Muons are fundamental particles created when cosmic rays
collide with particles in Earth’s atmosphere. They are way more powerful than
x-rays—but can be used in similar ways to CT Scan devices; but with
larger/thicker objects. Instead of CT Scanning someone’s body, you can use the
stronger muons to image entire Egyptian/Mexican pyramids and find hidden, undiscovered
rooms. Likewise, instead of just x-raying a suitcase at an airport—you can scan
an entire plane’s cargo hold. Its first real-world use was in 1970 when scanning
the Pyramid of Khafre, looking for hidden rooms.
So, of course I want to try detecting a muon. Not a whole
bunch—just one. Reliably.
How do scientists detect muons? They can follow the tracks in a spark chamber
(which I have, but it’s too small). They can use photographic film—I have a
darkroom. They can use scintillation crystal sensors—which I have, but I don’t
have dozens of them.
How will I detect muons? By using the humble Geiger counter with a
Geiger-Muller tube. A Geiger counter is basically a lightbulb with a tiny gap
in it filled with gas. When a particle passes through the lightbulb (tube) it
completes an electrical circuit and you get a click noise.
Basically Geiger-Muller tubes are just like those bug zapper
lights people have in their backyards.
I'm using the humble Soviet era, 400 volt SBM-20 tube for this. Two of them.
But Geiger counters will click when hit by X-rays, gamma
rays, alpha particles, beta particles, etc. Plus all the background radiation
makes them click around 20 times a minute in a normal room.
So, we cover the Geiger-Muller tube in a lead sheet, which
will block most of those particles.
Then we get a second Geiger-Muller tube and cover that with
lead.
Then, and here is the genius part, we stack the tubes on top of each other so that the only particles/rays that would trigger both tubes would have to be: traveling
at 90 degrees to the Earth’s surface; be powerful enough to pass through not
one, but two sheets of lead and two tubes; and be moving so fast as to trigger both tubes virtually
at the same time via relativistic motion.
How do we know if
the tubes are triggered at the same time (not just close)? With a humble
AND-gate microchip. This microchip has two inputs and one output. Only if it receives
two input signals at the same time
will it output its own triggered signal (and light an LED or make a click on a
speaker or a spike on my oscilloscope).
If you look at the pinout of the chip (a 74HC08 AND gate logic chip) you will see that if PIN1 and PIN2 get a signal at the same exact time = it will output a signal to PIN3.
Pin1 and Pin2 also each get a 100k resistor to ground (this lets the chip reset to off).
The yellow wires are input Pins 1 and 2. They have 100k resistors going to negative rail.
The other end of the yellow wires go to the 3rd pin of the NE555 chips on the geiger boards. One yellow goes to one pin3 of geiger board, the other yellow wire goes to pin3 of the other NE555 chip on the other board.
Here are the yellow wires going to Pins 1 and 2 on the 74HC08 chip. Notice the 100k resistors going from each one to the (blue) negative rail on the breadboard.
Here is the other end going to Pin3 of the NE555 chip. According to the schematics it should be possible to tie into this via a jumper or LED or speaker connection of the geigers, but that doesn't to work.
Thus, this is what's known as a coincidence detector.
So, stack two cheap Geiger counters sandwiched in lead plates.
Output from both go to a $2 logic AND-gate chip.
AND-gate chip outputs to: LED or Oscilloscope or Speaker.
Literal back of an envelope layout:
Any output basically must equal a muon. They occur about once per minute on Earth.
To make things easier I connected one geiger board to a 5v USB wall plug that had a higher (2 amp) current rating than usual. Not sure this is necessary.
Then I connected the always on screw tightening power block things together from both units. That poweeed the other board.
Then I ran a second pair of power wires to the breadboard.
So, USB wall plug to first geiger board. Then red and black wires between the boards:
Then another pair of red and black split off to power the breadboard:
Those go to the red and blue power rails on the breadboard. I also ran a short black wire from the right blue negative to the left vlue negative rail on the breadboard. That way I could easily run 100k resistors from the 74HC08 1 &2 Pins to the negative easily.
I'll probably reroute that more cleanly in the future. It was a quick test. My breadboard LED came on with mo input! That's why the resistors are needed to zero everything out and start with an OFF LED. Weird, but thats how logic chips like the AND gate work. You have to tell them what 0 or off looks like or else they go nuts.
So, it works.
If I test by turning both on I get an occasional light up by the LED on the breadboard. Especially if I put a radioactive substance near them. Even then, it doesn't light up often relative to the individual Geiger counters clicking and blinking. Good!
In the future I might pull each Geihlger board's J1 jumper to disable their speakers--and then put a speaker on thr breadboard.
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.
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.
