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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
X-Ray Spectrometer
X-Ray Sources - Svetlana BS-5
UNDER CONSTRUCTION
The Svetlana BS-5 is a low power end-window transmission-target X-ray tube designed for very close anode-to-sample coupling by means of an integral collimator. The tube is available in two anode flavours, copper and tungsten, but are identical in all other respects. A transmission target means that X-rays produced at the focal spot must pass through the target - which needs must be thin - to exit the tube .
The made-by-hand engineering is evident when we see that the tungsten anode type has been inscribed as a W with a carbide pencil.
And Cu for copper on this one.
The beryllium end window through which the collimated X-ray beam emerges has a diameter of 2.5mm.
This hole in the base allows the dielectric oil to enter, and to expand and contract without damaging pressure disequilibrium.
Filament current increases with filament voltage reaching its maximum design limit of 3.9A at 1.5V, and is one of the ways in which the anode current of an X-ray tube may be altered without changing the anode voltage and output photon energy.
Target transmission X-ray tubes will quickly fail if they are overdriven. This graph of anode voltage against anode current shows the maximum current in µA permitted across the tube's working voltage range from 10kV - 30kV.
Unlike diagnostic Xray tubes, which typically operate in the millisecond regime, and have a very layed back duty cycle to chill out between exposures, analytic X-ray tubes must be able to run continuously for long periods if needs be, and require attention to detail in their cooling.
The BS-5 is a true triode, by which I mean that the anode current can be controlled to cut-off by a negative bias on the grid. The top graph shows the anode current in µA across a range of negative grid bias with 30kV on the anode, and filament current 3.9A.
The lower of the graphs above shows anode current variation for a given grid bias range when there is 10kV on the anode, and filament current is again 3.9A.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
X-Ray Spectrometer
X-Ray Sources - Svetlana BH-4
UNDER CONSTRUCTION
The Svetlana BH-4 series of low power target transmission X-ray tubes feature Au, Ag, and Ge anodes. I've laid my hands on the silver and germanium types for this project.
The Ag mark hand-stamped onto the anode block of one....
And the Germanium stamp on the other.
The end window is at the tip of a short collimator emerging from the conical copper anode block.
The beryllium exit window in the centre circle has a diameter of 3mm.
The three pin phenolic resin base has a large oil hole to flood the innards with dielectric fluid.
The focusing cup marked "3" in the diagram keeps the electrons on the straight and narrow as they fly towards the anode. It is kept at negative potential relative to the filament.
The graph at the top shows the filament current for different values of filament voltage. The lower graph shows the maximum current in µA allowable for anode voltages up to 50kV to keep the tube within its safe dissipation limit.
Germanium anode: X-ray spectrum The strong Ge Kα line at 9.9keV is an excellent marker for ~10keV.
Silver anode: X-ray spectrum.
Silver is my absolute favourite X-ray anode because the strong Kα line at 22.2keV - the left-hand spike in the highlighted region - is relatively easy to isolate as a nearly monochromatic X-ray beam.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
X-Ray Spectrometer
Proportional Counter Tubes and PSU
UNDER CONSTRUCTION
The disintegrating protective foam is typical of Soviet era packaging after prolonged storage.
Nonetheless, the Russian manufacturer, Bourevestnik of St. Petersburg, waxes lyrical about the prowess of these particular tubes:
"Gas-filled proportional counters of SI series were applied in space projects focused on X-radiation of the Sun and the stars, the Venus surface matter, on orbital spacecrafts ("Lunohod-1", "Vega-1", "Vega-2" stations, "Astron", "Salut-4" space observatories, Satellites Series "Kosmos" and "Interkosmos")."
So we are keeping good company.
Each tube has two beryllium windows - one on either side - to allow both entry and escape.
Type: SI-11R-1 Fill gas: so called "P-10 gas" - Ar 90% CH4 10% Working Voltage: 1650V Energy range: 5-15keV Max Counting Speed: 10E5 cps Shielded Count Rate: 40 cpm
Type: SI-11R-2 Fill gas: Kr 90% CH4 10% Working Voltage: 1700V Energy range 15 - 40keV Max Counting Speed: 10E5 Shielded Count Rate: 300 cpm
Type: SI-11R-4 Fill Gas: Ne 90% CH4 10% Working Voltage: 1250V Energy Range: 2 - 5keV Max Counting Speed: 10E5 cps Shielded Count Rate: 20 cpm
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
UNDER CONSTRUCTION
CALIBRATION TOOLS
X-Ray Photometer
Back panel: SHV socket for PMT EHT supply. BNC PMT signal input. Mains input, mains fuse. Remote control cable socket.
Photometer NaI(Th) scintillator/PMT head
***
NE Technology Farmer Dosemeter model 2570/1
Here is my NE Technology Farmer Dosemeter model 2570/1 which can automatically compute dosage v. time calculations.
It is completely portable, and runs off two standard 1.5V D cells inserted into a holder in the back panel.
The ionization chamber polarising voltage is nominally 250V (251V shown), while the sign of the polarising voltage may be reversed by a switch on the back panel, which also offers switchable voltage division options of V/2, V/4, and V/8.
The great joy of the instrument is that when standard Famer-type chamber sizes are selected - 0.03 cc, 0.6 cc, or 600 cc, the instrument will function as a direct reading dosemeter.
Using the keypad, one can select a dose - say 1 Gy - and press the start button, and the instrument will trip the X-ray source via a back panel port when that dose has been delivered.
Use of free-air ionisation chambers is made possible by the correction keys - T for temperature, P for pressure in mBars, and CF for the chamber correction factor, the most troublesome of the corrections. Happily, parts of the chamber correction aggregate are available pre-computed in tabular form for 10 keV, 25 keV, 50 keV and above, thanks to the BNIP.
The instrument can produce results for both Absorbed Dose in Gy, and Exposure in C/kg.
The back panel is home to the battery locker, which houses 2 x 1.5V D Cells. A serial port can directly feed results to a printer, as well as tripping a relay to start and stop an X-ray or other source at a given dose or time.
The dessicator cartridge may be inspected for colour change, and replaced with a fresh one if necessary, via the perspex plate at right.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
UNDER CONSTRUCTION
X-Ray Tube 25kV 1mA Variable Dual PSU
PSU Overwiew
A dual +ve/-ve independently controllable dual PSU - effectively two separate PSUs - allows me to power both grounded anode and grounded cathode tubes up to 25kV, or to supply up to 50kV across a common mode load without needing to have more than 25kV worth of insulation-to-ground.
The high voltage transformer primaries are powered by two GE 220V 1A Variacs, allowing stepless indendent change of both EHTpositive and negative rails.
The transformer outputs both feed 2-stage full-wave Cockcroft & Walton voltage multipliers. The transformers Vpk being 11kV, the full-wave 2-stage multiplier would have an output of about 40kV at 1mA, but as this would mean exceeding the safe working voltage of the 16kV capacitors, I will set the spindle stop on the Variacs to limit the output voltage to 25kV. I work on the basis that the capacitor rating must be double the peak voltage for safe operation, i.e. 8kV for the 16kV capacitors I have available.
Multiplied X-ray tube anode volts also means multiplied shielding costs, and as this setup working under reduced power can still supply 50kV across a tube if needs be, I see no reason to buy more costly capacitors. Were I to need more milliamps, my minimalist approach to voltage multiplication at 50Hz would become unreasonable - like Henning Umland's baroque X-ray machine that takes up a good part of his cellar:
At the output of the full-wave multiplier, I have added what is both a low pass 16Hz pi filter, and a safety resistor, to keep the maximum possible short-circuit current through the diodes to 25mA - 5mA below their maximum current rating.
I was lucky to buy four of these excellent transformers brand new on ebay for £16 (18.94 EUR) because no one else bid for them. They came with Cofi logo three pin mains connectors.
Many ignition transformers are rated only for intermittent operation, but these beauties are designed for continuous use. What a shame that Cofi's 17kV peak model can only run in 4 minute bursts without getting all hot and bothered.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Grenadier wrote ...
How will this be designed?
How will it be designed? By intuition, with occasional calculation to sharpen the image in my mind.
All is 'under construction,' so previous posts - in fact, episodes - are all subject to further development, modification, clarification, exegesis, and the like, until the Fat Lady sings, as they say!
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
UNDER CONSTRUCTION
X-RAY TUBE PSU FULL-WAVE C&W SECTION One of Two
The 20nF capacitors shown in the first stage are formed from two 10nF in parallel. Using 16kV rated capacitors is not ideal. It limits my maximum peak C&W input voltage to 8kV, but was all that I could find in my stack. An 8kV input will still produce about 30kV at the full-wave C&W output, which is more than enough for the low power analytical X-rays tubes I have to hand - indeed the maximum anode voltage permitted for BS-5.
Each of the two C&W multipliers in the PSU requires nine capacitors.
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X-Ray Spectrometer
