N2

P61 P46

G1

Study of different SiPM as a photodetector for scintillator applications

D.J Herbert1, N. D'Ascenzo1, V. Saveliev2, N. Belcari1, A. Del Guerra1, F. Morsani1

1 INFN Sezione di Pisa & Dipartimento di Fisica, Università di Pisa, Pisa, Italy2 Obninsk State University of Nuclear Engineering, Obninsk, Russia

IntroductionPositron Emission Tomography (PET)

• A β+ emitting radiotracer is injected into a “patient”

• The radiotracer marks a specific function (e.g. glucose metabolism)

• The positron annihilates with an electron and produces a pair of opposite collinear 511keV photons

• A set of detectors surrounding the “patient” detects the pair of photons in time coincidence

• 3-D reconstruction gives the activity density within the body

PET camera requirements [1]- position information (~mm)- timing (~ns) - energy resolution (<20%) - all this with maximum possible sensitivity

Detectors for PET

Classic block detector (BGO then LSO) is still used but limited in spatial resolution and timing

For higher resolution, PSPMTs and pixellated scintillator arrays are used [2] but they are of limited size and expensive.

For the future there is interest in arrays of semiconductor detectors- ie) APD matrices.

- limited gain- expensive- limited size

A BC D

4 PMTs

Scintillator block(partially cut)

Proposed gamma camera for PET We plan on using a fine APD matrix to form a miniature, high-resolution gamma camera, based upon the Anger design. The scintillator is a continuous piece and scintillation light produced from gamma rays is detected by a number of the detector elements in the matrix. By calculating the centre of mass of these signals, we can reconstruct the interaction coordinates (X,Y). By stacking multiple copies of this camera, we can have high detection efficiency, whilst having depth-of-interaction (Z) information by using the first interaction.

The SiPM (Silicon Photomultiplier) is preferred over conventional APDs for a number of reasons, such as high gain at low bias, they are rugged and stable and cheap manufacturing process. In fact in many ways their performance is comparable to a conventioanl PMT. A sytstem based upon a matrix of these such dvices would permit th econstruction of a gamma-ray imaging system with the following characterisics;

Benefits of this SiPM γ-camera design [3]- low bias (50V)- signals have high S/N- compact- fast- good XYZ resolution- no dead space- cheap and simple design

SiPM Characterisation

Scintillator measurements - LSO

Summary References

The SiPM (Silicon Photomultiplier) APD represents an interesting advance in photodetection and could soon be a rival to traditional PMTs in many applications. The MRS (Metal-Resistor-Silicon) structure SiPM produced by CPTA, Russia has been characterised and tested for possible use in scintillator light detection in medical applications.

Measurement set-up:- CPTA* MRS SiPMs (600 – 1440 microcells)(4 Samples: G1, N2, P61, P46)- High stability bias supply (TTI QL564)- Custom made fast voltage preamp (based on Gali5 chip) with second output for self-triggering- CAEN V792 charge integrating ADC

SiPM is a type of avalanche photodiode operated in Geiger mode (GAPD) [4], [5], [6]. Geiger mode: large breakdown current is generated whenever a electron enters the depletion zone. The breakdown signal is constant regardless of the number of electrons present and is dependent on the quenching resistance. In this way, it gives just logical rather than proportional information. However, if many small GAPD are formed in a matrix,each with its own quenching resistance, and with the outputs multiplexed together, the total output signal is the sum of the signals from all of the microcells that were fired. The SiPM therefore gives for proportional information for moderate photon flux (Nphotons < Ncells).

Geiger breakdown

SiPM Gain

Detection efficiency

[1] J.L. Humm, A. Roszenfeld, A. Del Guerra, “From PET detectors to PET Scanners. A Review”, European Journal of Nuclear Medicine (2003), 30(11), 1574-1597[2] A. Del Guerra, A. Bartoli, N. Belcari, D.J. Herbert, A. Motta, A. Vaiano, G. Di Domenico, N.Sabba, E. Moretti, G. Zavattini, M. Lazzarotti, L. Sensi, M. Larobina, L. Uccelli, “Performance evaluation of the fully engineered YAP-(S)PET scanner for small animal imaging”, IEEE NSS-MIC Conference Record, Rome 2004 and submitted to IEEE Transactions on Nuclear Science [3] S. Moehrs, N. Belcari, A. Del Guerra, D.J. Herbert, M.A. Mandelkern, A. Motta, V. Saveliev, “A Small animal PET design using SiPMs and Anger logic with intrinsic DOI”, IEEE NSS-MIC Conference Record, Rome 2004[4] V.Saveliev, “The Recent Developments and Study of Silicon Photomultiplier”, NIM A535 (2004) 528-532[5] V.Golovin, V.Saveliev, “Novel Type of Avalanche Photodetector with Geiger Mode Operation”, NIM A518 (2004) 560-564[6] D.J. Herbert, V. Saveliev, N. Belcari, N. D'Ascenzo, A. Del Guerra, A. Golovin, “First results of scintillator readout with silicon photomultiplier”, IEEE NSS-MIC Conference Record, Rome 2004 and submitted to IEEE Transactions on Nuclear Science [7] D.J Herbert, L.J. Meng, D. Ramsden, “Investigating the energy resolution of arrays of small scintillation crystals”, IEEE Trans. Nucl. Sci., 49 (2002) pp 931 - 936

The gain is determined purely by the series quenching resistance which was calculated to be in the range of 10MΩ for each microcell. We measured the gain for different SiPM as a function of bias using precise fitting of the single photoelectron peaks. The peak spacing corresponds to the charge generated from the avalanche. From the plots it can be seen that the gain is linear and in the range 1-5 x 105

The accuracy of our measurements is currently limited by the errors on the gain calibrationof our preamplifier.

We studied breakdown process in the SIPM by plotting the current as a function of the applied bias voltage for four different SiPM samples. The current measured here is dominated by the contribution of the thermally generated carriers in the depletion region, which is studied further.... We can clearly see the voltage at which multiplication starts to occur, although the exact shape is different for each device. The region after the onset of avalanche represents the region that APDs operate in and is dominated by electron multiplication. At higher bias (higher junction field strength) holes also contribute to the avalanche, causing a positive feedback, leading to true breakdown.

From the scintillation spectra measured, it is obvious that the detection efficiency is very low in comparison with other photodetectors. The detection efficiency of the SiPM is a combination of 3 factors

The quantum efficiency of silicon (~80%) The active area of the microcells (~30%) The avalanche probability – this is a function of bias

We made measurements of the average number of photoelectrons detected, as a function of bias, for a fixed light level. A Red LED was used. In this way we obtain information on the behavior of the avalanche probability as a function as a function of over-voltage (Vbias - Vbreakdown). We see two types of behavior within the SiPM we tested, examples of which are shown on the right. In general the relationship was approximately linear in the region studied. However, the P61 showed an exponential dependence on the over-voltage. This would seem to indicate a different internal doping structure. Further analysis is required, and in addition we will repeat these measurements for other wavelengths of light.

SiPM has been shown to be a very promising photodetector. We have measured high gain at low bias voltage (5x105 at ~50V), fast rise time (800ps), excellent single photoelectron resolution, noise limited to the single photoelectron level (for moderate bias levels). The samples were found to be stable and rugged, operating over a period of several months with gain variations no more than 1% standard deviation. We have observed other interesting effects in our study of the SiPM that require further study. We saw evidence of the presence of sub-singles level counts. Evidence for these small pulses is also observed in the dark count rate plots and the dark count spectra. The presence of optical cross talk between microcells is evident in the dark count rate. We are working to isolate the contribution solely due to the cross talk from these data. The scintillation studies found light yields that were relatively disappointing, less than 100p.e, although the energy resolution was a reasonable 30%. The low light yield implies a low detection efficiency. The detection efficiency is a product of different factors. We studied the behavior of the avalanche probability with over-voltage and found a very different dependance for the P61, indicating a different structure.

Rise time: 800psDecay time: 2ns

Sub-singles pulses

Singles pulses

SiPM: matrix of n pixels (~1000) in parallel each pixel: GM-APD + Rquenching

Output signal is independent of

number of photoelectrons (binary device)

Output signal is the sum of the

signals from all fired pixels (analogue device)

GM-APD + Rquenching

- Vbias (> Vbreakdown)

n pixels

This oscilloscope screen capture shows the level of the single photoelectrons from the G1SiPM. The high 'signal-to-noise' is clearly evident, even for single photon detection. The signal is fast, even here limited by the bandwidth of our preamplifier. We measure an average signal rise time of 800ps and decay of 2ns. In addition to the single photoelectron peaks, we observe smaller peaks, that could be due to avalanches that fail to propagate fully.

This excellent single photoelectron spectrum was achieved using low light level pulsed LED. The narrowness of the pedestal shows the low noise of our system. Narrowness of the photoelectron peaks is indicative of low microcell variations and high gain stability.

Dark Rate The main source of noise from the SiPM is from thermally generated electrons that reach the depletion region and create an avalanche. Therefore, this noise should be on the level of 1 photoelectron. These events are called dark counts. We measured the dark count rate as a function of threshold and bias, using a low level discriminator and counter on the SiPM output pulses in the absence of light. Dark rates for different SiPM are shown below. A number of observations are made;

Dark rate increases significantly with bias The good single photoelectron resolution allows us, even in these rate plots,

to observe that we are seeing signals of a level higher than just 1 photoelectron.

This can be explained by the presence of optical cross-talk between microcells

• The requirement for high sensitivity at 511keV implies the use of dense scintillation-based detectors

To study the optical crosstalk, we acquired the spectra above, in the absence of light signal, at different bias, for the SiPM P46. The presence of a second peak at 45V and a third peak at 46V bias is indicative of the increase in crosstalk with increased bias. The ratio of the areas of second to first peak give an indication of the amount of crosstalk however..... Since the spectra are acquired with an integration window (~50ns), there is a probability of observing 2 counts as the sum of two dark counts instead of just cross talk. The data on the left can be used to correct for this.....

Having studied the characteristics of the SiPM we employed them in the readout of small LSO scintillation pixels of size 1x1x10mm3 to match the 1x1mm2 area of the SiPMs. The scintillator was placed in contact with the SiPM but no optical couplant was used. They were then illuminated with 511 keV gamma rays from a 22Na source. The resulting spectra for different SiPM are shown. The light yields are seen to be very low, less than 100 photoelectrons, but resulting in an energy resolution of 30%. This is consistent with expectation for LSO with light yield ~80 p.e [7]. Low light yield implies poor detection efficiency

Dubna SiPM: Energy resolution of 25% implies a light yield of about 140 p.e.

Energy resolution

~25%

Energy resolution

~30%

G1 P46 P61

* Certain SiPMs were acquired through Photonique, Switzerland.

The CPTA SiPM and a close up of the active area showing 1440 microcells