What is the Advantage and Disadvantage of Light Guide For Scintillator Array

In an earlier blog post, we took a closer look at the scintillation mechanism – from the absorption of ionizing radiation to the production of light pulses within inorganic crystal materials since that’s Hilger Crystals’ area of expertise. In this post on detecting scintillated light, we discuss how that scintillated light is detected and transformed to a format in which it can be usefully managed.

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When a scintillation crystal is excited by an external charged particle (e.g. alpha, beta, neutron, gamma radiation) it subsequently decays emitting light in all directions. It is important to direct that light towards an attached external detector to maximize its, well…detection. We do that by undertaking a variety of surface treatments such polishing, etching, and grinding the scintillator, as well as applying various optical reflectors. Naturally, these treatments are specific to the material in question because its geometry, emission wavelength, refractive index, and other characteristics all influence how the photons travel through the material.

Assortment of Scintillation Detectors

Getting as much scintillated light (as possible) out of a scintillator so it can be detected is no trivial task. You may recall that denser materials absorb more radiation. However, dense materials also tend to have a high optical refractive index which frequently “traps” light within the scintillator due to its multiple surface reflections. Here, too, we can employ surface treatments to reduce this effect, but there can still be a major loss of overall efficiency. Plus, not all crystals are capable of detecting all types of radiation. For example, Caesium Iodide is used to detect protons, and alpha particles and gamma radiation. Sodium Iodide (NaI) is used to detect gamma radiation, zinc sulfide is used to detect alpha particles but not protons.

Types of Scintillation Detectors

Once the light particle (aka photon) leaves the crystal it needs to be converted to an electronic signal that can be manipulated and analyzed. There are several types of optical photon light detectors that exist, including photomultiplier tubes (PMT), photodiodes (PD), avalanche photodiodes (APD), and silicon photomultipliers (SiPM) among others. Each technology converts optical photons to an electrical signal that can be manipulated as an analog or digital signal and used to determine the type and intensity of incident radiation.

How a PMT Works Source: https://commons.wikimedia.org/wiki/File:PhotoMultiplierTubeAndScintillator.jpg

Photomultiplier tubes are the oldest type of such detectors based on photosensitive materials and vacuum technology inside a glass envelope. Photodiodes, avalanche photodiodes, and silicon photomultipliers are silicon-based technologies that operate by producing electrons and holes from absorbed photons within the silicon, which is then detected with the application of low voltage.

PMTs sensitivity to light is unrivaled, even today, but its relatively large size and fragility, along with its operational requirement of high voltage impedes its flexibility and portability. On the other hand, silicon-based detectors operate at lower voltage compared to PMTs but they suffer from lower inherent gain (except APDs). They also lend themselves to mass production, and therefore lower per unit costs.

Comparison of Detector Technologies

PMT PD APD SiPM
Wavelength Sensitivity Range (nm) 150-1,700 190-13,000 190-1,700 300-900
Gain High Low High Medium
Radiation Flux Low Low Medium Low
Voltage High Low Med Low
Response Timing Fast Fast Slow Fast
Magnetic Field Sensitivity Yes No No No
Temperature Sensitivity Low High Medium High
Size Large Small Small Small
Cost High Low Med Low

Choosing a Scintillation Detector

Ultimately, the choice of scintillator and its associated photodetector would be determined by your application. No one type of detector is better than the other. Each offers advantages, and disadvantages, so it’s upon the researcher to carefully evaluate key parameters of the scintillator and the related application. The photodetector’s range of wavelength sensitivity, speed of response, signal to noise ratio and more should be matched to the emission wavelength of the scintillator, otherwise the combination of the scintillator and photodetector will not be optimized for the intended application. Additionally, physical and environmental variables such as size, sensitivity to magnetic fields, temperature, and cost factor into the decision-making process.

To help get you started, we’ve developed “Crystal Compass” – an easy 4-step tool to help you determine the best scintillator material for your application.

As an example, pixelated Lutetium Yttrium Silicate (LYSO) in the form of an array can be coupled to SiPMs to produce a high density fast detector suitable for PET imaging or any other application that requires high speed detection. Another example of a SiPM-based detector is Thallium-doped Caesium Iodide (CsI(Tl)), which offers a spectroscopic quality detector for the identification of radio isotopes. It can also be pixelated to provide positional sensitivity.

Photomultiplier Tubes (PMTs)

Light (photons) are converted into photoelectrons by absorbing them in a thin photocathode layer inside a (glass) vacuum tube. Most often a photocathode is semi-transparent and usually consist of a thin layer of evaporated Cs, Sb, and K atoms
or a mixture of them. Each photoelectron is pulled by an electric field towards a dynode and subsequently amplified. In a 10 stage PMT, the net amplification is of the order of 5 . 10(5). Each scintillation pulse produces a charge pulse at the anode of the PMT.
The process is illustrated below.

Besides in the above described pulse mode, PMTs can also be operated in current mode in which case the anode current is a measure for the radiation intensity absorbed in the scintillator. This can only be done when the photocathode is at negative potential. This allows to operate a scintillation detector is high radiation fields. The disadvantage is that all spectroscopic information is lost.

The energy resolution, coincident resolving time and stability of a scintillation detector depend to a great extent upon the type of photomultiplier tube. The selection of a proper type is fundamental to a good detector design.

The light conversion efficiency of a photomultiplier cathode is a function of the wavelength; the Quantum Efficiency (Q.E.) is defined as the chance that one photon produces one photoelectron. In the amplification process, one photoelectron produces per dynode step about 3 4 secondary electrons. With a 12 stage PMT, a typical gain in the order of 106 can be obtained. Fig.1 below shows a schematic of a PMT. It should be noted that PMTs are sensitive to magnetic fields; a μmetal shield provides adequate protection from the earth magnetic field. For operation in high magnetic fields, special PMTs are available.

There exist a number of PMT dynode structures, each with their typical characteristics. Important PMT parameters are :

•   Amplification as a function of voltage
•   Dark current
•   Pulse rise time
•   Physical size
•   Gain stability
•   Radiological background

Gain, stability and dark current depend on the used dynode materials and are a function of temperature. Pulse rise time depends on the dynode structure. For fast timing applications, so called “linear focused” PMTs are advised.

A very important factor is the sensitivity as a function of the position on the PMT entrance window. A large variation can cause a degradation of the energy resolution of a scintillation detector. This variation can be caused by a change in quantum efficiency of the photocathode or a non-uniform photoelectron collection efficiency from the cathode onto the first dynode. The above effects can be important for both small and large diameter PMTs.

From the scintillation properties table is clear that each type of scintillator has a different emission spectrum. It is important for a good performance that the emission spectrum of a scintillator is well matched to the quantum efficiency curve (for definition see above) of the PMT. To detect the fast scintillation component of BaF2 for example, it is necessary to use a PMT with quartz window since glass absorbs all light below 280 nm. The figure below shows the quantum efficiency (Q.E.) of a standard PMT with a bi-alkali photocathode. The emission spectrum of the most common scintillator NaI(Tl) is shown too. It can be seen that the overlap is very good. For other scintillation materials such as BGO, the match is less ideal.

Quantum efficiency curve of a bialkali photocathode
together with the scintillation emission spectrum of NaI(Tl).

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The gain of a PMT is temperature sensitive. The variation in gain, which depends on the photocathode and dynode material, amounts to typically 0.2 – 0.3 % per oC.

Due to their dynode stages, PMTs are usually quite bulky devices although some short versions and miniature types have been developed.

Care must be taken when PMTs are used inside magnetic fields. Although there are PMT types that have a high magnetic field immunity, this effect remains a problem.

The material of a PMT is usually glass. Glass has an intrinsic amount of 40K which contributes to the radiological background of the scintillation detector. 40K emits as well keV gamma rays as β-particles. The face-plate of the PMT can be constructed of special low-K glass. Furthermore, this background can be limited by using light guides absorbing the β-particles and creating a distance between the crystal and the PMT. The above techniques are used in so-called “low background” scintillation detectors.

On this webpage we would like to summarize the advantages and disadvantages of PMTs in conjunction with scintillation Crystals. Please see the Tabs on the top of the webpage.

For more information regarding PMTs we refer to the PMT manufacturer’s literature.

Below we would like to summarize the advantages and disadvantages of PMTs in conjunction with scintillation crystals : For more information regarding PMTs we refer to the PMT manufacturer’s literature.

Photodiodes

In a photodiode, the scintillation photons produce electron-hole pairs that are collected
at respectively the anode and the cathode of the diode. Most frequently, reverse biased PIN photodiodes are used having a low capacitance and low leakage current.

When photodiodes are optically coupled to a scintillation crystal, each scintillation light pulse will generate a small charge pulse in the diode which can be measured with a charge sensitive preamplifier. Alternatively, the current produced in the diode can be measured.

The quantum efficiency of silicon photodiodes is typically 70% between 500 and 900 nm but decreases rapidly below 500 nm as shown in the figure below. It is clear that the highest signals can be expected from scintillation crystals that have an intense emission above 500 nm. CsI(Tl), characterized by a large scintillation intensity with a maximum at 550 nm, are therefore well suited to couple to photodiodes.

In contrary to photomultiplier tubes, photodiodes do not require a high voltage (HV) power supply but only a bias voltage of about 30 V. Photodiodes are thin, rugged and insensitive to magnetic fields. Furthermore, the output signal from a crystal/photodiode detector is very stable due to the absence of drift of the diode gain since no charge amplification takes place in the device itself. Photodiodes are thin (several mm) which can be advantageous.

.

Quantum efficiency curve of a silicon photodiode
together with the emission spectrum of CsI(Tl).

Due to the small signal generated by the photodiode, it is necessary to employ a high quality charge preamplifier in order to keep the noise level as low as possible. Noise is an intrinsic problem to standard photodiodes. In silicon PIN photodiodes, the created number of primary electronhole pairs (eh pairs) is not increased by amplification. The PIN photodiode is a unity gain device. The thickness of the silicon used is typically 200 500 μm. Coupled to a conventional (low noise) charge sensitive preamplifier, the substantial capacitance of the device (40 50 pF/cm2 for 200 and 300 μm wafer devices) is mainly responsible for the noise which determines for a large part the energy resolution of the detector. Also the dark current of PIN photodiodes (1 3 nA/cm2 at full depletion) may contribute significantly to the noise, especially at larger shaping times. The dark current increases as well with increasing surface area as with increasing temperature.

As long as there is enough light per event available, every scintillation event can be detected using photodiodes. However, due to the intrinsic noise there is a lower limit on the energy of the radiation that can be detected. For a small (1 cm3) CsI(Tl) cube coupled to a 10 x 10 mm2 photodiode the best lower energy limit reported amounts to approx. 40 keV. From the above noise numbers and the electronhole pair yield of the scintillator / photodiode combination, the noise contribution to the energy resolution can be calculated. The figure below shows a pulse height spectrum measured with a photodiode scintillation detector.

Example of a pulse height spectrum of 662 keV gamma rays
absorbed in a 10x10x50 mm3 CsI(Tl)scintillation crystal
read out by a 10×10 mm PIN photodiode at 20 degrees C

At increasing temperatures, the dark current of the photodiode increases. This limits the use of scintillation photodiode detectors to temperatures below 50 C.

Below we summarize the advantages and disadvantages of photodiode scintillation detectors in conjunction with scintillation crystals for pulse counting :

Photodiodes can also be used in DC mode to read out a scintillation crystal. Capacitance and leakage current are less important then since the diode is used unbiased. This mode of operation is used for applications where radiation intensities are high and close packing of arrays is scintillation crystals is required such as in medical CT scanners.

The low level noise limit can be overcome by using so called “Avalanche Photodiodes“, APDs. In these devices an internal amplification enables to detect also X-rays of lower energy. However, an external voltage of at least several hundred Volts is required and the amplification is a strong function of temperature (gain stability). Also the leakage current of APDs at room temperature is relatively high. APDs are currently available in approx. 1 cm diameter size maximum. APD signals are much faster than signals from PIN diodes (ns range) and are mostly used for fast timing with small scintillation crystals or when operation in a magnetic field is mandatory.

All diodes are susceptible to radiation damage induced by particles or gamma-rays which usually results in an increase in the dark current.

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