Introduction

The teardown of a Fujifilm FCR XG-1 X-ray image scanner for computed radiography became the basis for this reverse engineering project. You can see the teardown of the entire Fujifilm FCR XG-1 machine here and the teardown of the polygon laser scanner module here.

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 Fujifilm PMT12A PMT module that contains the photomultiplier tube, high voltage power supply and analog amplifier.

Safety

WARNING!: Working with electricity is dangerous, all information found on my site is for educational purpose and I accept no responsibility for others actions using the information found on this site.

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

Reverse engineering

The photomultiplier assembly is mounted on a sheet metal holder with the tube going out through a grounded layer of mu-metal for shielding the tube from electrostatic interference. The PCB has a output amplifier and power supply part on the left side of the socket and a high voltage power supply module on the right side.

The Fujifilm PMT12A board has two connectors, which both went to the SCN12A card in the main computer of the Fujifilm FCR XG-1 scanner. CN1 is a 34-pin flatband pin header and CN2 is a coaxial output signal from the output amplifier.

The output amplifier is shielded, the differential amplifiers on the board is using +12/-12VDC from a LM2940CT and LM2990T regulators, as the +15/-15VDC input goes directly to the HV module. The Hamamatsu C7775 from 2007 does not have any datasheets available online, but browsing through the newest modules shows that they still make a C-series 5 pin HV module, that turns out to use the same pin layout as its earlier modules.

From the pin 5 of the C7775 comes the negative high voltage supply for the photomultiplier tube. The cathode of the tube is connected to the 4 blue capacitors 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 is 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.

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%. The Hamamatsu R1848-07 photomultiplier tube is a 10-stage 14-pin version that can use up to -2500V dynode supply.

It took a great deal of cutting with a new sharp box cutter, to get in-between the acrylic light guide and the blue glass filter window of the photomultiplier tube. It took about an hour to free the two parts from each-other, without harming tube or light guide.

Schematics

Schematic from the service manual. I marked the needed pins for the module to run without the computer/scanner analog-to-digital conversion modules.

PCB test points and connections schematic

Experimentation: Getting the unit to run and scintillation plastic for radiation detection

6th June 2021

After having seen the connection diagrams in the manual, I was confident to test the module with power on. I was using a power supply with +15 VDC, -15 VDC and GND. I shielded the window of the PMT with aluminium foil to block out any light. A powered up photomultiplier tube that is flooded with light risk burning holes in its dynodes! Protect and treat your PMT with great care.

Judging from the computer power supply of +5 VDC and the datasheet for the Hamamatsu C9619 high voltage power supply, I tried my luck with putting +5 VDC on the input pins I had located. It was also mentioned in the manual that the module has a “high voltage hardware ok” and “high voltage software ok ” signal, that both had to be high for the high voltage to get active. It turns out that the input pin 5, labelled HVSH is “High Voltage Software High”. When +5 VDC is applied to pin 5 the high voltage supply is generating -510 VDC.

The output signal is sine wave like waveform with 1, 2 or 3 peaks, depending on the signal strength. The highest peak was up to 70V, but I am not sure that the scaling was correct from these measurements. I unfortunately lost all my oscilloscope screenshots to a defective USB stick 🙁

A photomultiplier tube can be a very sensitive instrument, depending on the negative supply voltage for the dynodes. From the PMT handbooks graphs show that common PMT behavior the amplification is highly dependent on the high voltage supply. The -510 VDC this module uses gives a mere amplification of 40.000 times. 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 to -2 kV is needed for a amplification of 20.000.000 times and up towards 1.000.000.000 times.

 Demonstration

Teardown, reverse engineering and test of the Fujifilm PMT12A board.

References

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

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