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Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
This thread is work in progress, all important news is announced in this first message, in order
Nov 29, 2010 I got my hands on a nice Scintillation probe
The NaI crystal (with thallium doping) is used to detect Gamma Rays. I assume it is usable for X-rays as well, but didn't test that yet. The photomultiplier is very sensitive, it is a Philips XP2008, but a XP2010 can also be used. The probe needs a bipolar -12V 0 12V supply, to power the high voltage inverter that puts out -1200V (Regularized) for the Photomultiplier.
The next thing to do is to create a 12V to the bipolar (-12,0,12) converter to make things easier, and to hook up a microcontroller to log the signal to an SDCard or direct interface to a computer via the serial port or Bluetooth.
The probe makes a high-frequency sound when operating so the ivnerter probably is in the 15-20KHz range.
June 03, 2011 Slow progress, but project alive: I managed to finish the bipolar supply needed for my probe (+12V,0V,-12V), and a basic board with a microcontroller , a UART Bluetooth Module set to 9600bps doubled by a MAX 232 serial connection as a backup.
Using bluetooth I can now establish wireless connections to the microcontroller, to read the scintillation probe data, and to send commands such as Start/Stop the High Voltage supply connected to the X-ray tube.
The microcontroller reads the probe data using a ADC port, with 10bit resolution! That's great news for accuracy measuring the events amplitude (values interval 0.. 2^10= 1024, mapped to a 0..5V input) The second picture shows data being transmitted wirelessly to my PC. My scintillation probe has a small crystal and seems to only respond to relatively low energy rays (but very responsive under 100KV). In case I get a probe with sensitivity to higher energy radiation, I would be able to apply a Fast fourier transformation pm the radiation data, and build a spectrum . This characteristic could be used to identify the exact radio-nucleotide that caused the event. Still I will try this for my x-ray tubes, maybe I can get a characteristic of the cathode metal or something useful.
June 07, 2011 A first video of the setup, showing a good response to x-rays:
June 08, 2011 It appears my initial assumption that the probe is insensitive to radiation of higher energy levels was incorrect. The probe's crystal is a cylinder of 3.2cm diameter and 2.5cm long. The Cesium 137 was not detected previously because of counter limitations. The radiation events caused by Cs137 are discreet in time, meaning that the counter must be capable of processing a high number of samples per second. The Atmega8 was reading data from the ADC port every 100ms which is totally inappropriate!
I've been running a few tests sending the probe data directly to the sound card of my PC (via line-in), since this card is capable of up to 48000 samples per second. The result is that the Cesium sample is now correctly detected: This graph shows 30 seconds of background radiation followed by another 30 seconds of events caused by the Cs137 brought close to the scintillation probe.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
They're designed to count one photon at a time. Room light is about 10 billion times stronger than they're designed for. Some models can't take it while they're powered, it causes enough photocurrent to burn the tube out.
Others can stand it fine, and yet others can't even take room light unpowered, and need days to recover from an exposure to it.
Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
I didn't know that , now I also see it on wiki "While powered (energized), photomultipliers must be shielded from ambient light to prevent their destruction through overexcitation. "
Do you have any idea on how I could check the tube for damage?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
radhoo wrote ...
Unfortunately I didn't know that (didn't think the anode is so sensitive), now I also see it on wiki "While powered (energized), photomultipliers must be shielded from ambient light to prevent their destruction through overexcitation. "
The photomultiplier+amplifier circuit gives different readings when the tube is exposed to light than when covered. Do you have any idea on how I could check the tube for damage?
Measuring the performance of PMTs is very difficult - for example, if it has been only slightly damaged, or is worn out from years of use, but still working to some extent.
A first very simple test: Disconnect the tube from all power and keep it in total darkness for 72 hours. Then, keeping it still totally dark - wrapped up carefully in aluminium foil, for example - measure the dark current, and compare this figure with the figure in the Philips data sheet.
As a second test, wrap the whole PMT/NaI assembly in aluminium foil and/or black paper, and observe the output on an oscilloscope. You should see random pulses from local gamma ray background, with very infrequent but high energy pulses from cosmic ray muon strikes on the scintillator block.
The ratio of the average gamma pulse height to the muon pulse height is predtictive of system health.
If this can not be done at the moment, you will have to see what happens when you bring the scintillator head close to a lively gamma source, such as a piece of pitchblende/uraninite (oxid de uraniu nativ). A piece of good pitchblende will produce a dose rate of about 80 - 100μSv/hr at 3cm - with headphones this will make a constant rushing sound, and not individual clicks.
Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
Hi Proud Mary, from your reply I understand that you also show your concern about the possibility of irreversible damage received by my PMT XP2008. I can't find the datasheet (help?) but judging that this tube is counting photons it is probably extremely sensitive.
However I managed to talk to the person I got this from. The device was used for Gamma detection (normally since it has the NaI crystal), for some weak sources that produced 20x the normal background radiation at 10cm away. He also said that he would be surprised the PMT is damaged , since they used to expose the PMT to light, by removing the NaI crystal compartment, while running, for various purposes, and they had no issues with that.
So the only way to know this for sure is to run some tests. If I can find the datasheet, it would be great. I don't have an oscilloscope (will buy one if I run into more problems like this), so I can try the dark currents approach (assuming I can measure them), and easier the exposure to gamma source approach. I have the side check source on a CDV 700.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
radhoo wrote ...
Hi Proud Mary, from your reply I understand that you also show your concern about the possibility of irreversible damage received by my PMT XP2008. I can't find the datasheet (help?) but judging that this tube is counting photons it is probably extremely sensitive.
However I managed to talk to the person I got this from. The device was used for Gamma detection (normally since it has the NaI crystal), for some weak sources that produced 20x the normal background radiation at 10cm away. He also said that he would be surprised the PMT is damaged , since they used to expose the PMT to light, by removing the NaI crystal compartment, while running, for various purposes, and they had no issues with that.
So the only way to know this for sure is to run some tests. If I can find the datasheet, it would be great. I don't have an oscilloscope (will buy one if I run into more problems like this), so I can try the dark currents approach (assuming I can measure them), and easier the exposure to gamma source approach. I have the side check source on a CDV 700.
What Steve says is correct, to which I will add that some tubes will have their performance degraded, but still work to some extent, which is the most difficult to diagnose without costly equipment. Imagine if the performance were to have been degraded by 15%, or had developed non-linearities. You would not notice unless you had access to calibration equipment.
Anyway, the thing is to get it working as best you can now.
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Scintillation Probe (Photomultiplier/NaI)
