How Do You Set The Voltage On A Geiger Counter?
Well, I'll show you how in this post. Here is my Ludlum Model 3 Survey Meter (it's the boxy thing on the right).
The box is a survey meter. The tube-shaped detectors (on top of the box) can be Geiger-Mueller tubes (GM), or they can be plastic crystals joined to a photo multiplier tube (PMT). So technically it's only a "Geiger Counter" when I have my other detector plugged into it-not the PMT in the photo.
So, in the above photo are what I use to set voltages. Most radiation detectors use 900 volts DC. Some do not. My Ludlum 3 can be adjusted anywhere from 400vDC to 1500vDC! Most true Geiger-Mueller detector tubes take 900vDC. So, we are dealing with electricity at high voltages. Which is why I have a HUGE Cal Test CT2700 high voltage probe, which has a 1000 to 1 voltage divider. 40,000vDC goes in, but only 40vDC would past through to my Extech MN36 multimeter.
Compared to the little probes (extreme left of first photo) you can see this beast means business! It can take up to 40,000vDC and 28,000vAC and only pass through exactly 1/1000th of the voltage, so my multimeter doesn't burst into flames!
I'll be setting the Ludlum survey meter to 900vDC, which will read as 0.900vDC on my multimeter. This high voltage probe needs a multimeter with at least a 10M Ohm input resistance. The Extech MN36 has exactly 10 Mega Ohms input resistance and works great!
So, on the Ludlum Model 3 survey meter there is a metal plate on the top of the case that says "CAL". It is just held on by two screws.
Here it is after removal, showing the 5 adjustment pots (potentiometers) that are screwdriver adjustable. ONLY adjust the top one labeled "HV" for high voltage. Some people put take over the other 4 holes which are used for adjusting the multiples meter readouts. Adjusting those takes a pulser devices which feeds a signal to the meter-if you don't own a pulser, you should never mess with those other pots.
Here's the steps I use to set the required voltage:
1. Unplug the detector cable from the box.
2. Clip the ground clip of the high voltage probe to the detector tubes holding bracket.
3. Turn the survey meter on and set to "Battery".
4. Put the point of the high voltage probe into the center hole where the detector cable is normally plugged into.
5. With a screwdriver, adjust the HV pot until your multimeter reads 1/1000 of your desired setting (900v would display as 0.900v).
6. Once done turn off the survey meter and wait 2 minutes before reattaching the detector cable. You'll hear the high voltage system "power down" a little while after turning the survey meter off.
Cautions:
1. You can get a shock by touching the center hole where the detector cable plugs in.
2. You can get a shock touching the other end of the cable.
3. You can get shocked touching the (red) portion of the high voltage probe below the (black) handle.
4. High voltage stays for a couple minutes (or more) even after removing the batteries from the survey meter!
5. Wait a few minutes after shutting off the survey meter before attaching our detaching the cable and/or a detector.
6. If your multimeter isn't 10M Ohms, your readings will be way off.
How do you know what voltage you need to set your survey meter to? Well, it depends on what detector tubes you want to use. The photo below shows my Ludlum 44-7 alpha/beta/gamma probe which takes 900vDC.
To the right is my Ludlum 42-2 neutron probe (Ludlum 47-1502 neutron scintillator), which is happier with a bit less than 1000vDC, even though the specs call for 900vDC. The meter pegs out at full and the clicks turn into a scream at 900vDC, so I set it around 600vDC when plugging in the neutron probe; but it's a fine art. Sometimes I dial it in to over 1000vDC just to get an occasional click as background. It's touchy! IT SHOULD BE NOTED THAT AT THIS POINT IT'S ONLY DETECTING GAMMA...a Ludlum Model 12 meter would be able to handle neutron probes because it has a threshold knob.
Just yesterday I plugged it in and without using a voltage meter I played around until I could (just barely) discern a slight difference when placing and removing an AmBe (Americium Beryllium) neutron source. I have to believe most of the clicks were gamma radiation noise, but the most usable setting just happened to be 600vDC after removing the probe and checking the actual settings with the multimeter/high voltage probe. At that setting there was a huge rise in clicks when I placed a Uranium source (alpha/beta/gamma) next to it too...so I'm just reading gamma at the moment. At some point a pure alpha check source (Polonium) will be acquired for definitive testing. I don't do much with neutrons at the moment so it's not a pressing issue.
Specs? Luckily, Ludlum is still in business and they have PDF files of many of their old user manuals online for free.
However, for the neutron probe Ludlum had nothing, so I had to find other sources of information. Other people actually contacted Ludlum, and all they could get was a confirmation of the model number. I had to dig deeper than that:
Below is some great information on some older NEUTRON DETECTORS (as opposed to Geiger Counters) which you may find used online, which I also put in an older post about "My Radioactive Dime". I snagged most of this info from a 1973 report to the US Atomic Energy Commission by Alex Lorenz. If you want to read the full report, it's available as a PDF online by searching "Review of Neutron Detection Methods and Instruments".
That document has more information on each device, including the method of detection (i.e. chemical composition of scintillator crystal, etc.). It' a great document to consult if you're like many people and find just a part/tube/probe of one of these devices and want to use it with a different base/amplifier/etc.
You'll want to pay attention as to whether your Neutron detector sees fast or slow neutrons--that makes a difference in whether or not you need to use paraffin or other moderators or actually have to remove those barriers and moderators from your experiment. You don't want to slow down your neutrons with paraffin if your device can only see the fast ones and vice versa.
That document has more information on each device, including the method of detection (i.e. chemical composition of scintillator crystal, etc.). It' a great document to consult if you're like many people and find just a part/tube/probe of one of these devices and want to use it with a different base/amplifier/etc.
You'll want to pay attention as to whether your Neutron detector sees fast or slow neutrons--that makes a difference in whether or not you need to use paraffin or other moderators or actually have to remove those barriers and moderators from your experiment. You don't want to slow down your neutrons with paraffin if your device can only see the fast ones and vice versa.
DEVICE RANGE VOLTAGE
| Ludlum | Fast neutrons | 900v |
| (Model 42-2) | ||
| Eberline | slow or fast neutrons | 900-1200v |
| (Model SPA-2) | ||
| Ludlum | 1/v for thermal neutrons | 900v |
| (Model 42-1) | ||
| Kaman | thermal & fast — | 120v |
| (Model A-300) | 0-14 MeV | |
| Ludlum | thermal - 12 MeV | 900v |
| (Model 42-4) | ||
| IiUdium | thermal & fast neutrons | 900v |
| (Model 42-5) | ||
| LND | thermal neutrons | ? |
| (Series 900) | ||
| Ortec | ? | ? |
| (System 525) | ||
| Nuclear Instruments | Linear between | ? |
| and Chemical Corp. | 10^7 and 10^12 nv | |
| (Model 3782) | ||
| Reuter Stokes Co | 1 0^15 nv | ? |
| Reuter Stokes Cd | 5X 0 ^014 nv | ? |
| Reuter Stokes Rh | 10^15 nv | ? |
| Reuter Stokes V | 10^15 nv | ? |
| Reuter Stokes | 10^10 nv | 1000-1400v |
| (RSN-337) | (thermal) | |
| Ludlum | thermal and fast | 500-2400v |
| (Model 15) | neutrons | |
| Centronics | <7.5x10^10 nv | 1000v |
| (Type D.C. 12) | ||
| Reuter Stokes | 3x10^4 to 2.5x10^5 | 800-900v |
| (RSN-17A/326/ | (thermal) | |
| 330/251/327) | ||
| Reuter Stokes | 10^4 to 10^11 | 800v |
| (RSN-229A) | (thermal) | |
| Reuter Stokes | 10^4 to 10^11 | 800v |
| (HSN-234A-M1) | (thermal) | |
| Reuter Stokes | 10^3 to 10^10 | |
| (RSN-15A/304/ | (thermal) | 100-1000v |
| 325/332/306) | ||
| Reuter Stokes | 10^3 to 10^10 | 200-800v |
| (RSN-314A) | (thermal) | |
| Reuter Stokes | 10^8 to 10^14 | 20-150v |
| (RSN-186S-M2 | (thermal) | |
| and 316S-M5) | ||
| LND | 3 decades | |
| (Series 30771) | 500v | |
| LND | 5 decades | 200-800v |
| (Series 3077) | Thermal (U235) | |
| or fast (U238) | ||
| (Series 3075) | Thermal | 200-500v |
| (Series 3000, | Thermal | 50-500v |
| Series 3050) | ||
| Centronics | 9x10^3 to 9x10^7 | 250-500v |
| (PFC 16A) | ||
| Texas Nuclear | Thermal | 800-1400v |
| (Series 9300 | ||
| Texlium) | ||
| Eberline | Dose response from | 1600-2000v |
| (PNR-4 and | thermal to 10 MeV | |
| NRD-1) | ||
| Eberline | 0.01-10^3 eV & | 1300-1800v |
| (PNC-4) | 0.2-18 MeV | |
| Harshaw | Thermal | 1700-3400v |
| (Model series | ||
| B3, B6, B12, B14) | ||
| Reuter Stokes | 10^-3 to 10^-5 | 2500-3500v |
| (RSN-7A/7S/44/ | Thermal | |
| 177S-M7/320-M2/ | ||
| 108S-MG) | ||
| N. Wood Model G | ? | 1100-2300v |
| Centronics | 3.3x10^3 to 6x10^6 | 900-1100v |
| (Series 5EB/6) | ||
| Texas Nuclear Series 9300 Texlium | Thermal | 800-1400v |
| LND | ||
| (Series 3000, | Thermal | 50-500v |
| 3050) | ||
| Centronics PFC 16A | 9x10^3 to 9x10^7 | 250-500v |
| Centronics PFC 16B | 10^11 | 200-400v |
By the way, another great place to creep around and find info like this is the Oak Ridge National Lab at http://web.ornl.gov/info/reports/ which has tons of DECLASSIFIED reports of various techniques for radioactive fun. The directories are by year--so just poke around. A cool file I found was "The Preparation, Properties, and Uses of Americium - 241, Alpha-, Gamma-, and Neutron Sources" in the 1962 folder.
INFORMATION FOR GAMMA SPECTROMETRY
For gamma ray spectroscopy NaI(TL) crystal scintillation detectors are best. Bicron, Rexon, Teledyne and a few other detector brands can share internal components with each other. Here are general crystal stats:
Type
|
Scintillation
Crystal Type
|
Density
(g/cm)
|
Emission
Maximum (nm)
|
Decay
Constant
|
Index of
refraction
|
Relative
conversion efficiency
|
|
BaF2
|
Barium Fluoride
|
4.88
|
310
|
0.63
|
us
|
1.50
|
|
BGO
|
Bismuth Germanate
|
7.13
|
480
|
0.3
|
us
|
2.15
|
15-20
|
CaF2 (Eu)
|
Calcium Fluoride
|
3.18
|
435
|
0.94
|
us
|
1.47
|
50
|
CdWO4
|
Cadmium Tungstate
|
7.90
|
470/540
|
20/5
|
us
|
2.30
|
25-30
|
CsI(Na)
|
Cesium Iodide doped with Sodium
|
4.51
|
420
|
0.63
|
us
|
1.84
|
85
|
CsI(Tl)
|
Cesium Iodide doped with Thallium
|
4.51
|
550
|
1.0
|
us
|
1.79
|
45
|
CsF
|
Cesium Fluoride
|
4.64
|
390
|
3.5
|
ns
|
1.48
|
5-7
|
GSO(Ce)
|
Gadolinium Silicate doped with Cerium
|
6.71
|
440
|
30-60
|
ns
|
1.85
|
20-25
|
LiI (Eu)
|
Lithium Iodide
|
4.08
|
470
|
1.4
|
us
|
1.96
|
35
|
NaI (T1)
|
*Sodium
Iodide doped with Thallium*
|
3.67
|
415
|
0.23
|
us
|
1.85
|
*100*
|
YAP
|
Yttrium Aluminum Oxide Perovskite
|
350
|
27
|
ns
|
|||
ZnS(Ag)
|
Silver activated
Zinc Sulfide
|
4.09
|
450
|
110
|
ns
|
2.36
|
25 - 30
|
Rexon Inc.'s Dr M. H. Farukhi has layed out an informative explanation of each crystal type here: http://www.rexon.com/crystalscintypes.htm
Lemme know when all the neutrons are gone and it's safe to come out! Meow.
