devices. The LiF TLD is the workhorse of space-based radiation monitoring. TLD measures total absorbed dose and cannot discriminate low and high linear energy transfer (LET) radiation. Furthermore, TLD records dose from high-LET particles ( ~> 10 keV/μm) with reduced efficiency. The CR-39 PNTD is used to measure LET spectrum, dose and dose equivalent from particles of LET∞H2O ≥10 keV/μm. Data from the CR-39 PNTD is used to correct the dose from highLET particles measured in TLD and is combined with dose measurements from TLD to yield total dose equivalent [5]. This combination of passive detectors, referred to as the Crew Passive Dosimeter (CPD), has the advantage over the current generation of active detectors in being compact, of low mass, and requiring no external power so that a crew member can wear the dosimeter on his body without it interfering with his normal activities. At present, few active detectors are small enough to be routinely worn on the human body. One such detector system, the Liulin-4 Mobile Dosimetry Unit (MDU), is currently being used aboard the Russian segment of the ISS [6]. The Liulin-4 MDU uses a small Si-diode to measure particle flux and energy deposition (dose). The Liulin-4 MDU is batterypowered, about the size of a pack of cigarettes, and weighs 0.23 kg; therefore, it is still too large and bulky for routine use by a crewmember working inside a spacecraft or habitat. In addition to its size and mass, drawbacks of the Liulin-4 MDU include the fact that it is not tissue equivalent and the fact that it is insensitive to particles of LET∞H2O > 40 keV/μm, making it unable to measure the dose and dose equivalent from the high LET component. Low LET incident radiation High LET incident radiation Tissue equivalent scintillator External interface

CMOS ‘DoseChip’

Few pixels fire Many pixels fire Fig. 1. Sample dosimeter-on-a-chip layout. The simple dosimeter on a chip design consists of a GPD array, the supporting detector electronics, storage capacity, and an external interface. A low LET event causes a few Geiger pixels to trigger, while a high LET event creates more photons in the scintillator, causing more pixels to trigger. Individual pixels are operated in Geiger mode; the number of pixels triggered is proportional to the energy deposited in the scintillator.

1.3 Detector operation The initial element of the dosimeter is tissue-equivalent scintillation plastic (Saint-Gobian BC-430), which has an emission spectrum that is well matched to the quantum efficiency of the SSPM. An array of photodetectors that count the individual photons created by an ionizing event in the scintillation material provides a very high internal

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amplification of the signal, facilitating its discrimination of small scintillation signals from baseline noise. This innovative approach is known as the solid-state photomultiplier (SSPM) and bears the promise of compact, lightweight, low-power integrated detector chips that provide real-time dose information on astronauts or as area monitors. The array of GPD pixels is monolithically integrated to the readout, data storage, and transfer electronics. Fig. 1 illustrates the digital dosimeter concept using a detector-on-a-chip, which includes the monolithic SSPM and readout electronics, coupled to a scintillator. The scintillator, which sits atop the detector-on-a-chip in the figure, converts the radiation into a light pulse that uniformly illuminates the SSPM in the chip. 1.4 Calculation of Dose The number of triggered pixels is proportional to the energy loss in the plastic. This energy loss is then used to determine the dose, but a response function is required to convert triggered pixels in to energy deposition. This response function for protons is determined through direct measurements with proton beams of known energy. The dose is calculated by using the following function: Dose =

LETH 2O ⋅ 3000μm ⋅ 1.602 x10 −16 kg 0.33 cm3 ⋅1x10 −3 cm3

J keV ⋅ Q( LET ) , H 2O

(1)

where Q, the quality factor, is ⎧ keV LETH2O < 10 1 ⎪ μm ⎪ keV keV ⎪ ≤ LETH2O ≤ 100 Q(LETH2O ) = ⎨(0.32 ⋅ LETH2O − 2.2) 10 μm μm ⎪ keV ⎪ 300 LETH2O > 100 ⎪ LETH2O μm ⎩

(2)

A calibration curve is generated to convert ADC channels into LET, and the dose for each bin in LET is calculated in terms of Sv. The total dose for an exposure is the sum dose over all LET bins. . The expected dose is calculated based on an expected number of particles incident on a 3 x 3 x 3 mm3 of water. The number of particles expected is determined from two detectors, which were located near the dosimeter during beam operation. One detector consisted of a 1-cm2 scintillator coupled to a PMT and readout using a counter, and the other detector was a 0.3 x 0.3 x 0.3 cm3 scintillator coupled to a PMT and readout using an MCA. The number of events from both of these detectors provided an expected number of incident particles on the dosimeter.

2. EXPERIMENT 2.1 Dosimeter construction The detector consists of a 3 x 3 mm2 SSPM with 2024 pixels, each 50 μm x 50 μm. The SSPM is optically coupled to a 3 x 3 x 3 mm3 of BC-430 scintillation material, which is wrapped in Teflon tape. The SSPM is connected to a readout PCB that supplies the operating bias and powers the processing electronics. The signal is amplified and connected to a peak-hold circuit that holds the maximum pulse height of the SSPM output while the signal is digitized by an ADC. The digitizing trigger signal also generates a delayed pulse that is used to reset the peak hold circuit. The microprocessor stores the data in a histogram. A block diagram of the readout process is shown in Fig. 2 Comp

SSPM +Vb

Amp

Delay/Reset Peak Hold

μProcessor

Fig. 2: Conceptual schematic of the dosimeter. The SSPM is coupled to an amplifier, and the signal is split, where one end is routed to the peak hold for processing and the other is sent into a comparator. The comparator is used to trigger the

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microprocessor ADC to start collecting data and to generate a signal to reset the peak hold circuit. The power to the existing device is generated from a wall adapter with a ±15V supply

2.2 Test Beams The dosimeter irradiated with ion beams from the HIMAC facility in Japan and the NSRL facility at Brookhaven National Laboratories. The HIMAC delivered He, O, Ne and Fe beams to the dosimeter, and the NSRL facility delivered protons. The LET ranged from 200 to 0.2 keV/μm, proving a large range for calibration as shown in Table 1. Table 1. Ions, Ion energy and LET for the dosimeter evaluation experiments described in this work.

Ion 1 H 1 H

Energy 1 Gev 150 MeV

LET 0.22 keV/μm 0.55 keV/μm

He Ne O Fe

144 MeV/n 180 MeV/n 100 MeV/n 420 MeV/n

2.25 keV/μm 48.33 keV/μm 46.96 keV/μm 201.1 keV/μm

3. RESULTS 3.1 Spectral Response Spectra for each of the six ion beams are shown in Fig. 3. The LET in water for each of the particles is shown in the legend, and the mean of the peaks for each spectrum is dependent on the LET. The proportional nature of the means indicates that the instrument can provide dose for charged particles with LETs from 0.2 to 200 keV/μm.

80

0

460

P: 1 GeV: 0.22 keV/μm P: 150 MeV: 0.55 keV/μm He: 144 MeV/n: 2.25 keV/μm Ne: 180 MeV/n: 48.33 keV/μm O: 100 MeV/n: 46.96 keV/μm Fe: 420 MeV/n: 201.1 keV/μm

70 60 Counts

Pixels Triggered 919 1379 1838 2298

50 40 30 20 10 0

0

500

1000 1500 2000 2500

Pulse Height (ADC Channels) Fig. 3: Spectra of six ion beams taken at the HIMAC facility and NSRL. The mean value of the peaks is proportional to the LET of the ions as seen in Fig. 4.

The conversion from ADC channels to LET in water is the most relevant conversion term for calculating the dose. This value can be used to measure the energy deposition and calculate the quality factor. The mean of the peaks in Fig. 3 is plotted against of the LET in water. Fig. 4 shows the dependence with the abscissa on a natural logarithm scale (note this is not a log base-ten scale).

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2500 ADC = 731.78 + 312.30*ln(LET) ADC Channel

2000 1500 1000 500 0

0.4

2.7 20.1 148.4 LET in H2O (keV/μm)

Fig. 4. The calibration curve to convert ADC channels into LET. The dependence of LET on ADC channels is roughly exponential, and the data is plotted with the abscissa in a natural logarithm scale.

. 3.2 Measurement of dose for protons The table below summarizes 32 measurements of 1 GeV protons at the NSRL with various beam intensities. Table 2. Calculated dose results for 1 GeV protons of various intensities.. The dose was calculated using equation 1 with the spectrum conversion from Fig. 4. The expected dose is determined using an external beam counter and the known LET.

Run Detected Expected Name Counts Dose (Sv) R00 1971 7.87E-06 R01 4530 1.79E-05 R02 4520 1.79E-05 R03 4523 1.79E-05 R04 4598 1.79E-05 R05 4565 1.79E-05 R06 4454 1.79E-05 R07 4057 1.79E-05 R08 4412 1.79E-05 R09 4078 1.79E-05 R10 11365 4.47E-06 R11 6554 2.68E-05 R12 4517 1.88E-05 R13 5247 1.79E-05 R14 4668 1.79E-05 R15 4593 1.79E-05 R16 4714 1.79E-05 R17 4970 1.79E-05 R18 4970 1.79E-05 R19 4519 1.79E-05 R20 5144 1.79E-05 R21 8434 3.26E-05

Measured Dose (Sv) 7.52E-06 1.72E-05 1.73E-05 1.73E-05 1.76E-05 1.74E-05 1.70E-05 1.57E-05 1.69E-05 1.56E-05 -2.50E-05 1.73E-05 2.01E-05 1.76E-05 1.75E-05 1.81E-05 1.89E-05 1.89E-05 1.75E-05 1.97E-05 3.24E-05

% Error -4.4 -3.8 -3.3 -3.3 -1.6 -2.7 -4.9 -12.2 -5.5 -12.8 --6.8 -7.9 12.4 -1.6 -2.1 1.2 5.7 5.7 -2.1 10.2 -0.4

Run Name C01 C02 C03 C04 C05 C06 C07 C08 C09 C10

Detected Counts 3138 1373 3850 4980 4656 2665 1561 4625 2442 2172

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Expected Dose (Sv) 1.21E-05 5.47E-06 1.47E-05 1.93E-05 1.81E-05 1.03E-05 6.35E-06 1.84E-05 9.35E-06 8.21E-06

Measured Dose (Sv) 1.38E-05 5.67E-06 1.48E-05 1.96E-05 1.80E-05 1.02E-05 5.95E-06 1.76E-05 9.29E-06 8.21E-06

% Error 14.2 3.6 0.4 1.5 -0.3 -0.5 -6.3 -4.4 -0.7 0

4. DISCUSSION 4.1 Dose Accuracy: The expected dose was compared to the measured dose, and the error for a number of data sets for 1-GeV protons is shown in Fig. 5. The measured dose ranges from 6 to 36 μSv, and the dosimeter reproduced the dose with better then 7% accuracy for 95% of the events.

Number of Data Sets

10 μ = -2.2% 8 σ = 3.3% 6-36 μSv

6 4 2 0

-30 -20 -10 0 10 20 Percent Error (%)

30

Fig. 5. Histogram of the percent error for 1-GeV protons for a number of data sets. The doses ranged from 6 to 36 μSv. There is no significant systematic offset in the dose reconstruction, and for these doses with 1-GeV protons incident at a normal angle, the dosimeter will reproduce the dose to within 7% for 95% of the events.

These results are an indication of the practical application of the dosimeter. The information is very encouraging, but we understand it is not complete. One factor that will be studied for future beam tests is to look for the limits of the device over particles with a variety of LET. An accuracy of 10% for small LET particles for low doses (~1μSv) is feasible since many particle interactions are needed to provide these doses. For high LET particles, far fewer particles are needed, meaning fewer data points in the spectra to provide a mean over some spectral distribution. In future runs, we will explore the limits of the dose accuracy as a function of mass, LET, and number of incident particles. 4.2 Direct Interactions: Since charged particles generate charges in silicon, it is possible that the signal from a particle detection event can contain components from the scintillation flash and from direct interaction. Since the magnitude of the direct interaction will be highly directional this effect is not desirable. Fortunately the high pixel gain drastically amplifies the scintillation signal so that the effect of direct charge generation is negligible. Fig. 6 shows the spectrum during high intensity and intermediate intensity beam flux for the worst case scenario, an intense beam of high LET particles. The small signals from Fe and other heavy ions are not expected to compete with the much larger scintillation signals, since each heavy ion can only activate a single or, in rare cases, two pixels, but the light generated in the scintillator is many thousands of photons. The large beam intensities mean that multiple interactions occurred within the silicon with the 250 ns integration window. Because the experiment was operated in a parasitic mode at the NSRL, the intense iron beam deposited a wide range of energies in the silicon due to fragmentation as the device was placed behind a number of targets. A flux of only 9x107 particles per cm2 per second would cause an average of two events per integration time in the 9 mm2 device. We estimate the actual beam intensity for the high-intensity measurement to be near 3.2 x 109 Fe per cm2 per second, or enough for several hundred direct interactions per integration period.

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Equivalent Triggered Pixels

0

161

484

645

806

High Intensity Reduced Intensity

10000 Counts

323

1000

Pulse Generator

100 10 1

0

500

1000 1500 2000 2500 ADC Channels

Fig. 6. Direct interactions recorded with no scintillation material. The beam intensity is high enough to create hundreds of interactions per integration period. For beams of these intensities, the signal from the scintillation material would saturate the pulse height range of the detector.

5. CONCLUSION These experiments demonstrate that the SSPM based particle dosimeter can accurately reproduce the dose from 1 GeV protons. We have successfully demonstrated autonomous operation of the prototype, while demonstrating the large dynamic range which is required for effective operation as a space dosimeter. Development of the next generation particle dosimeter is underway. The effects of direct interaction are minimal except in extremely high and unlikely dose situations. These results are an indication of the practical application of the dosimeter.

REFERENCES [1] E. B. Johnson, E. Chapman, P. Linsay, S. Mukhopadhyay, C. J. Stapels, and J. F. Christian, "Tissue-Equivalent Solar Particle Dosimeter using CMOS SSPMs," presented at IEEE Aerospace Conference, Big Sky, Montana, 2009. [2] "Space Radiation Hazards and the Vision for Space Exploration," National Reaserch Council, Washington, D.C. 2006. [3] E. R. Benton and E. V. Benton, "Space Radiation Dosimetry in low-Earth Orbit and Beyond," Nuclear Instruments and Methods in Physics Research. Section B, vol. 184, pp. 255-294, 2001. [4] F. A. Cucinotta, P. B. Saganti, J. W. Wilson, and L. C. Simonsen, "Model predictions and visualization of the particle flux on the surface of Mars," J Radiat. Res. (Tokyo), vol. 43, 2002. [5] E. R. Benton, E. V. Benton, and A. L. Frank, "Passive Dosimetry Aboard the Mir Orbital Station: Internal Measurements," Radiation Measurements, vol. 35, pp. 443-460, 2002. [6] T. P. Dachev, F. Spurny, G. Reitz, B. T. Tomov, P. G. Dimitrov, and Y. N. Matviichuk, "Simultaneous Investigation of Galatic Cosmic Rays on Aircrafts and on Iternaltional Space Station," Adv. Space Res., vol. 36, pp. 1665-1670, 2005.

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