YSO and SiPM Radiation Detection Advantages

1. Introduction

1.1 An Introduction to the Scintillation Detector

A gamma ray interacting with a scintillator produces a pulse of light, which is converted to an electric pulse by a photon sensor. Scintillation detectors are one of the oldest and most widely used means of radiation detection and measurement.

1.2 Comparison with Other Radiation Detectors

• High detection efficiency for different types of radiation
• Capability to measure energy spectra
• Very good timing
• High counting rate capabilities
• Great variety in size and constitution

2. Definition

A scintillator is a material which produces visible light when ionizing radiation interacts with it, a process known as fluorescence. By measuring the light output or something proportional to it (e.g. voltage pulses), we have a measurement of the incident radiation intensity.

2.1 An Introduction to the YSO(Ce)

Y₂SiO₅ doped with Ce (YSO:Ce) is a member of the rare earth oxyorthosilicates that also includes the well-known Lu₂SiO₅:Ce (LSO:Ce). YSO:Ce itself is potentially useful as a detector of relatively low energy X-rays or gamma rays.

2.2 Comparison with Other Scintillators

	NaI	BGO	LSO	YSO
Effective Atomic No. (Z)	51	74	66	34
Density	3.67	7.13	7.4	4.54
Relative Light Output	100	15	75	120
Peak Wavelength (nm)	410	480	420	420
Decay Const. (ns)	230	300	40	56
Fragile	Yes	No	No	No
Hygroscopic	Yes	No	No	No
Radioactivity	No	No	Yes	No

 

3. Photon Sensor

3.1 An Introduction to the Silicon Photomultiplier

The silicon photomultiplier (SiPM) addresses the challenge of detecting, timing and quantifying low-light signals down to the single-photon level. Traditionally the role of the photomultiplier tube (PMT), avalanche photodiode (APD), or photodiode (PIN) with high-gain amplifier; the silicon photomultiplier now offers a highly attractive alternative that closely mimics the low light detection capabilities of the PMT while offering all the benefits of a solid-state device. The SiPM offers low voltage operation, insensitivity to magnetic fields, mechanical robustness and excellent uniformity of response. Due to these traits, the SiPM has rapidly gained a proven performance in the fields of radiation detection and imaging.

3.2 Comparison with Other Photon Sensors

Low-light photon detectors constitute the enabling technology for a diverse and rapidly growing range of applications: nuclear medical imaging, radiation detection, spectroscopy or meteorology. All these fields require detectors that serve to quantify and/or time stamp light signals with anywhere from 1 to ~1000 photons per event. The ideal detector provides a response proportional to the incident photon flux and incorporates an internal gain mechanism, yielding signals of sufficient magnitude to be easily processed. It should offer sub-nanosecond response times and broad spectral sensitivity, be robust, easy to operate, and only generate manageable amounts of noise or dark count rates.

To date the photomultiplier tube (PMT), a well-established and widely available vacuum tube device, has been the detector of choice for such applications. The semi-transparent photocathode deposited inside the entrance window inherently limits the PDE they can achieve, with typical PMTs having about 20% at 420 nm. A gain of 1–10 e6 is achieved at the cost of a high bias voltage of 1–2 kV, which requires the use of costly high-voltage power supplies. PMTs are generally stable and low noise, but are bulky and delicate due to their vacuum tube structure. They can also be adversely affected by magnetic fields, which will limit their suitability for some applications.

Solid state devices have many practical advantages over the PMT, and this led to the PIN diode being used in applications where PMTs were too bulky or delicate, or where high voltages were not possible. However, PIN diodes are severely limited by their complete lack of internal gain.

The avalanche photodiode (APD) is a more recent technology, an extension of the simple PIN diode. Here the reverse bias is raised to a point where impact ionization allows for some internal multiplication, but is below the breakdown bias where the Geiger mode would take over. In this way, a gain of around 100 is achieved for a bias of 100–200 V. With special manufacturing, it is possible for gains of several thousand to be reached using an HV bias of greater than 1500 V. While the gain may be lower than that of a PMT, APDs have the advantage of a PDE which can be greater than 65 percent. Additionally, a compact size, ruggedness, and insensitivity to magnetic fields are other advantages. The main drawbacks are the excess noise (associated with the stochastic APD multiplication process) and in an important trade-off: The capacitance increases with increasing device area and decreasing thickness, whereas the transit times of the charge carriers increase with increasing thickness, implying a performance trade-off between noise and timing. They are limited in size to ~10 mm diameter.

The SiPM has high gain and moderate PDE (~20%), very similar to the PMT, but has the physical benefits of compactness, ruggedness and magnetic insensitivity in common with the PIN and APD. In addition, the SiPM achieves its high gain (1 e6) with very low bias voltages (~30 V) and the noise is almost entirely at the single photon level. Because of the high degree of uniformity between the microcells the SiPM is capable of discriminating the precise number of photoelectrons detected as distinct, discrete levels at the output node. The ability to measure a well resolved photoelectron spectrum is a feature of the SiPM which is generally not possible with PMTs due to the variability in the gain, or excess noise. Despite the fact that the SiPM is sensitive to single photons, its dark count rate of ~100 kHz/mm² at room temperature renders it unsuitable for use for applications at very low light levels.

3.3 Comparison Table of Photon Sensors

	PIN	APD	PMT	SiPM
Gain	1	102	106	106
Operational Bias	Low	High	High	Low**
Temp. Sensitivity	Low	High	Low	Low
Mechanical Robustness	High	Medium	Low	High
Ambient Light Exposure?	OK	OK	No	OK
Spectral Range	Red	Red	Blue/UV	Green
Readout/Electronics	Complex	Complex	Simple	Simple
Form Factor	Compact	Compact	Bulky	Compact
Large Area Available?	No	No	Yes	Yes
Sensitive to Magnetic Fields?	Yes*	Yes*	Yes	No
Noise	Low	Medium	Low	High
Rise Time	Medium	Slow	Fast	Fast
	* Due to the requirement for the external electronics to be located close to the detector
	** SiPM from SensL, having an operational bias of 30 V, meeting the requirement of the Extra Low Voltage Directive