# S

### Scintillation counting

Scintillation counting is one of the most sensitive and effective methods for detecting radiation. It uses a photomultiplier tube coupled to a scintillator that produces light when struck by radiation. In radiation measurements, there are two parameters that should be measured: the energy of individual radiation particles and the amount of radiation. When radiation particles enter the scintillator, they produce visible light in response to each particle. The amount of light is extremely low, but is proportional to the energy of the incident particle. Since individual light flashes are detected by the photomultiplier tube, the output pulses obtained from the photomultiplier tube contain information on both the energy and amount of pulses. By analyzing these output pulses using a multichannel analyzer (MCA) a pulse height distribution (PHD) or energy spectrum is obtained and the amount of incident particles at various energy levels can be measured accurately.

### Scintillator

A material that emits light when exposed to radiation such as X-rays. Scintillators are divided into inorganic and organic scintillators. Well-known inorganic scintillators are crystals or powder of CsI (cesium iodide) doped with a small amount of activator such as Tl (thallium) to enhance the emission efficiency. Organic scintillators include naphthalene, anthracene, plastic, liquid scintillator, and lumogen. The lumogen is a material that emits light in response to UV rays, and is therefore sometimes coated on front-illuminated CCDs having no UV sensitivity.

### Short circuit current (Isc)

This is the output current that flows in a photodiode when load resistance is zero. This is called “white light sensitivity” to differentiate it from the spectral response, and is measured with light from a standard tungsten lamp at 2856 K distribution temperature (color temperature). Our product catalog lists the short circuit current measured under an illuminance of 100 lx.

### Shunt resistance

This is the voltage/current ratio of a photodiode operated in the vicinity of 0 V. In our product catalog, the shunt resistance is specified by the following equation, where the dark current (ID) is a value measured at a reverse voltage of 10 mV. $$Rsh[ohms]=\frac{0.01 [V]}{ID[A]}$$ Noise generated from the shunt resistance becomes dominant in applications where a reverse voltage is not applied to the photodiode.

### Single-mode fiber

An optical fiber designed to transmit light in the single transverse mode (electromagnetic field distribution). Single-mode fibers have low transmission loss and are not affected by modal dispersion, making them suitable for long-distance transmission. However, they require precise core alignment when connecting to a light emitter since their core diameter is small.

### Smear

In image sensors, smear is a phenomenon where a signal charge generated by intense input light leaks to the adjacent pixels or CCD transfer region and causes the original signal to become smeared (blurred). In contrast to “blooming” that occurs following saturation, smears occur even before saturation. Smears tend to occur from light at longer wavelengths rather than light at shorter wavelengths.

### SOA (semiconductor optical amplifier)

An optical amplifier using a semiconductor. The structure is very similar to a Fabry-Perot laser diode but is designed not to cause reflection at the edge. SOA enables amplification over a wide spectral range and requires fewer components than EDFA, which makes the amplifier device smaller and reduces power consumption.

### SONET/SDH

These are international standards for high-speed digital communication methods using optical fibers. SONET (Synchronous Optical Network) is a North American standard specifications established by ANSI (American National Standards Institute) based on technology developed by Bellcore Technologies (now Telcordia Technologies). SDH (Synchronous Digital Hierarchy) is an internationally standardized interface by ITU (International Telecommunication Union) based on SONET. Although differing on some minor points, SONET and SDH can be considered as nearly the same standard and allow interconnections with each other. SONET is well-known in North America, while SDH is mainly used in Europe.

### Space charge effect

When light incident on a photosensor is blocked, the carrier distribution in the depletion layer is disturbed. The carriers are then drawn to the electrodes and generate an electric field in the depletion layer in the direction opposite to the applied bias voltage. This phenomenon is referred to as the space charge effect and might degrade the response characteristics (fall time) when the incident light level is high.

### Spatial resolution

The ability of an image sensor to faithfully capture the details of an object. MTF (modulation transfer function) is usually used to evaluate the resolution of an image sensor. When an object with a sine-wave brightness distribution is imaged, the MTF indicates how the sine-wave brightness contrast varies with the spatial frequency. The spatial frequency is the number of times that a sine wave is repeated per unit length. Since the active area of a CCD consists of discrete pixels, CCDs have a limiting resolution determined by the Nyquist limit based on the discrete sampling theorem. For example, when a black-and-white pattern is viewed with a CCD, the difference between the black and white signal levels decreases as the pattern becomes finer, and finally reaches a point at which the pattern can no longer be resolved. The ideal MTF is expressed as follows: sinc* {(π x f ) / (2 x fn)} (f: spatial frequency, fn: spatial Nyquist frequency). However, because of the difficulty in creating an optical sine wave, a rectangular-wave response test chart is generally used instead. In this case, the spatial frequency response is called the CTF (contrast transfer function).
* sinc: Fourier transform of an ideal rectangular function

### Spectral distribution

The wavelength range of energy emitted by a lamp. The wavelength range varies according to the input energy, gas pressure, type of lamp (continuous vs. flash mode) and transmittance on the window material.

### Spectral half-width

Full width at half of the output maximum of the emission spectrum, expressed in wavelength (nm).

### Spectral response

The photocurrent produced by a given level of incident light varies with the wavelength. This relation between the photoelectric sensitivity and wavelength is referred to as the spectral response characteristic and is expressed in terms of photosensitivity or quantum efficiency versus wavelength.

### Stealth DicingTM process

Stealth DicingTM process is a new dicing method developed by HAMAMATSU. It uses a laser beam to form a modification layer inside a wafer and cut the wafer into chips with high quality. Since light that transmits into the material is used, no thermal damage occurs on the wafer surface. Stealth DicingTM process does not produce any cutting loss, so the chip yield per wafer can be increased to the maximum. There is absolutely no contamination such as from flying debris which is unavoidable in conventional dicing techniques, and Stealth DicingTM process is a completely dry process because no cleaning water is required.