Photomultiplier tubes output a small amount of current even when operated in a completely dark state. This output is called the dark current and the resultant noise is an important factor in determining the lower detection limit. The above graph shows typical dark currents from micro PMTs.
Photomultiplier tubes have a fast time response and can capture very short events. The above graph shows a typical output waveform when a light pulse with a width of 70 ps is input to a micro PMT.
The time interval between the arrival of light at the photocathode and the instant when the anode output current reaches its peak amplitude is called the electron transit time. The transit time spread usually called T.T.S., indicates fluctuations in the electron transit time measured when the photocathode is fully illuminated with single photons and is defined as the FWHM (full width at half maximum) of the fluctuations in the histogram.
An intense light pulse input to the photocathode causes a large current to flow in the latter dynode stages that induces current saturation. This causes the output current to deviate from its ideal linearity. The above graph shows pulse linearity characteristics of micro PMTs.
Ripple noise is caused by the electronic oscillator of the built-in power supply. This noise signal can be observed on an oscilloscope along the baseline in a low voltage range by feeding the output signal to the oscilloscope while no light is incident on the micro PMT.
This uniformity is the variation in sensitivity relative to the incident light position on the photocathode. The above graph shows an example of anode output measured by scanning a 1 mm diameter light spot over the photocathode surface of a micro PMT at a pitch of 0.1 mm in the X and Y axis directions.
The anode sensitivity of photomultiplier tubes is affected by the ambient temperature. Temperature characteristics for anode sensitivity are wavelength-dependent and the temperature coefficient generally changes from a negative value to a positive value near the long wavelength limit. The above graph shows temperature coefficient data for each photocathode.
An external magnetic field causes photoelectrons in a photomultiplier tube to deviate from their normal trajectories, causing a loss of gain. The extent of the loss of gain depends on the direction of the magnetic field. The above graph shows effects from magnetic fields on the output of a micro PMT, indicating that the magnetic field in the Z direction has the largest effect on the output.
Some dark current pulses are generated in a photomultiplier tube during operation even if no light is incident on it.
Dark count is the number of dark current pulses per second (s-1) and indicates the approximate lower limit of signal detection.
When light is randomly incident on a photomultiplier tube, the output pulses begin to overlap each other as the light level increases and the count value is no longer proportional to the light level. If the incident light level greatly exceeds the count linearity, a signal is output to indicate an excessive light input.
When the number of measured pulses exceeds 106 s-1, counting errors start to appear due to pulse overlap. One method for improving the count linearity utilizes a correction formula to find the approximate values. The above graph shows improved count linearity characteristics obtained by applying this correction formula.
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