Muon Decay

[alan sailer]

A while back I decided to try and detect muons and hopefully their decay. Muons are a species of secondary cosmic rays that account for about half of all sea level cosmic rays. The idea behind the experiment is to set up a large enough detector that can both detect the initial muon event, slow down the muon enough so that it stops and then record the energetic electron that results from the decay. The detection is doen using some sort of material which emits light when hit by and energetic event and then a photomultiplier tube (PMT) to detect and amplify the faint flash.

The papers I was able to find all used fairly large blocks of plastic scintillator, at least six inches in all directions. I could not find any Ebay listings for such thick blocks so I bought a big chunk 35x14x1.5" of Bicron BC412 and planned to cut it up and stack (using optical coupling grease).  Cutting and then polishing all this was not a job I looked forward to.

A friend of mine who works with optics had a good idea. Just try the slab as is and see what happens. I still needed to polish the ends of the slab as it was cut from a larger piece. I used sandpaper starting at 180 grit and went down to 20,000. The plastic is very soft so this took less time than if the plastic was Lexan.

The scintillator must be light tight and reflect light back into the slab when it tries to escape. Most papers descibe using aluminum foil but one paper I found used white Tyvek sheet. I used heavy white paper as it was easy to work with.

Here is a picture of the white wrapped slab inside a wooden box.



The PMT was attached to one side of the white paper wrapped slab. The end of the PMT was optically coupled using index matching grease.  The PMT is shown in this picture.



The PMT circuit is the same as found in the Hamamatsu paper on PMTs. the circuit is the diagram shown for fast pulse detection.
The PMT I used is a very old RCA 6342A. I have always wanted to wire up and use a PMT and I find them fascinating devices. I also happened to have an acient HV supply that turned out to have very low noise, perfect for a supply.

After I mounted the PMT to the paper wrapped slab I had to build a lightweight box to seal away any outside lights sources. I used very thin plywood with heavier bracing. The inside of the box was light sealed using roofing tar.  The lid of the box as wel las all the other sides were sealed using gaffers tape. Two coax cables brought the HV (~1200V) in and the detected pulse out.

The final sealed box.



The "first light" went well. I immediately saw pulses on my scope, about one per second.  However the were quite wide, which puzzled me. I also saw no double pulses indicating capture and decay. I was showing my wife the pulses when I saw a strange looking pulse.



It looked like it could be a muon decay. But still way to wide. I found out that using a 10X probe for such tiny charge events was integrating the pulse. When I went to direct coax into a fifty ohm load I got better results.



A muon detection/decay is shown here.



And finally I put the scope on infinite persistence and let it run for quite a while (hours).




It was a fun experiment. If I enjoyed working with computers the next step would be taking data and calculating the muon half life (~2usec). But I don't.

Cheers.

[Mads Barnkob]

Nice experiment, to read about something invisible occuring and then building a setup to detect it yourself is something special

I am a little puzzled by the need for such a large scintilator, maybe its because those papers are about deep sea detector arrays?

My own experiments just used half-beer-can sized crystals and could also detect muon decay: https://highvoltageforum.net/index.php?topic=7.0 and

[alan sailer]

As I mentioned I found several papers describing undergraduate built muon decay experiments. They all used detectors that were very thick. So I assumed that a thick detector was needed.

After doing this experiment I have no doubt that a smaller scintillator would work just fine. I simply had no idea. I've made way worse mistakes in my life than buying a large block of plastic.

But now I have to find some way to get rid of it. If anyone else has other  experiments that I could do with this slab then please let me know. Or wants me to cut up and sell it....

At work they have a Gen III image scope that I am trying to borrow. I'd love to see if I can actually see that flashes with this tool. That would be really neat.

If this post duplicates stuff that has already been done I'll be glad to delete it and move on. It was just a moderately entertaining project that I did, nothing earthshaking.

Cheers.

[Mads Barnkob]

No need to delete a great thread. Some times the small experiments are more vital to understanding our world than the large experiments!

You could chop up the scintillator and build a muon coincidence detector, there is even some open source software/microcontrollers for that. I was just googling around a bit and found this: https://muonpi.org/ a RaspberryPi based muon network online! I might even join up my not-used-pi and my PMT for that

[alan sailer]

Mads,

Thanks for the suggestion. I have seen muon telescopes based on paired detectors.

The use of microcontrollers and computers is usually enough to turn me away from a project.
Programming has always either bored or frustrated me. It's like work but without the paycheck.

Cheers.

[Da_Stier]

Hi Alan,

very nice.
Even if an experiment isn't new or original it always is very interesting to see what details, problems and interesting things you face when actually doing it.
I like the big scintillator block, it provides a nice surface area.

I guess your infinite persistance scope screenshot shows different particles being captured, since the peaks have different amplitude?

If you decide to cut up the slab and sell it, I would definately be interested in getting a piece or two. 

Greetings,
Michael

[Uspring]

Lovely experiment 
Big scintillators increase the probability of stopping muons within the scintillator. Most muons have a high energy and will zip right through the block and decay far underground, where you won't see the decay. You will see the traversal, though, and depending on the energy and the length of path within the scintillator, there will be more or less light generated. For accurate statistics it is helpful to not have too many background (e.g. traversal) events.

[alan sailer]

Da_Steir,

Anything I'd say about the different amplitudes would be speculation but theoretically with such a large block particle that decay further away from the PMT would have smaller amplitudes.

A piece of data that initially confused me was the event that occurred before the triggered pulse. If you look at the final infinite persistance image you can see this. I believe that there is no rule that the initial muon detection has to be brighter than the decay pulse. The geometry of the slab should allow a far away (and dim) initial pulse and a brighter (closer) decay event. The trigger would pick up the decay and thus the initial entry pulse would appear before.

I'll keep you in mind for chunks.. I have another experiment to try and then I am done with the slab. A little warning, the dealer said that he was unsure if it was Bicron 412  or 408. It does detect muons and gets more frisky when I bring a piece of U ore near it, so it does do gammas.

Cheers.

[Da_Stier]

Da_Steir,

Anything I'd say about the different amplitudes would be speculation but theoretically with such a large block particle that decay further away from the PMT would have smaller amplitudes.

That's a pretty good point, with such a large slab, the distance to the PMt is definately not negligible, I totally did not think about that.

A piece of data that initially confused me was the event that occurred before the triggered pulse. If you look at the final infinite persistance image you can see this. I believe that there is no rule that the initial muon detection has to be brighter than the decay pulse. The geometry of the slab should allow a far away (and dim) initial pulse and a brighter (closer) decay event. The trigger would pick up the decay and thus the initial entry pulse would appear before.

For this it might be pretty interesting to have several PMTs around the slab.
I guess like that you would be able to triangulate the exact location of the event and therefore calibrate out the "light transmission losses" through the slab.
If you push this to the extrem, a spherical scintillator with a grid of PMTs might give the best results, since it has the least sharp edges and the smallest distances to every PMT.

I'll keep you in mind for chunks.. I have another experiment to try and then I am done with the slab. A little warning, the dealer said that he was unsure if it was Bicron 412  or 408. It does detect muons and gets more frisky when I bring a piece of U ore near it, so it does do gammas.

Thanks a lot. The exact type wouldn't matter for me, since I would also just play around a bit without the need of trustworthy output data. 



Greetings,
Michael

[alan sailer]

Michael,

The light paths inside a slab must be crazy. The surfaces of the slab would cause all sorts of internal reflaections and escapes. The white paper is not well coupled to the slab either. When I was trying to think of a good way to cover the slab I wondered if just painting it white with latex house paint would be a good idea. The trouble of trying to remove it (if just soaking in water didn't work) and having to polish such a lrage surface kept me from trying the experiment. If I do end cutting it up (quite likely) it might be fun to try the white paint on a smaller chunk.

The multiple PMTs is a fun idea. Do enough and with a proper computer could you do a single tomography slice and draw a reconstruction of the event? I know enough to ask the question but not enough to answer it. Story of my life...

Cheers.

Cheers.

[Da_Stier]

Hi Alan,

The light paths inside a slab must be crazy. The surfaces of the slab would cause all sorts of internal reflaections and escapes. The white paper is not well coupled to the slab either. When I was trying to think of a good way to cover the slab I wondered if just painting it white with latex house paint would be a good idea. The trouble of trying to remove it (if just soaking in water didn't work) and having to polish such a lrage surface kept me from trying the experiment. If I do end cutting it up (quite likely) it might be fun to try the white paint on a smaller chunk.

Well that is of course another very good point, that I didn't even think about.
(This is also what I meant by my first post, to learn about all the little details of doing the experiment, since I would have missed a lot of these details)
I guess the problem is acutally pretty similar to an electric impedance mismatch so you need to optically match the impedances of the slab to the "termination wrapping".
This also makes me wonder if it is possible to get multiple readings from the same event by literally having reflections in your optical medium with an incident and reflected light wave, however I guess they would be so close together that it should not be an issue.

The multiple PMTs is a fun idea. Do enough and with a proper computer could you do a single tomography slice and draw a reconstruction of the event? I know enough to ask the question but not enough to answer it. Story of my life...

I would expect, that you can do a full 3D view of the event if you have enough sampled data from all the PMTs and know the exact behaviour of the optical system.
This idea acutally made me wonder how they generate the kind of muon decay graphs.
I expect they are all just simulations and not actual measurements but at something like a particle accelerator you face a similar problem, don't you?
I guess the hardest thing would be to differentiate between different events and or particels if they occure simultaniously, since each PMT can only measure the overall brightness of everything that happens at the same time.

Very fascinating to think and read about. 

Greetings,
Michael

[Uspring]

Alan wrote:

I believe that there is no rule that the initial muon detection has to be brighter than the decay pulse.

Yes, during the initial event the muon is stopped within the scintillator. The light flash brightness depends on the kinetic energy of the muon entering the scintillator. There is an upper limit to this energy, since the high energy muons will leave the scintillator before they are stopped. My guess would be maybe a few tens MeV for your setup.

The decay event is detected by the electron or positron emitted during the decay. That particle can have an energy up to the muons rest mass, i.e. ~100 Mev. But the electron won't leave much energy in the scintillator, since electrons at high energies aren't braked that much and will leave the scintillator before they have lost very much energy. Typical energies deposited by an electron/positron in the scintillator might be around a few MeV.
So you can have a low energy stopping event, followed by a higher energy decay. But that is less frequent.

[alan sailer]

Uspring,

Thanks for the analysis. I was thinking more about geometry ie events happening further away from the detector. But particle energy and the stopping power of the plastic for different types of particle would have an effect also as you point out.

By the way I did get a chance to look at the slab with a night vision set-up. There were flashes visible but these "flashes" were present looking at non-scintillator objects. They look like random noise from the image tube. I suspect that if I could somehow compare the flash rate with and without the scintillator I would see an excess that would be due to actual detection events. But with just a casual look I cannot claim to have seen muon events.

Too bad. It was worth trying.

Cheers.

[Uspring]

You might be able to detect events with your bare eye. In this article https://www.nature.com/articles/ncomms12172 it was reported, that humans can detect single photons, provided they have good vision and their eyes are well adapted to darkness. Depending on the muon energy and scintillator efficiency there might be hundreds or even thousands of photons emitted by an event. I dunno specifics about your scintillator and whether it emits photons in the visible region. Also considerable patience might be required due to the scarcity of muons.

[Mads Barnkob]

You might be able to detect events with your bare eye. In this article https://www.nature.com/articles/ncomms12172 it was reported, that humans can detect single photons, provided they have good vision and their eyes are well adapted to darkness. Depending on the muon energy and scintillator efficiency there might be hundreds or even thousands of photons emitted by an event. I dunno specifics about your scintillator and whether it emits photons in the visible region. Also considerable patience might be required due to the scarcity of muons.

Interesting enough... so interesting that you had my go under all my blankets on the bed and stare into the deep darkness of nothing for 10 minutes.

Did I see anything, while trying to stare into my plastic scintillator maybe. I am not sure what I am looking for. Is it a bright flash or just a tiny peck, is it going to draw a line.. is it just a faint dot...

I did experience several small lines and s single flash, BUT I can not say it was a muon, it could just be noise on the eye/brain or my eye lids flashing.

I did get to lay there and ponder about the universe and I did get nice warm and comfy, I say go try it! At least you get a good rest and some warmth

[alan sailer]

Uspring,

Strangely enough this whole looking at the scintillator project started because my hiking partner (who is a professional optics guy) wondered if the human eye could see the flashes. I never tried the experiment because I don't have any place that is really dark. When I was using the night vision goggles I noted all the little lights that are on inside my garage. Light from power strips etc. The light from those little guys cast visible shadows six feet away while wearing the googles.

Also my vision is pretty poor. I guess I have nothing to loose trying the experiment.

Mads,

when I saw flashes using the night vision goggles I only saw pinpoint flashes. Had I seen any trails then I would have been very happy.  Incidentally my muon detection rate* with my large scintillator/PMT was higher than one per second.
So there should be lots of flashes to see.

Cheers.

* the muon decay detection rate was a very orders of magnitude lower.

[Uspring]

@Mads
As Alan said, you should be looking for lines, which are the visible remains of the ionisation trails created by the muons. If the muon is headed toward you, it might look more like a point flash.

@Alan

So there should be lots of flashes to see.

The energy loss of muons and therefore the brightness of the lines depends strongly on their energy. See e.g. page 6 of this doc: https://pdg.lbl.gov/2021/AtomicNuclearProperties/adndt.pdf .
The diagram is for copper, but the stopping power for plastics looks similarly. Energy loss decreases strongly between 10 keV and 100 MeV. The human eye might be just sensitive enough to notice the muons around the energy which can just be stopped in the scintillator. And these are the rare ones, where you also see a muon decay. That's a bit speculative, since I don't know the visibilty threshold for such an experiment.
A histogram of your PMT pulse heights should show these brightness variations.