performance of rena-3 ic with position-sensitive solid ... · the rena-3 specifications are listed...

Performance of RENA-3 IC

with Position-Sensitive Solid-State Detectors

Tumay O. Tumer*, Victoria B. Cajipe, Martin Clajus, Satoshi Hayakawa and Alexander Volkovskii

NOVA R&D, Inc, 1525 Third Street, Suite C, Riverside, CA 92521, USA

ABSTRACT

The RENA-3 (Readout Electronics for Nuclear Applications) is a multi-channel mixed-signal integrated circuit (IC) developed for the readout of position-sensitive solid-state detectors with excellent energy resolution. We will present results of experiments characterizing its performance as used with a variety of spectroscopy-grade detectors currently available in the industry, notably CZT pixel arrays as well as other detector configurations. The merits of specific RENA-3 design features vis-à-vis different detector applications will also be discussed.

Keywords: Mixed signal Application Specific Integrated Circuit (ASIC), Multi channel mixed signal Integrated Circuit (IC), CdZnTe (CZT) detector, Imaging x-ray detector, Position sensitive detector, Solid-state detector, Radiation detection, and Gamma-ray spectroscopy.

1. INTRODUCTION

The RENA-3 is a one-dimensional (1D), low-noise, 36-channel, self-resetting, charge sensitive amplifier/shaper integrated circuit (IC) (Figure 1) designed for use with position-sensitive, spectroscopy-grade radiation detectors and based on the original RENA-2 IC [1,2]. It has in recent years been chosen to provide the front-end electronics in advanced prototype instruments for various applications – from space physics research [3] through medical imaging [4] to homeland security [5-7]. These instruments for the most part employ CZT in the form of planar sensor assemblies or monolithic pixel/strip arrays. Whereas the RENA-3 was developed largely to address the needs of the CdTe, CZT and Silicon community, a level of adaptability was designed into this IC to allow its use across a broad range of charge output solid state and scintillation radiation detectors.

Correspondence: Email: [email protected]; Telephone: (951) 781-7332; & Fax: (951) 781-0178.

* [email protected]; phone (951) 781-7332; fax (951) 781-0178; www.novarad.com

Figure 1: On the left is the actual silicon layout of the RENA-3 IC using 0.6 micron process and on the right is the photograph of the RENA-3 IC placed on top of a dime to show its size.

Hard X-Ray, Gamma-Ray, and Neutron Detector Physics X, edited by Arnold Burger, Larry A. Franks, Ralph B. James,Proc. of SPIE Vol. 7079, 70791F, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.797750

Proc. of SPIE Vol. 7079 70791F-1

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Figure 2 shows photographs of the RENA-3 IC wire bonded inside a CQFP package. The RENA-3 die size is approximately 6.9 mm 6.4 mm.

RENA-3 IC features include low-noise performance, its self-trigger capability, and the versatility it offers by providing many different peaking times, different readout modes, and the daisy-chain option. The key design features for the RENA-3 IC are listed in Table I.

Table 1. Key design features of the RENA-3 ASIC.

Signal range Two full-scale ranges; 56 and 338 ke, externally selectable for each channel,(9fC and 54fC) or (256keV and 1.5Mev for CZT)

Self-trigger output User selectable comparator threshold for self-trigger output

Input polarity Positive or negative, selectable channel-by-channel

Number of channels 36 (2 with test points for characterization)

Noise 150 e RMS and 280 e RMS for low and high signal ranges, respectively

Noise optimization Preamp optimized for 2 pF and 9 pF detector capacitance, externally selectable

DC leakage current Tolerant to 5nA per channel

Power consumption 6mW per channel with all features (reduction possible by powering down certain features)

Fast timing output 30ns (for timing/coincidence measurements @ peaking time 0.29 s)

Channel-to-channel time difference Implemented to enable determination of time difference between 2 coincident pulses for 5 ns accuracy

Bias current Adjustable through power supply current into input transistor

Trigger comparator thresholds Individually adjustable by internal 8 Bit DACs for each channel

Peaking (shaping) times 0.29 s to 38 s in 16 steps

High count rates Using pole zero cancellation, capable of over >200,000 counts per second per chip with digital readout system (external ADC will determine actual rate)

Key gamma signals 14 keV, 60 keV, 141 keV, 511 keV, 662 keV, up to 1.33 MeV

System components Pipeline A/D converter, FPGA state machine controller, data FIFO

Interface Minimum pin count and support component count. Up to 8 ICs can be daisy-chained.

Readout mode Maximum flexibility through programmable hit register

Deadtime per event <5 s for digital interface. Overall deadtime depends on the external ADC used.

Other, innovative features include user-selectable dynamic range, fast trigger output for coincident event detection and the ability to provide channel-by-channel time difference information. The comparator thresholds are individually adjustable through an 8 bit DAC on each channel. This allows accurate and uniform threshold setting throughout the

Figure 2: On the left is the RENA-3 IC wire bonded inside a CQFP package and on the right is the photograph of the packaged RENA-3 IC.


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1 I I

detector. Two very important new features for space deployment are the adjustable power consumption by limiting the current flow to the input transistor and the radiation hardness inherent to the 0.6 micron CMOS process. The peaking times are made adjustable from about 0.4 to 40 microseconds, which makes it suitable for a wide range of detectors, from CdZnTe to HgI2. The new IC incorporates a pole zero cancellation circuit to handle large rates without significant pileup. The input amplifier is designed tolerant to leakage current so that the IC can be used DC coupled which eliminates the need to use capacitive AC coupling. The functionality of the new RENA-3 IC is dramatically improved by eliminating unnecessary connections and interface. Another important new feature is the inclusion of 32+4 extra channels to allow the connection of the cathode side into the same IC. The two edge channels, 1 and 36, include internal test points for testing. However, it is not recommended to use the two edge channels, 1 and 36 for data acquisition because they are tagged with pads at different sections of the internal circuitry for testing.

2. RENA-3 DESCRIPTION

RENA-3 IC is a 36 channel mixed signal ASIC [1] with low noise self resetting charge sensitive preamplifiers at the input of each channel (Figure 3). The extra channels are built in to allow connection to cathodes of the solid state detectors such as CdZnTe. It has two externally selectable dynamic ranges for wide energy range applications ( 9 fC &

54 fC). It is designed for ultra low noise (<150 & <280 e rms input referred noise for each range, respectively, corresponding to dynamic ranges 256 & 1,500 keV). The inputs are single ended to improve noise performance. It has selectable input signal polarity at each channel. Channel-by-channel selection of negative or positive input polarity allows signal detection from both anode & cathode on the same IC. It has sparse readout mode for reading out only the channels which contain data. It has on chip built in generalized neighbor readout mode which can be applied for both strip and also 2-D pixel array detectors. RENA-3 IC features low power operation < 6mW/Ch for space based astrophysics applications. For example, power consumption is adjustable and power down is available for channel-by-channel basis and on unused features. It is optimized for two input capacitances (2 pF & 9 pF externally selectable). One of the most important features of RENA-3 is its self-resetting charge-sensitive input preamplifier (Figure 3). The self-resetting function restores the increase in pedestal produced by the detector leakage current and eliminates the

Figure 3: New RENA-3 IC single channel block diagram showing all the analog circuitry and some of the control system.


need for AC coupled connection between the detector channel and the input amplifier. Therefore, RENA-3 has been designed with built-in tolerance for detector leakage current (< ±5 nA) and it can be DC coupled to most position sensitive solid state detectors.

The RENA-3 IC is designed to have a self trigger output capability using a comparator (Figure 4). The self trigger output is especially important for nuclear and radiography applications where the incident photon is random and its arrival time is not known. Figure 4 shows a more complete block diagram of most of the IC and not only the analog section which is shown in more detail in Figure 3. Some applications require accurate (low jitter) fast timing signal for coincident imaging such as positron emission tomography (PET) and double Compton scatter detectors. RENA-3 is designed to accommodate such applications by

incorporating a fast low jitter shaping and comparator circuits on each channel (Figure 4). This circuit produces a fast timing trigger output which is separate from the slow event trigger output. The slow trigger output provides accurate pulse-height discrimination for event triggers and must be used for recording an event, and the fast trigger should be used in conjunction with the event trigger for coincidence determination. Another interesting functionality built into theRENA-3 IC is the time stamp circuit at each channel (Figures 3 and 4). This circuit utilizes user-supplied time variablesignals to produce data on channel-to-channel pulse arrival time difference. This feature can be useful if information on the arrival time differences for multiple simultaneous events is required.

RENA-3 also has a second slow comparator (Figure 3) that triggers whenever the input signal exceeds the dynamic range of the amplifiers.

Figure 4: Preliminary schematic diagram of the RENA-3 ASIC. Only one channel (Channel k) is shown; connections to adjacent channels are indicated where applicable; the slow and fast signals are shown multiplexed.

Peak Detect Out

Fast Out

Analog Signal Processing

k-1

k

k+1

setk

from ch. k-1

to ch. k+1

Read Sequence Logic

ANALOG OUT (to A/D)

HRDI

NOTES:

1. In general, configuration control

logic is not shown here.

2. All signals are shown here as single-

ended, although in reality some might

better be differential.

3. Signal names in boxes are external

connections.

HRDO NEXT

EVENT TRIGGER

DETECTOR

TEST

VU Input

VV Input

Threshold DAC

Comparator

ACQUIRE

enablek

S Q

Slow

Out

Mux

Fast

Comparator

TIMING TRIGGER

overloadkOVERLOAD

Hit/Read

Shift Register

AOUT Bus

AOUT Bus

CHANNEL k


In RENA-3 provisions were made to enable fine tunable compensation for channel-to-channel and chip-to-chip fabrication variations. For example an adjustment of comparator thresholds for each channel is achieved by using a DAC at each comparator threshold.

The RENA-3 IC may be used in fast photon counting applications. To achieve smooth fast photon counting at short peaking times, a pole zero cancellation circuit which reduces pulse pileup for high input signal rates was incorporated into the IC; this. Up to eight RENA-3 ICs can be wired together to be read out as a single chip with 288 channels. This feature will be useful for large channel low rate applications. It has a low noise differential analog output to preserve its low noise high energy resolution capability.

3. RENA-3 SPECIFICATIONS AND APPLICATIONS

The RENA-3 specifications are listed in Table 1, which lists the many new features of the RENA-3 IC. The new features shows that the RENA-3 IC is a versatile and flexible IC with multiple application capability for different fields.

RENA-3 can therefore be used for many different applications in medical, industrial, NDI and security fields. Most of these applications are for imaging x-rays and gamma rays. However, it can be used with a variety of detectors and systems that produce charge pulses in the time frame of picoseconds to microseconds. It can be used as a single IC solution for applications that require depth of interaction determination for thick detectors by measuring signal from both the anode and the cathode on the same chip due to selectable input polarity. The built in fast timing signal with low jitter and time stamp circuit can be used with detectors that have fast coincidence or fast timing requirements such as PET and Compton double scatter detectors.

4. RENA-3 EVALUATION SYSTEM

A RENA-3 Evaluation System is developed to test the new IC. Figure 5 shows a photograph of the RENA-3 Evaluation System. The mother board can service several ICs. The daughter board shown on the right can hold up to two RENA-3 ICs with top receptacle can be used for removable mounting. The daughter board can be customized.

Figure 5: RENA-3 Evaluation System mother and daughter boards. The daughter board can accommodate up to two RENA-3 ICs as shown on the right with two standard edge connectors to mount detector(s). Using the top receptacle RENA-3 IC can be mechanically clamped to the board for initial acceptance tests. The large XILINX IC on the left is the FPGA that controls the evaluation system and the RENA-3 IC.


Noise vs. Input Capacitance

450

400

350

300

250a)

2000z

150

100

50

0

,—

-- .—

I-___•Low input

capacitance(2 pF)

• High InputCapacitance(9 pF)

0 5 10 15

Inp.t Cpcitnce (pF)20 25

5. RENA-3 DIAGNOSTIC FEATURES

Every channel of the RENA-3 has a test pulse input (707 mV step injects full scale signal 53 fC). The test inputs can be turned on and off individually for each channel. This capability allows for testing RENA-3 without a detector. RENA-3 also has several diagnostic modes. For example, the follower mode bypasses the peak hold circuit and connects the shaper output directly to the output of the channel. The force-enable mode provides the capability of continuous monitoring of the peak detector or shaper output of any externally selected channel. There are also probe pads provided on the chip layout to test critical sections of the IC using probes.

6. RENA-3 TEST RESULTS

Tests were performed on the fabricated RENA-3 ICs using the above-described evaluation system (Figure 5). Figure 6 shows noise measurements for different detector input capacitances plotted for the two externally selectable detector capacitance ranges, 2 and 9 pF. The preliminary measurements show that 0 pF detector capacitance results in approximately 140 and 150 e rms noise for the externally selectable 2 an 9 pF detector capacitance ranges.

Figure 7 shows the RENA-3 linearity measurement. Linearity measurement is made by measuring the output pulse height against set input test pulse height. RENA-3 was configured for maximum channel gain. The low energy range (9 fC) is used for this measurement.

Figure 8 demonstrates the effect of the Pole Zero Cancellation circuit. The undershoot after the event pulse is normally produced in these type of circuits. The pole zero cancellation circuit eliminates the undershoot and allows for faster counting at higher rates.

Figure 9 shows 57Co spectra taken from 32 channels simultaneously. The spectra from 16 channels are shown here. The differences between the spectra are due to the response of the CZT pixel detector and not due to the RENA-3 IC. There

were a few channels, which did not show any spectra such as channel 7. These channels were the same channels, which also did not produce spectra using other test methods or readout electronics. Variation of spectra from pixel to pixel due

to CZT detector response is clearly seen.

Figure 6: Noise for different detector input capacitance values are measured and plotted for the two externally selectable detector capacitance ranges, 2 and 9 pF.


1800 - —-—-—-—-—

'VV

0 1 2 3 4 5

Pulser amplitude [fC]

1600=1400

01200

- 1000=a 800E

600

400

200

06 7 8 9

Single-element CZT detectors equipped with Frisch collars have been developed by the group of D.S. McGregor at Kansas State University (KSU) [8,9]. The group recently presented new methods designed for the commercial scale production of detectors of this type. Frisch collar detectors have reached an energy resolution as low as 1.3% FWHM at 662 keV [8], or 1.15% with very moderate cooling [8], without the need for correcting the data for the depth of interaction (DOI) of the photons in the detector. This allows for simpler electronics and data processing – desirable in most commercial applications - compared to monolithic pixel detectors, for which somewhat better energy resolution has been achieved with the help of DOI corrections. Exploratory spectroscopy measurements using KSU Frisch collar CZT detectors read out with a standard RENA-3 evaluation system were performed jointly by KSU and NOVA. The spectral results for tests using Co-60, Na-22 and Cs-137 sources with 3 3 5 mm3 and 6 6 7 mm3 Frisch collar CZT detectors are shown in Figures 10a, 10b and 11, respectively. The data clearly show that the RENA-3 higher amplitude range stretches out to 1.5 MeV. This is important relative to nuclear monitoring (e.g., weapons grade uranium is 7% U-235 which produces about 1 MeV gamma ray line). Figure 10b shows a Cs-137 spectrum from one of the preliminary measurements. The energy resolution at 662 keV obtained in the particular example of the 3 3 5 mm3 detector is 2.6% FWHM, which leaves room for improvement. It should be noted that these were preliminary measurements and the RENA-3 system board layout for connecting these detectors was not optimized for use with these detectors and that the detector connection and experimental conditions during these tests were also not optimal.

a) b)Figure 8: a) Pole zero cancellation circuit on the RENA-3 IC is turned off and b) Pole zero cancellation is turned on.

Figure 7: RENA-3 IC linearity measurement. Linearity measurement is made by measuring the output pulse height against set input test pulse height. RENA-3 was configured for maximum channel gain. The low energy range (9 fC) has been used for this measurement.


Ch 2M Ch 3M Ch 4MCh 1M

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Ch 14M Ch 15M

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Ch h6M

Bin

200

15O

Cs-I 37 V-I 350

Energy ]ADC values]

Na-22 V-I 350

Energy ]ADC Values]

Figure 9: Simultaneously acquired Co-57 spectra from a 2x16=32 pixel linear CZT detector array. All 32 pixels of the detector were tested simultaneously but only 16 are shown here. RENA-3 IC is set as follows: Feedback C=15 pF, Gain=5. Shaping = 1.9 us. Chip 0 channel 4 was selected for high count spectrum. Figures show the spectra with only 65k points. The actual file has about 200k points but the Excel can only process 65k of them. Spectra for channels 1, 2, 3, 4, 5, 6, 8, 9, 10, 11, 12, 13, 14, 15, 16 and 17 are produced for Chip 0 and multi-pixel spectra results are shown here.

(a) (b)

Figure 10: (a) Co-60 and (b) Na-22 spectra obtained from KSU’s Frisch collar CZT detectors.


450

400

350

300

250

200

150

100

50

0

5000 6000 7000 8000 9000 10000 11000 12000 13000ADC value

7. CONCLUSION

Test results of the RENA-3 IC are encouraging. Full characterization is currently underway with an eye on performance parameters and functionalities that can find immediate use in instrument prototyping. RENA-3 can be modified and optimized for specific x-ray and gamma-ray spectroscopy and imaging applications in the fields of medical, small animal, security and industrial imaging, and astrophysics.

8. ACKNOWLEDGMENTS

The RENA-3 development work was supported in part by OTTC-CCAT grants GT30420 and GT60615 administered by the OTTC of CSUSB. We acknowledge contributions to this work through detector use and information sharing from Dr. J.L. Matteson at University of California San Diego (UCSD) and Prof. D.S. McGregor at Kansas State University (KSU). We also thank D. Ward and G. Visser for their contributions to the design.

9. REFERENCES

[1] Kravis, S.D., Maeding, D.G., Tümer, T.O., Visser, G., Yin, S., “Test Results of the Readout Electronics for Nuclear Applications (RENA) Chip Developed for Position-Sensitive Solid State Detectors,” SPIE Symp. Proc. 3445, 374 (1998), available at http://www.novarad.com/pages/documents/RENA_test_results_SPIE_1998.PDF.

[2] a) Tümer , T.O., V.B. Cajipe, M. Clajus, H. Flores, C.A. Shirley, G. Visser, and D. Ward, Preliminary Test Results of RENA-2 ASIC Developed for Position-Sensitive X-ray and Gamma-Ray Detectors, contribution to the 2002 IEEE Nuclear Science Symposium, Norfolk, VA, November 2002; b) Tümer, T.O., V. Cajipe, A.C. Shirley, M. Clajus, S. Hayakawa, G. Visser, J. Matteson, and D. Ward, Preliminary test results of a low-noise integrated circuit (IC) developed for position sensitive solid state detectors, contribution to the International Symposium on Optical Science and Technology (SPIE’s 48th Annual Meeting and Exhibition), San Diego, CA, August 2003.

[3] J.L. Matteson, R.T. Skelton, M.R. Pelling, S. Suchy, V.B. Cajipe, M. Clajus, S. Hayakawa and T. Tümer, Three-dimensional readout of CZT detectors with the RENA-3 ASIC, presented at the 15th International Workshop on Room-Temperature Semiconductor X- and Gamma-Ray Detectors, San Diego, CA, Oct. 29 – Nov. 4, 2006.

[4] Craig S. Levin, Angela M.K. Foudraya and Frezghi Habtea, Impact of high energy resolution detectors on the performance of a PET system dedicated to breast cancer imaging, Physica Medica, Volume 21, Supplement 1, 2006, Pages 28-34.

Figure 11: Cs-137 spectrum obtained from KSU’s Frisch collar CZT detectors.


[5] Y. Du, W. Li, B. Yanoff, J. Gordon, and D. Castleberry, 4 pi direction sensitive gamma imager with RENA-3 readout ASIC, Proc. SPIE 6706, 67060V (2007).

[6] W. Li, Y. Du, B. D. Yanoff, J. S. Gordon, “Impact of Temperature Variation on the Energy Resolution of a 3D Position Sensitive CZT Gamma-Ray Spectrometer” IEEE Nuclear Science Symposium, Honolulu, HI, Oct. 26-Nov. 3, 2007, Conference Record, Vol. 3, 1809-1815 (2007).

[7] G. Cardoso, J. L. Matteson, M. A. Capote, G. J. Batinica, E. Stephan, F. Duttweiler, T. Skelton, T. Gasaway, R. E. Rothschild, G. Huszar, M. R. Pelling, “Second Generation Directional Gamma Radiation Spectrometer,” presented at the IEEE Nuclear Science Symposium, Honolulu, HI, Oct. 26-Nov. 3, 2007.

[8] A. Kargar, A.M. Jones, W.J. McNeil, M.J. Harrison, and D.S. McGregor, CdZnTe Frisch collar detectors for -ray spectroscopy, Nucl. Instrum. and Meth. A 558, 497 (2006).

[9] W.J. McNeil, D.S. McGregor, A.E. Bolotnikov, G.W. Wright and R.B. James, Single-charge-carrier-type sensing with an insulated Frisch ring CdZnTe semiconductor radiation detector, Appl. Phys. Lett. 84, 1988 (2004).


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