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AGFA PMT module AGFA-IUP3, reverse engineering

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Mads Barnkob

Wed Mar 30 2016, 08:54AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

The original article on my website, has more information on similar modules and more pictures:

Schematics are not yet done.

I got this module from a AGFA ADC Compact Plus computed radiography developer machine.

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 SUBD 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 SUBD 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.

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.

1459327735 1403 FT0 Pmt Current Amplification

Future experiments involves:
- Scintillation radiation detection with current power supply
- Adjust high voltage power supply for higher voltage to increase amplification

Conundrum

Thu Mar 31 2016, 06:00AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

Nice work, thanks for sharing!
Apparently you can still buy PMTs on Ebay, although often they don't work some useful work can be done with even a damaged unit depending on what went wrong.

Proud Mary

Thu Mar 31 2016, 04:47PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

"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."

Advanced photomultiplier applications using costly tubes very often contain one or both of these protective elements:

(1) An electro-mechnical switch that will disconnect the HT when the case is opened.

(2) A fast over-current detector that will reduce the HT if excess current is drawn due to excess light.

A single flash of light from a torch would damage some PMTs with high gain settings. Damage may range from increased dark current and shot noise - but still serviceable - to major changes in parameters and total failure.

Conundrum

Fri Apr 01 2016, 07:49AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

Similar to how some early CRTs could be damaged by not driving them correctly. Seem to recall stripping the emitter on one by trying to drive the heater negative by mistake (2KV negative to be specific) resulting in very low emission.

Apparently on PMTs the failure modes can be quite interesting, including increases in shot noise and failure to generate any output. Typically this is due to user error but can sometimes happen due to a duff connection to one or more dynodes as they are only spot welded and can (and do) break just as with any vacuum tube.

in other news, does anyone have 4 identical EL34s please?

edit: found this one

Proud Mary

Fri Apr 01 2016, 08:23AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Conundrum wrote ...

edit: found this one

Not to hijack Mads' thread, but anyone buying IP21 hoping to get it to work in a scintillation counter will be disappointed. It is an early side-entry design with a very modest current amplification of about 3*10-4 and enjoyed great popularity in 'electric eye' intruder alarms, door openers, and other interrupted beam applications such as counting items as they flew past on a conveyor belt in automated production.

Mads Barnkob

Sat Apr 02 2016, 11:58AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Proud Mary wrote ...

"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."

Advanced photomultiplier applications using costly tubes very often contain one or both of these protective elements:

(1) An electro-mechnical switch that will disconnect the HT when the case is opened.

(2) A fast over-current detector that will reduce the HT if excess current is drawn due to excess light.

A single flash of light from a torch would damage some PMTs with high gain settings. Damage may range from increased dark current and shot noise - but still serviceable - to major changes in parameters and total failure.

This is clearly not one of the expensive tubes :) There are no shutdown mechanisms like the ones you described, I captured no turn off of high voltage, but also I never flooded it with more than a torch blink through a needle hole in the foil, maybe it has a shutdown at much longer max peak signals.

Mads Barnkob

Thu Jun 09 2016, 09:27PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

I had the housing put back together and the scintillator and open end of the tube was covered in 3-4 layers of aluminium foil.

From a square piece of unknown scintillator plastic with the measures 32 x 32 x 100 mm ~ 102 cm^3

1465507379 1403 FT176242 Ds1z Quickprint11

1465507379 1403 FT176242 Ds1z Quickprint12

1465507379 1403 FT176242 Ds1z Quickprint13

1465507379 1403 FT176242 Ds1z Quickprint14

From a cylinder piece of BC408 scintillator plastic with the measures 30 x 50 mm ~ 59 cm^3

1465507435 1403 FT176242 Ds1z Quickprint15

1465507435 1403 FT176242 Ds1z Quickprint16

1465507435 1403 FT176242 Ds1z Quickprint17

1465507435 1403 FT176242 Ds1z Quickprint18

There certainly is difference in detected energy levels, from 2 to 5.6 VDC, but I need to make a counter with different filters to see how many counts I get, these were all captured in single shot mode.

radhoo

Fri Jun 10 2016, 01:33PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

Hi Guys! Interesting thread. I was hoping to convince Steve Sesselmann to change direction towards digital with his nice detectors, and maybe even derive a product that would push data online.
I hope to see some nice results from this thread's work which I find it interesting. I didn't finish mine, but hopefully will someday.

LE: about capturing the data. You'll need a sample and hold mechanism (with an OpAmp) before feeding the pulse to the ADC converter of a microcontroller, because the pulse is too short lived.

Mads Barnkob

Tue Jun 14 2016, 09:07PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

radhoo wrote ...

Hi Guys! Interesting thread. I was hoping to convince Steve Sesselmann to change direction towards digital with his nice detectors, and maybe even derive a product that would push data online.
I hope to see some nice results from this thread's work which I find it interesting. I didn't finish mine, but hopefully will someday.

LE: about capturing the data. You'll need a sample and hold mechanism (with an OpAmp) before feeding the pulse to the ADC converter of a microcontroller, because the pulse is too short lived.

I got a LM358 precision rectifier / low loss hold and sample circuit that I made for another project, it will do just fine here I hope.

I recently got a STM32F7 discovery board, 216 MHz clock and has a 4.something" touch screen on the back of it, that would be perfect for sampling and visualizing in one package.

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