# A

### Activation energy

This is the energy required for a substance to perform a reaction. In the Arrhenius equation for calculating the speed of a chemical reaction, activation energy is used as an indicator for expressing how easily a chemical reaction occurs. The Arrhenius equation is used to calculate the service life of an LED, etc. The activation energy of LED degradation is obtained from the failure rate under several temperature conditions.

### A/D converter

Device for converting analogue signals into digital signals.

### Afterpulse

Afterpulses are pseudo signal pulses following the true signal output pulse. In an MPPC, this indicates a phenomenon that produces pulses other than signals when the generated carriers are trapped by crystal defects and then released at a certain time delay. Afterpulses can cause detection errors the same as from crosstalk and dark pulses. The lower the temperature, the higher the probability that carriers may be trapped by crystal defects, so afterpulses will increase.

### Amorphous

Noncrystalline state having no definite form. For example, when a liquid or gaseous semiconductor is cooled and solidified so rapidly that no crystals are formed, it becomes amorphous. In this state, the crystal structure has a short-distance order but does not have a long-distance order, and a tail level appears at the band gap edge, making the optical characteristics different from those of monocrystalline or polycrystalline materials.

### Amplifier/Preamplifier

A device for increasing the signal size from a photodector to make it easier to digitize.

### Amplifier bandwidth

An amplifier's bandwidth (BW) is defined as the difference between the upper and lower frequency cutoff points. The cutoff points are where the signal drops by 3 dB.

### Anisotropic etching

An etching process in which the etching speed in a particular direction is different from that in other directions. For example, when a (100) silicon substrate is alkaline etched, V-grooves are formed due to the fact that the etching speed on the (100) plane is faster than that on the (111) plane. Etching in which the etching speed is the same in all directions is called isotropic etching.

### Anode luminous sensitivity

Anode luminous sensitivity is the anode output current (amplified by the secondary emission process) per incident light flux (10-10 to 10-5 lumens) on the photocathode. A tungsten filament lamp, operated at a distribution temperature of 2856K, is used to provide the incident light. Cathode and anode luminous sensitivity are particularly useful when comparing tubes having the same or similar spectral response.
Anode luminous sensitivity is expressed in A/lm (amperes per lumen). Note that the lumen is a unit used for luminous flux in the visible region and therefore these values may be meaningless for tubes that are sensitive beyond the visible light region.

### Anodic bonding

When the flat surface of glass containing alkali metal is attached to the flat surface of silicon and heated while a voltage is being applied, an electrostatic attractive force is generated at the interface between the glass and silicon. Anodic bonding is the bonding technique that makes use of this phenomenon. During anodic bonding, the silicon side is used as the anode.

### Arrhenius equation

The equation (see below) describing the temperature dependence of chemical/physical reaction speeds, proposed by S. A. Arrhenius (Sweden) in 1889. This equation is used to calculate the expected life of a component when a major cause of degradation of the component is probably temperature.$$K=A\cdot exp \bigl( -\frac{Ea}{k\cdot T} \bigr)$$   K: reaction speed
A: constant
Ea: activation energy [eV]
k: Boltzmann's constant [eV/K]
T: absolute temperature [K]

### ASE (amplified spontaneous emission)

An optical amplifier amplifies signal light by induction radiation. However, it emits energy little by little even under conditions where no signal light is input. This is optical amplifier spontaneous emission and is referred to as ASE. This ASE is noise and degrades characteristics.

### Average/typical life - Lamp

Length of time the lamp usually operates to with 50% of stated specifications, including output energy, drift and fluctuations.
Using a lamp with a longer life leads to the reduction of maintenance cost as well as the time and running cost of equipment. Due to unique electrode structures with minimum electrode wear, Hamamatsu lamps feature unprecedented high stability over extended periods of operating time.