RatioMaster

Click here to view Detector Technical notes.

Click here to view the Model 810/814 PMT Housing User's Manual.

General Information

For spectroscopy experiments in the ultraviolet-visible-near infrared region of the spectrum, a photomultiplier tube (PMT) is perfect for the detection of light. PMTs can be used from 180 to 1200 nanometers. A PMT is extremely sensitive, allowing the measurement of very low levels of light. It has a wide dynamic range, so it can also measure high levels of light. It is very fast, so rapid spectral events can be reliably monitored.

PTI's 800-Series Photomultiplier Detectors include everything needed to measure the intensity of light in a compact, attractive package that attaches conveniently to your instrument.

The Model 814 Analog/Digital PMT Detector allows you to choose between analog and photon-counting detection. In analog mode, you don't even need any external data gathering, displaying or recording equipment--the analog signal is displayed directly on the unit's built-in LCD meter. Other unique features of the 800 Series includes the ability to measure the signal directly from the PMT anode.

The signal from an 800-Series PMT Detector can be processed in a variety of ways. Depending on your application, you can use a digital multimeter, a recorder, or a computer. PTI's new SID-101 monochromator controller accepts the output from any 800-Series housing and sends it to the com port of any PC.

Detection Techniques: Which is Best?

Photon counting is the best way to operate a photomultiplier tube to measure low light levels. In photon counting mode, individual photons that strike the photocathode of the PMT are measured. Each photon event gives rise to a pulse, or a count, at the output. The number of pulses, or counts per second, is proportional to the light impinging upon the PMT. The tube is typically operated at a constant high voltage where the PMT is most sensitive.

Because of noise from various sources in the tube and the electronics, the output of the PMT will contain pulses that are not proportional to the light input. The detection system must reject these spurious pulses. It does this with a discriminator that electronically "discriminates" a low-level noise signal from a higher level signal from a photon event.

As the number of photon events increase at higher light levels, it will become difficult to differentiate between individual pulses and the photon counting detector will become non-linear. This usually occurs at 1-1.5 million counts per second. In that case, decrease the light level or switch to analog detection.

In analog detection, the pulses from the photocathode are effectively averaged by the high impedance at the anode and the stray capacitance. The resultant photocurrent is converted to a voltage that is proportional to the intensity of the light striking the photocathode of the tube.

The high voltage driving the tube may be varied to change the sensitivity of the PMT. Analog detection is usually used to measure medium-to-high light levels. At low light levels, analog noise may prohibit measurements of highest-quality.

Photomultiplier Tubes

A PMT consists of a light-sensitive photocathode that generates electrons when exposed to light. These electrons are directed onto a charged electrode called a dynode. The collision of the electrons with the dynode produces additional electrons. Since each electron that strikes the dynode causes several electrons to be emitted, there is a multiplication effect. After further amplification by (typically) nine dynodes, the electrons are collected at the anode of the PMT and output as a current. The current is directly proportional to the intensity of light striking the photocathode.

Because of the multiplicative effect of the dynode chain, the PMT is a virtually noiseless amplifier. The result is very high sensitivity and remarkably low noise. PMTs are also quite robust. When properly cared for, a PMT will typically function for 10,000-100,000 hours.

PTI's 800-Series will accept a wide variety of side-on 1 1/8-inch diameter photomultiplier tubes. They are optimized for PMT's with uniform dynode chains. PMTs are normally selected based on their wavelength response and their noise and/or sensitivity characteristics. PTI offers several PMTs for use with our housings.

Model 814 PMT Housing

Model 814 is a compact PMT housing for analog or digital (photon-counting) detection. The detection mode is switchable, and the built-in LCD can also selectively display high voltage or analog signal level.

Features power on/off switch, high voltage adjust, LCD high voltage/analog signal display, detection mode and display mode switches, gain, time constant and offset adjustment controls, BNC signal output connection, BNC external high voltage adjust input connection, light-tight RF shielded case, internal direct anode output mode control, and socket for most 1-1/8 inch side-on photomultiplier tubes. Includes a separate wall-mount line voltage power supply.

Analog Mode: This mode can be used for medium to high levels of light intensity. The maximum PMT current is 10 microamperes. The output is a DC signal between +13 Volts and -13 Volts. This signal is usually measured with a voltmeter, a recorder, or the analog-to-digital converter in a general-purpose I/O board in a computer.

Photon-Counting (Digital) Mode: This mode can be used for low to medium light levels up to 4 million counts per second. The output is a TTL level, active low signal. The signal line must be terminated at 50 ohms. The output is a frequency corresponding to the number of photons impinging upon the photocathode, and is usually measured with a counter in a general-purpose I/O board in a computer.

Direct PMT Output Mode: This mode applies the signal from the photomultiplier anode directly to the output BNC. This is generally used only when the highest-speed response is desired. Internal digital or analog signal processing is completely bypassed. The peripheral equipment connected to the housing must be properly configured to accept the signal directly from the PMT itself.

Specifications

High Voltage Power Supply
Input: +/-15VDC, 250mA
High Voltage: –200 to –1,100VDC manually adjustable (LCD display indicates actual cathode voltage)
External High Voltage Adjust: 0 to +5VDC (0 = –200V, 5 = –1,100V), continuously adjustable
Input Regulation: +/-0.05% max. (for 15V +/-1V input)
Load Regulation: +/-0.05% max.
Ripple: 100mV p-p max.
Temperature Coefficient: +/-0.01% max. (+5 to +40 deg. C)
Drift: +/-0.03%/hr max. (after 15 minute warm-up)

Dimensions

7.39" H x 4.04" W x 4.02" D