Ex-Royal Air Force Contamination meter NIS322XA by D.A Pitman LTD

[gravelmonkey]

Hi All, I hope everyone is well,

As promised in my introduction, below are some photos of a scintillation detector I bought on eBay a number of years ago, I've not seen another like it since.

Instrument is housed in a large, custom, Zarges aluminium case. Military reference number for the kit is 6Z/1116866.


Square on floor for scale is 26cm long.

Inside the case is custom cut foam:


Top left are the adjustment/maintenance tools, bottom left are the original peizo headphones.
Top right is the a frame to hold the detector at a distance from one's body- photo to illustrate this down the page.
Parts in the middle, from left to right are: Battery cap, 6v battery (disassembled as I had to reverse engineer a new one), pistol grip handle.
Instrument at the bottom right.



With pistol grip handle attached:


With the larger frame handle attached:


The instrument face consists of a log 0 to 250 CPS dial, rotary selection switch: (O)FF - (B)attery - C1 - C2 - (T)otal counts, BNC connector for headphones, adjustment screw ("EHT Control"?? according to paperwork, more on this later).


An original battery was included with the instrument- it's an obscure 6V alkaline battery, apologies for the terrible photo:


Inside is 4x normal AA sized cells, with battery tabs welded in series. I've made a battery carrier from a 4S AA holder, the anode and cathode are thin sheet metal- unlike "normal" devices, in this instrument, the positive terminal is sprung and requires a large flat surface for contact:


Outer blue housing is easily removed by unscrewing the slotted screw at the bottom of the battery compartment (just about visible in photo 2 and visible below).



Instrument is composed of (I believe NaI) scintillation crystal and PMT housed inside the black coloured aluminium which is held down by two firm springs. The detector electronics are distributed on two PCB boards, either side of the arrangement






Foil window at the end of the crystal/PMT housing:


Black aluminium housing is removed by unhooking the two springs. The (?NaI) crystal and foil are fixed in the housing - I'm not disassembling the housing further for fear of damaging the crystal. Crystal size is approximately 45mm diameter by 10mm length.

PMT is an EMI 9757A:


Overall, the instrument exudes high quality- I do not doubt that this was extremely expensive in it's day!

The paperwork which was included in the aluminium case included a calibration certificate and instructions card for operators checks.

The certificate, dated 1998, is addressed Royal Air Force Armament Support Unit, RAF Wittering.

According to Wikipedia, the Armament Support Unit were based at RAF Wittering between 1971 and 2000. This unit appears to have been involved with the UK nuclear armament, a few hits on the internet bring up a 1970's Armament Support Unit fire engine (http://www.ukemergency.co.uk/category/o ... t-support/).

I'm not sure of the original purpose of this device, the calibration certificate mentions it's a "Surface Contamination Monitor". Does anyone have any suggestions what it might have originally been designed to do?

The C1 and C2 settings on the instrument appear to be energy windows- is it likely these were for detecting specific isotopes?

The operators instruction card refers to the screw potentiometer on the lower left of the face of the instrument as "EHT Control". 'Calibration' involves a 241Am source (not mentioned on the card, but a google search appears to suggest this was a 43.8 kBq Am-241 source) in a jig 95mm away from the detector. In setting 'C2', the EHT control screw is turned completely anti-clockwise then clockwise. This gradually increases counts then, with continued clockwise turns, the counts decrease.

Operators were instructed to turn EHT screw in a clockwise direction, through the maximum counts then continue until half of maximum counts were reached. This number was to be noted.

The screw was then turned anti-clockwise, counting the number of rotations, back through the maximum value, until the half-value is reached.

The screw was then turned clockwise, half the number of rotations counted previously.

Once 'calibrated' C2 was supposed to read 80 - 120 cps; C1 45-67 cps and T not less than 180 cps.

Happy to take more photos if anyone is interested, and I'm keen to hear thoughts from people who know more about this than I!

Thanks!

Will

[Svilen]

Interesting device indeed. Never seen one like this before and unfortunately I can't tell you more about it, but thanks for sharing.
Is this PMT housing so easy to remove, just the springs? Isn't it light-tight? Be careful not to power up the PMT without both the housings anyway, it will destroy the cathode.

[Sesselmann]

Thanks for posting, very interesting and slightly complicated build.

What is the size of the crystal?

Is the spring arrangement intended for quick changing the crystal?

[gravelmonkey]

Interesting device indeed. Never seen one like this before and unfortunately I can't tell you more about it, but thanks for sharing.
Is this PMT housing so easy to remove, just the springs? Isn't it light-tight? Be careful not to power up the PMT without both the housings anyway, it will destroy the cathode.

Thank you for the warning!

The PMT housing is composed of the two black aluminium parts, there is about 2cm of overlap when they are assembled. In addition, the blue outer housing is sealed with an O-ring at the face end and this arrangement at the detector end:


This is a rubberised plastic with a thin layer of low-density foam behind it. When assembled, the crystal and PMT press against the back of the foam.

As an aside, it's almost impossible to power the instrument up without the outer blue housing as the battery cap, which forms the positive battery terminal, threads into the housing.

Thanks for posting, very interesting and slightly complicated build.

What is the size of the crystal?

Is the spring arrangement intended for quick changing the crystal?

I don't think the spring arrangement is intended for quick changes, appears to be intended to hold the crystal against the face of the PMT and to hold the PMT into the socket.

Crystal is mounted in the end of the black aluminium housing which attaches with spr approx. 40mm diameter by 10mm length.

I wonder, with the plastic/rubber window and relatively thin crystal, if this was specifically designed for low energy gamma?

Given this was (possibly) intended for use with the UK nuclear weapons arsenal, what is it likely they were looking for?

Calibration procedure uses an Am-241 source, snip from the .pdf I found on google gives some strange information:


The part number relates to two (slightly) different Am-241 sources (??)

I think (happy to be corrected) that this means the instrument can be (approximately) calibrated/energy gated using an ionising smoke detector source.

What I also don't understand is the instrument name (NIS 322XA), but not part number, is associated with a Pu-239 source(!).

Thanks for all your feedback so far!

[Iaiinspringer]

Ch1 = Pu239
Ch2 = Am241
Ch3 - total Ch1 + Ch2
RAF had fairly large numbers of these as instruments to support weapon transport activities and the like. RAF ASUPU were simply the maintenance unit for these and other similar instruments. Allegedly one of the prototypes of these was dangled out of a helicopter following the windscale fire to try and track the fallout. Legend has it that it distinguished itself following the B52 going into the ice at Thule when the americans couldnt find the the 'residues' of the payload very effectively whereas the Danes could using the 322's they'd bought presumably from Pitman.

[gravelmonkey]

Thanks for the additional information, very interesting!

From what I've learnt thus far, I would assume that this would have been detecting 59.5keV Am241 and 129keV Pu239?

The shape of the detector crystal makes sense if this designed for lower energies.