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Model 810/814 Photomultiplier Detection Systems

 

CONTENTS

1

DESCRIPTION

Introduction

Specifications

Controls

Inputs and Outputs

Modes of Operation

2

INSTALLATION

Attaching the Housing

3

OPERATION

Digital Mode

Analog Mode

4

SERVICE

Replacing the Photomultiplier Tube

 

 

 

DESCRIPTION

Introduction

810 PMT Detector from PTI The PTI Model 810 and 814 detection systems consist of integrated photomultiplier tube (PMT) housings that include high voltage supply, controls and signal processing circuitry. All units feature a direct-read digital high voltage display. Various tubes are available to suit a variety of detection needs, and are easily installed within minutes.

Model 810 is a photon-counting (digital) type detector system with a built-in discriminator. Users of the 810 digital detector may upgrade it at any time to selectively perform analog detection with the addition of an optional analog circuit module.

814 PMT Detector from PTIModel 814 is a combined digital and analog detection system, with all of the electronics and controls of the 810 plus an analog type detector system, with controls for gain, time constant, offset and output mode. Detection/output mode of the 814 is set by a selector switch.

Both models are capable of direct (PMT anode) output. This mode is set by an internal jumper.

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Specifications

All units have power on/off switch, high voltage adjust, LCD high voltage display, BNC signal output connection, BNC external high voltage adjust input connection, RF shielded case, output mode control, and socket for most 1-1/8-inch side-on photomultiplier tubes. All models share the same internal high voltage power supply and come with a separate line voltage power supply.

High Voltage Power Supply - Both Models

Input +/-15 VDC, 250 mA
High voltage -200 to -1,100 VDC manually adjustable
LCD display indicates actual cathode voltage or analog signal level
External high voltage adjust 0 to +5 VDC, continuously adjustable (0 = -200 V, 5 = -1,100 V)
Input regulation +/-0.05% max. (for 15V +/-1V input)
Load regulation +/-0.05% max.
Ripple 100 mV p-p max.
Temperature coefficient +/-0.01% max. (+5 to +40 deg. C)
Drift +/-0.03% / hr max. (after 15 minute warm-up)

 

Model 810 Photon-Counting (digital) Unit

Dimensions 7.39 in. H x 4.04 in. W x 2.23 in. D
Maximum count rate 4 MHz
Pulse pair resolution 250 ns
Output TTL level active low pulse
Rise time
20 ns
Fall time
100 ns
Pulse width 220 ns

 

Model 814 Digital/Analog Unit - Analog Section

Dimensions 7.39 in. H x 4.04 in. W x 4.02 in. D
Gain settings 1 µA = 1 V
0.1 µA = 1 V
0.01 µA = 1 V
0.001 µA = 1 V
Time constant settings

0.05 msec
0.5 msec
5 msec
50 msec
500 msec (0.5 sec)

Offset correction +/-50 nA
Inherent noise
50 pA p-p typical, with 0.05 msec time constant
Digital section electronics specifications same as for Model 810

 

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Controls

Controls, external

Power ON/OFF - Both Models

Power ON/OFF controls the power supply to the PMT housing only. The analog section is always powered as long as the external (line) power supply is connected to it.

Voltage Adjust - Both Models

This dial adjusts the amplitude of the high voltage applied to the photocathode of the PMT. This voltage determines the gain of the PMT, and therefore the value of the output current from the PMT's anode caused by light incident on the photocathode. The voltage is adjustable from -200 to -1100 volts. When adjusting the voltage, care must be taken to prevent too much current from flowing in the PMT. Excessive current will cause vaporization of the anode and the destruction of the tube. The maximum current permissible is 10 microamperes (uA). The value of the high voltage can be seen on the LCD VOLTAGE display on the front of the PMT housing.

Mode Selector - Model 814 Only

The Mode Selector allows the user to choose between (processed) analog and photon counting (digital) output. The switch is a locking type to prevent accidental switching. To change positions, pull the lever out and swing into position. Note that this switch is present on the Model 810, but is not functional.

Display Selector - Model 814 Only

The Display Selector controls the LCD display, and allows the user to choose between the PMT high voltage or the analog signal. This option is only available when the unit is operating in analog mode. The switch is a locking type to prevent accidental switching. To change positions, pull the lever out and swing into position. Note that this switch is present on the Model 810, but is not functional.

Gain - Model 814 Only

Selects the electronic gain of the analog circuitry. Four gain settings are available:

1 µA = 1 Volt (x 1)
0.1 µA = 1 Volt (x 10-1)
0.01 µA = 1 Volt (x 10-2)
0.001 µA = 1 Volt (x 10-3)

Time Constant - Model 814 Only

Selects the time constant of the filters in the analog circuitry. The choice of time constant affects both the noise filtering and the settling time of the output signal. A longer time constant will cause the output to be less noisy, but will increase the time necessary for the output to reach its final value. The settling time is approximately equal to five times the time constant.

Five time constant settings are available:

0.05 milliseconds (msec.)
0.5 msec.
5 msec.
50 msec.
500 msec. (.5 sec)

Offset - Model 814 Only

Adjusts the amount that the output is offset from 0. This control can be used to eliminate the effects of dark current and other additive noise. The offset is adjustable +/-50 nanoamperes (nA).

Direct Output - Both Models

Direct PMT output is available via internal jumper settings. The cover of the main housing must be removed in order to gain access to these jumpers.

NOTE: Both jumpers must be set to the same position or the unit will malfunction.

Controls, internal

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Inputs and Outputs

Power In

This input connects with a female 9-pin “D” connector from the power supply which carries the voltages and grounds necessary for proper operation of both the PMT housing and the optional analog circuitry. The voltages and connections are as follows:

1. No Connection
2. +15 Volts for High Voltage Supply
3. -15 Volts for circuitry
4. +15 Volts for circuitry
5. NC
6. NC
7. HV Supply Ground
8. Circuitry Ground
9. NC

Ext. Volt. Control

This BNC connection accepts analog input from an external device such as a computer, allowing a control source for the high voltage other than the internal potentiometer of the PMT housing. The control source, either external or internal, is selected by a jumper on the printed circuit board inside the PMT housing. The lid must be removed to access this jumper. The external input voltage levels should be between 0 Volts for -200 Volts and +5 Volts for-1100 Volts.

Signal

This BNC connection provides the output to a measurement or display device, such as a computer, a recorder, or an oscilloscope.

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Modes of Operation

Model 810 operates in photon-counting (digital) mode or direct output mode. Model 814 can be operated in digital, analog or direct output modes. Digital/analog mode is controlled by the Mode Selector switch on the side of the housing. Direct Output is selected by a pair of jumpers located inside the housing. The lid must be removed to access these jumpers.

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, and the controls will have no effect. The peripheral equipment connected to the housing must be properly configured to accept the signal directly from the PMT itself.

WARNING: Direct Mode operation requires that the user have a full understanding of photomultiplier tube operating characteristics. Damage to the PMT, housing or connected peripherals may result if this mode is improperly used.

 

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INSTALLATION

Attaching the Housing

Remove the four screws from the housing cover and take the lid off the box. Verify the proper jumper settings. Using four flat-head machine screws, mount the housing onto the instrument. Plug the photomultiplier tube into the socket. Place the shroud over the tube. Check that all connectors for the cables that run from the PC board to the housing lid are secure. Replace the lid and close the housing tightly. Connect the power supply line and the signal cable(s).

NOTE: To ease insertion of the tube, orient the indexing pin so that the rib faces the back of the housing.

CAUTION: To avoid possible damage to the sensitive photomultiplier tube, it is suggested that the tube be kept in darkness as much as possible until it is installed in its socket and the housing is closed tightly.

 

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OPERATION

DIGITAL MODE - Both Models

Before applying power to the unit, verify that the Mode Selector is set to Digital and the Display selector is set to High Voltage (Model 814 only).

  1. Turn the High Voltage control to minimum, and turn the On/Off switch to “On”.

  2. With the tube in total darkness, adjust the High Voltage control until the display reads between 1000 and 1100 volts.

  3. Expose the tube to the light source and verify that a signal is present.

CAUTION: To avoid possible damage to the photomultiplier tube, observe the following guidelines when operating the unit.

 

Under test conditions, the photon counting circuit will produce up to 4 million counts per second, provided that the input pulses are separated by 250 nanoseconds (nsec = 10-9 sec); a pulse arriving less than 250 nsec after the previous one will not be detected. Due to the random nature of photon arrival from a typical light source, the maximum possible output is approximately 3 million counts per second. When operating at this “threshold” level, it is possible that a large number of photons striking the tube will not be detected, and this excess light may result in tube damage. Therefore, avoid operating the unit at or near this output level.

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ANALOG MODE - Model 814 Only

Before applying power to the unit, verify that the Mode selector is set to Analog and the Display selector is set to High Voltage.

  1. Turn the High Voltage control to minimum, and turn the On/Off switch to "On."

  2. With the tube in total darkness and the Gain set to 10e-3 µA/Volt, adjust the offset control so that the output signal is 0 Volts. Return the Gain to 1 µA/Volt.

  3. Expose the tube to the light source and slowly increase the High Voltage until a signal is seen on the display device.

If no external display device is connected, the output signal can be displayed by changing the Display selector to Signal.

CAUTION: To avoid possible damage to the photomultiplier tube, observe the following guidelines when operating the unit.

 

Keep the PMT operating current below 10 µA. This can be done by setting the Gain to x1 and ensuring that the output voltage remains less than 10 Volts.

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SERVICE

Tube Replacement

DANGER: The PMT housing contains dangerous voltages during operation. Be sure to turn off the power and disconnect all cables before opening the housing.

 

To replace the photomultiplier tube, turn the power off, disconnect the power supply line and all signal lines. Remove the four screws from the main housing cover and take the lid off the box. Remove the PMT shroud and carefully pull out the tube. Plug the new tube into the socket. Replace the shroud and check that the connectors for the cables that run from the PC board to the housing lid are secure. Close the housing lid tightly.

Note: To ease reinsertion of the tube, orient the indexing pin so that the rib faces the back of the housing.

CAUTION: To avoid possible damage to the sensitive photomultiplier tube, it is suggested that the tube be kept in darkness as much as possible until it is installed in its socket and the housing is closed tightly.

 

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