Photomultiplier detection system

DC operation (contiuous light beam) AC operation (chopped light beam)

[Cell definitions and equations] [Student assignment handout] [OpenOffice and Excel Version]

A simulation of measurement of light intensity by a photomultiplier tube (PMT). Includes the effect of load resistance, integration time, wavelength, light flux, applied voltage, and phototube temperature on signal and signal-to-noise ratio of light intensity measurement with photomultiplier tubes. Students compare difference types of phototubes, measure spectral characteristic, observe effects of amplifier overload, display resolution limits, phototube overload, determine lowest flux that can be measured, attempt to improve the SNR by cooling the phototube. There are versions for DC operation (with a continuous light beam) and AC operation (with a chopped light beam). The DC version shows the signal and signal-to-noise ratio numerically; the AC version shows the signal and signal-to-noise ratio graphically.

When used in a lecture-demonstration environment with a computer video projection system, where it is often difficult to use the keyboard data entry in a darkened room, these models can be operated using only the mouse-activated on-screen sliders, pop-up menus, and radio buttons.

Download links.
WingZ versions:
DC operation (with a continuous light beam): pmtDC.wkz;
AC operation (with a chopped light beam): pmtAC.wkz.
Wingz player application and basic set of simulation modules, for windows PCs or Macintosh

OpenOffice and Excel Version

Other simulations that employ a photomultiplier detection system:
Signal-to-noise ratio of absorption spectrophotometry
Fluorescence Spectroscopy Signal-to-Noise Ratio
Comparison of Calibration Curve Fitting Methods in Absorption Spectroscopy
Effect of Slit Width on Signal-to-Noise Ratio in Absorption Spectroscopy
Scanning Fluorescence Spectrometer
U.V.-Visible Spectrophotometer
Dual Wavelength Spectrophotometer
Effect of Slit Width on Emission Spectroscopy SNR
Spectroscopy of Atomic Absorption

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References:
Photomultiplier handbook (PDF format)
Getting the best out of photomultiplier detectors (PDF format)
Building your own photomultiplier system

Spreadsheet cell definitions and equations


Student handout (WingZ Version)

Light Measurement with Photomultiplier Tubes

1. Download the WingZ player application as described above. Launch wingz.exe, then open pmtDC.WKZ.

2. Select Photomultiplier 1, 1 Megohm load resistance, and 1 sec integration time from the pop-up menus (right side of screen). Using the slider controls, set the wavelength to 300 nm, light flux Phi to 10-9 watts (e.g. log(Phi) = -9), percent flicker to .1%, the applied voltage to 800 volts, and the phototube temperature to 300 K.

3. Select different values of load resistor. Note the effect on the signal voltage and noise voltage. The amplifier saturates at 10 volts, so the load resistor must not be so high as to exceed this value. On the other hand, the readout display has a resolution of only 0.001 volt; so the load resistor must not be so low that the

display resolution is a limitation. Does the load resistor have a significant effect on the signal-to-noise ratio (SNR)? Why or why not?

4. Vary the applied voltage. (Select the load resistor as required to make the signal voltage as large as possible without exceeding the saturation level of the amplifier). Does the applied voltage have a significant effect on the signal level? On the SNR? Why or why not?

5. What is the lowest flux F that can be measured by this phototube with an SNR of 3; choose the applied voltage and load resistance in an attempt to improve the SNR as much as possible. Select Photomultiplier 2 and repeat. How does this tube differ from the first one in terms of gain and low light level performance?

6. What is the longest wavelength that can be measured by this phototube (the long wavelength threshold)? Explain. Calculate the cathode work function, in Joules.

7. Can you improve the SNR by cooling the tube (reducing the phototube temperature)? Explain the observed effect.

8. Select Photomultiplier 3 and repeat steps 6 and 7. Why is this phototube called a red sensitive tube? How does cooling the tube effect this tube? Why?

OpenOffice Calc and Excel Versions

Click to see larger graphic

Computes the detector signal current and signal-to-noise ratio, given the phototube characteristics and the incident light power (watts) on the photocathode. Includes source flicker, photon, and thermionic emission noise.

Assumptions: Quantum efficiency of photocathode, gain per stage, and collection efficiency are independent of light level and detector current.

View Equations (.pdf)
Download spreadsheet in OpenOffice format (.ods)
Download spreadsheet in Excel format (.xls)

(c) 1991, 2015. This page is part of Interactive Computer Models for Analytical Chemistry Instruction, created and maintained by Prof. Tom O'Haver , Professor Emeritus, The University of Maryland at College Park. Comments, suggestions and questions should be directed to Prof. O'Haver at toh@umd.edu. Number of unique visits since May 17, 2008: