Photomultiplier tube

Introduction

The teardown of a AGFA ADC Compact Plus computed radiography developer became the basis for a range of reverse engineering projects, a complicated system like a automated developer for computed radiography has a lot of different parts inside.

Just for a short introduction, computed radiography is based on reusable phosphor imaging plates, the plate is used instead of film. The x-ray exposed imaging plate is scanned with a red laser timed to a photomultiplier tube to read out the single pixels on the imaging plate. The returned light from the scanning laser has a different intensity dependent on the amount of absorbed x-ray energy and thus the photomultiplier tube can translate this into a 12-bit grey scale resolution.

This article covers the reverse engineering of the AGFA PMT module that contains the photomultiplier tube, high voltage power supply and analogue amplifier.

Safety

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Read this document about safety! http://www.pupman.com/safety.htm

Reverse engineering

The photomultiplier assembly sits in a aluminium housing with the tube going out through a plastic cover, the inside of the plastic cover has a grounded layer of mu-metal for shielding the tube from electrostatic interference.

The 15-pin SUB-D connector is the only connection to the module and is both for supply voltages, input and output signals.

Opening the lid of the aluminium enclosure, the entire  assembly can be taken out with 4 screws undone. The photomultiplier tube is soldered directly to the circuit board with a supporting aluminium base ring to sit tight in, along with another grounded layer of mu-metal around the rest of the base.

It is not possible to see any kind of markings or stickers on the tube to identify its exact type. From the number of pins only, I guess this tube only has 8 dynodes and in the computed radiography application this gives plenty of sensitivity. The thick blue glass front window makes is impossible to make out how the tube is constructed on the inside.

The tube is soldered to the circuit board on which the 15-pin connector connects to, upwards sits another circuit board on which the high voltage power supply is. Underneath the tube there is a LED coupled with a light receiving diode. This LED is used for testing the tube.

The different markings on the boards are as follows: AGFA-IUP3 on the main circuit board along with serial number SN 02583 and photomultiplier tube number PMT 111296. High voltage power supply circuit board is marked with AGFA-HV1.

On the following picture of the bottom side of the main circuit board I have marked pin header for the 15-pin connector and the pins connecting the high voltage board to the main circuit board. These markings are essential in relation to the attached schematic.

The arrangement of pin 1 and 9 quickly shows that it is a meant to be supplied from a positive and negative power supply, from the polarization of diodes and electrolytic capacitors. The +/- 15 VDC power supply is the only power source connected and the +5 VDC house keeping power supply is generated with the LP2951 IC.

Pin 2, 6, 10 and 12 is connected directly to ground plane and pin 13 is connected to the ground plane through a 1K resistor.

Pin 3, 4 and 11 goes straight to the high voltage power supply circuit board. Pin 3 through a 1K resistor. Pin 3 is the high voltage enable input.

Pin 5 is the processed data output from the AD823 op-amp, a signal between ground to +15 VDC is outputted on this pin in regard to light detected by the tube. There is also a pin header near the tube connections where the raw signal from the tube can be taken out.

Pin 7 and 14 are not connected to anything.

Pin 8 and 15 connects to a DG202 IC which is a quad SPST CMOS analogue switch. Two switches each have their inputs connected in parallel, but only one of the outputs are utilized. Pin 15 activates the test LED with its auxiliary circuitry of the coupled light receiving diode on the second op-amp of the AD823 IC. Pin 8 imposes a +400 mV reverse bias on the negative input of the first op-amp of the AD823 IC and is used to reset the tube output data faster than the fall time of the tube itself so it is ready to read the next pixel faster.

The upside of the main circuit board only has pin header connector for the 15-pin SUB-D connector, 4 electrolytic capacitors and 2 inductors.

From the pin HV9 comes the negative high voltage supply for the photomultiplier tube. The cathode of the tube is connected to a large orange painted capacitor that is tied to ground, to ensure a stable voltage at the start of the high voltage chain.

As more electrons are accelerated from the first dynode to the next and so on, less current is needed to drive them on, this can normally be done with a pure resistive divider network between the dynodes, but it has its drawbacks with loss of linearity and output deviation in the region of 10-20% at high output current. A improved divider network uses capacitors to insure stable supply voltage at peaks and even individual power supplies for each dynode can be used to do the same.

The solution on this circuit board is however much more elegant and complicated. A network of resistors are used between the first 4 dynodes and there after capacitors, but load dependent driven transistors are used to linearise the output depending on load and that brings the output deviation to around 1-2%.

The high voltage power supply circuit board has no components on the backside, only a few traces that is marked on the following picture.

From the photomultiplier tube handbooks from Hamamatsu, the requirements to a PMT high voltage power supply reveals that quality control is needed to achieve a line/load regulation at +/- 0.2% and ripple noise/temperature drift at 0.05%.

Pin 3 and 11 from the main circuit board connects to pins HV1 and HV2 and are somehow related as they both goes to the positive and negative input of the TL032A op-amp and the output from this goes to a LM393 op-amp positive input and with negative tied to ground through a 50K resistor. The output of the LM393 drives a transistor that ties the ZVS Royer oscillator to ground and thus enables it to oscillate. The ZVS oscillator uses two IRFL110 MOSFETs, 100 VDC/12 A rating, driving the small transformer through a isolation toroid and the output of the transformer is fed through a Cockcroft -Walton multiplier to generate around -600 VDC.

When the circuit is applied with power it is however only pin 3 that has to be pulled high with +5 VDC to enable the high voltage generator and get the photomultiplier tube circuit running.

From the high voltage output end of the Cockcroft-Walton multiplier there is two 100M resistors, one leads back to the -15 VDC supply rail and the other to the second part of the TL032A op-amp, possible for high voltage measurement feedback.

 

Schematics

Schematics are not yet completed.

no images were found

no images were found

Experimentation: Getting the unit to run

5th March 2016

After having drawn a fairly accurate schematic from reverse engineering the circuit boards, I was confident to test the module with power on. I was using a power supply with +15 VDC, -15 VDC and GND. On the oscilloscope I have a differential probe on the high voltage supply output on channel 1 (yellow), raw signal output from the photomultiplier on channel 2 (blue) and processed signal output from pin 5 on the main circuit board on channel 3 (purple).

I shielded the window of the PMT with aluminium foil to block out any light

Judging from the house keeping power supply IC that generated +5 VDC and the datasheet for the DG202 analogue CMOS switch IC, I tried my luck with putting +5 VDC on the input pins I had located. This is how I found the high voltage enable on pin 3.

When +5 VDC is applied to pin 3 the high voltage supply is generating -600 VDC.

Now that the module was up and running it was no problem doing a few tests, as the test LED was controlled by the CMOS switch, it was merely to pulse + 5 VDC on pin 15 to activate the test LED and capture a shot of the output pulse. The result was a raw signal of -20 mV and processed signal of +4.2 VDC on pin 5.

To test the sensitivity and assumption of maximum processed output of being positive supply voltage for the op-amp, +15 VDC, I used a LED flash light to blink at the tube through a pin hole in the aluminium foil covering the window. The result was a raw signal of -770 mV and a processed signal of +12.6 VDC on pin 5.

As a photomultiplier tube can be a very sensitive instrument, there must be a explanation on why a blink of a flash light is not completely drowning the tube and giving a maximum output. From the PMT handbooks we can see on graphs for common PMT behavior that the amplification is highly dependent on the high voltage supply.

The -600 VDC this module uses gives a mere amplification of 40.000 times, marked with a red line on the graph. This level of amplification is sufficient for the computed radiography, but for a more interesting task like radiation detection with a scintillation crystal, it is suggested that 1-2 kV is needed and as it can be seen in the marked blue area that gives a amplification of 20.000.000 times and up towards 1.000.000.000 times.

Experimentation: Scintillation plastic for radiation detection

9th June 2016

To conduct this experiment I knew that I needed a crystal or plastic scintillator of a fair size to make up for the low tube voltage with more detector volume. Plastic are the cheapest and for a reason, in the detection of gamma rays, organic plastic scintillators have a low absorption coefficient and exhibit less probability for the photoelectric effect, making them unsuitable for energy analysis applications. BC408 is however very good for fast neutron detection.

I bought a BC408 cylinder piece that measures 50 mm in diameter and is 30 mm thick, this corresponds to a volume of 59 cm3.

I borrowed a unknown square piece from a friend with the measures 32 x 32 mm and a length of 100 mm, this corresponds to a volume of 102 cm3.

The test setup was fairly simple, power supply, oscilloscope and photomultiplier tube with a scintillator on it and covered in 2-3 layers of aluminium foil for blocking out light.

I captured the waveforms in single shot mode and just kept raising the trigger limit to see how high output levels I would get. It was however necessary to use a scan range of about 200 uS to catch the signals and then zoom in for the following screenshots.

In the following screenshots from the oscilloscope is the test done with the square plastic scintillator and  I was able to catch up to 4.16 VDC pulse out of the processed output pin.

In the following screenshots from the oscilloscope is the test done with the round plastic scintillator and  I was able to catch up to 5.6 VDC pulse out of the processed output pin.

The noise floor is well below 200 mV with this preliminary test I do know for now that I have a sensitivity between 200 mV to 5600 mV. To be rough and assume this is the maximum values I had found already, have a step of 100 mV that actually gives me a resolution of 54 steps. This is a much better result than what I expected from the standard setup of the tube, future experiments with a proper data collection setup will reveal much more information about its sensitivity.

 

Additional information about other PMT modules [3]

Circuit boards AG-IUP32-8 has a special 15-pin connector which includes a coaxial output.

The following 15-pin SUB-D connector pinout is not verified by myself, but use it as a help if you have to work with the same unit

Coaxial connector is for the signal out

Pin 1 Ground
Pin 2 -15 VDC supply
Pin 3 Ground
Pin 4 +5 VDC supply to PIC and EEPROM
Pin 7 I2C data connection to PIC
Pin 8 Ground
Pin 9 +15 VDC supply
Pin 10 Ground
Pin 11 Ground
Pin 15 I2C data connection to PIC

Pinout of the connections for the high voltage circuit board when looked at with SUB-D connector pointing downwards.

Pin 1 is connected to PIC Pin 13 (RC2/CCP1)
Pin 2 adjust? connected to PIC DAC 1A
Pin 3 feedback? connected to PIC ADC? PIC Pin 7 (RA5)
Pin 4 Ground
Pin 5 +15 VDC supply
Pin 6 -15 VDC supply
Pin 7 High voltage out
Pin 8 Ground
Pin 9 -15 VDC  supply
Pin 10 +15 VDC supply

 

Circuit boards AGFA-IUP16 has a normal 15-pin connector and looks very similar to the AGFA-IUP3.

The following 15-pin SUB-D connector pinout is not verified by myself, but use it as a help if you have to work with the same unit:

Pin 1 +15 VDC supply
Pin 2 Ground
Pin 3 High voltage enable
Pin 4 Connects to LP2951 +5 VDC power supply IC
Pin 5 Processed signal output
Pin 6 Ground
Pin 7 Not connected
Pin 8 CMOS analogue switch, either test LED or positive bias
Pin 9 -15 VDC supply
Pin 10 Ground
Pin 11 Connects to LP2951 +5 VDC power supply IC
Pin 12 Not connected
Pin 13 1K resistor to ground
Pin 14 Not connected
Pin 15 CMOS analogue switch, either test LED or positive bias

Conclusion

Reverse engineering a circuit board with only discrete and analogue components is not hard, just time consuming and it takes thinking and experience to make the proper guesses on SMD components as they do not have a unique identification system.

 

Demonstration

Not yet.

 

References

[1] Hamamatsu, “Photomultiplier Tubes – Construction and Operating Characteristics”, January 1998.

[2] Hamamatsu, “Photomultiplier Tubes – Basics and Applications”, Third edition, Handbook, 2006.

[3] V. Baumann and Benedikt H. – http://www.mikrocontroller.net/topic/288933, 2013.

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