Archive for the ‘ Physics projects ’ Category

Geiger Counter/ scintillation detector

Description:

A device that can measure radiation using a variety of different detection instrumentation. An adjustable high voltage supply, PMT pre amp, counting circuitry, digital display, and PC interfacing systems are included. It is compatible with many different tubes, including geiger muller tubes, PMT based scintillators, or even Boron 10 lined proportional tubes.

Introduction:

Shortly after I first made the wind bottle CRT, My dad started to get concerned about radiation. Even though I was sure there was absolutely no significant amount of radiation being produced, and even if there was it was very low energy, but I had to get quantitative experimental data. The only problem was I had nothing capable of measuring radiation. Luckily I did know someone who’s main hobby was radiography, and he agreed to help me. He brought out a Ludlum scintillation unit, and I was intrigued by it to say the least.

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The unit had an a digital readout, adjustable timer for total dose counts, and adjustable power bias and amplifier settings to allow for compatibility with various different scintillation tubes.

After that, I knew I wanted to make my own unit, but I wanted to be able to detect basically any type of radiation. Pretty much the only way to do this is to use a lot of different types of tubes with it.

Design:

I decided that to start designing with the probe. I could have bought an SBM-20 very cheaply, and been done, but the first thing that I wanted to be able to do was measure background radiation, something that Geiger tubes are not the best at doing, although I wanted to return to it, I decided to start with a scintillation tube.

I chose to base the probe around the R1307 photomultiplier tube, coupled to some scintillation plastic. I chose this tube because of the large front view window, and because its peak sensitivity is at about 350 nm, that matched the scintillation plastic’s emissions. 008 012 013

The PMT has a board built into it with the divider circuit built in, which is a definite time saver. I did have to pick apart all the connections on the board myself, as no schematics were available. next is the power supply based on a CCFL inverter, and the amplifier for the PMT.

Construction:

Beginning work on the probe construction, the first thing I had to do was polish the sides of the scintillation plastic. They were very rough from machining, and needed to be more reflective for the plastic to be most effective.

To get the sides smooth I sanded the sides progressively with 60,120, and 400 grit sand paper before rubbing it with firm pressure on a brown paper bag laid flat on my work bench. Despite not having the best selection of sand paper, it worked pretty well. Not perfect, but surprisingly good.

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Before…

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After.

Next I had to make some modifications to the PMT’s board. I began unsoldering the resistors in the divider circuit, which actually proved pretty difficult (they were bent over the pad making desoldering difficult), and replaced them with higher value resistors, and rewired a few things using the following schematic that I found after searching the part number on my PMT.

R1307-10

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Above is the board with the original resistors

 

 

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The 10meg resistors (only 8 were needed).

Since the resistors were so difficult to desolder, I eventually removed the entire board to finish making all the modifications marked on the schematic.

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Above you can see all the resistors in the divider have been replaced.

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I made the rest of the necessary modifications and…

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I replaced the board on the PMT which turned out to be a less than trivial task. Since I had to clip the leads to remove the board, they became too short to support the board, so I had to solder some longer leads to them prior to replacing and re soldering the board.

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Above: The tube ready for coupling to the scintillation plastic.

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The scintiallation plastic came with quite a bit of optical coupling compound, which I made good use of. I applied a small amount of the compound to the face of the PMT to help make a good connection to the plastic.

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Above: I prepared the plastic by wrapping it in paper to keep the adhesive on the electrical tape from affecting the plastic, and then I wrapped both the tube and plastic with black electrical tape excluding the sides that will be coupled.

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I then put the plastic on the PMT window, and used a gentle swirling motion to evenly spread the optical compound without creating bubbles. I used a few long pieces of tape to hold the scintillator to the PMT, and then finished wrapping both of them with black tape.

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I decided to use a paint can to act as an enclosure for the detector because it was large enough to hold the PMT, and it was also metal which means it is easy to ground and can effectively shield the detector from electrical noise.

I wrapped the scintillation detector (now barely short enough to fit into the paint can in some recycleable packaging material digikey always uses in their shipments. I made sure to fold a little over the front of the scintillation tube to help protect it.

After drilling a hole in the lid of the can, I tightened a female BNC connector into the hole, and soldered the ground and voltage wires to it in their respective spots (ground outside of the connector and power+signal inner pin). I then taped around the connector with black tape on the inside to stop any light leaks.

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I closed up the can, which concluded the construction of the detector.

Now It is time to work on the amplifier, power supply and counter electronics.

Results:

Conclusion:

Proving Heisenberg’s Uncertainty Principle: the work in progress

The uncertainty principle is one of the most coveted laws in quantum mechanics. Its mysterious properties have mystified physicists for decades, and you may be surprised to find out that its counter intuitive affects can be observed somewhat easily. Shine a laser through a slit. what you see projected behind it is a spot from the laser. As you close the slit, you see the spot get thinner and thinner, and you expect this to continue until the spot is too small to see, but it doesn’t. Once the slit gets very thin (on the order of a few hundredths of an inch), you see something completely unexpected. The projected light from the laser begins to fan out, getting wider and wider until the slit closes and no more light gets through.

This experiment is a somewhat common demonstration, and if you don’t believe it, try it your self. You will get the same surprising results that so many have in the past. Heisenberg came up with a very specific law to prove explain this, but how do we prove it exactly? you could measure the amount of dispersion and the width of the slit, using the unchanged output of a laser pointer, but to me this seems like it would invite a large percent of error.

I wanted to be able to prove this myself, but in a more profound way. I got thinking. Would it be possible to shine a single photon source through a slit and prove the uncertainty principle on a level of individual particles? It seemed difficult, but I actually already had a lot of the necessary materials to do this. I wanted to perform the double slit experiment with individual photons a while ago so I bought 5 ND 3.0 neutral density filters and a 1 mW laser pointer in hopes of making a single photon source. ImageImage

I intended to test the setup by letting my eyes get fully dark adapted and then try to see the characteristic uniformly bright flashes from individual photons, but I didn’t have enough patience to do it successfully. So the setup sat for a long while, until it made its way up on my “to do list.”

I reassembled the single photon source and tried to get results with my Dad’s DSLR camera (a Rebel T1i (?)).

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I wasn’t about to just shine things through slits before a control though, So I built a quick setup to hold the laser in a stationary position with an external 3.3v supply for the long exposures I made using a bulb setting on the camera with a remote shutter control.

I mounted everything on a small wood frame I made and put it in a dark exposure box.

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After successive 1, 5, 10, 15, 30, and 60 minute exposures with both 5 and 4 filters, I couldn’t see any photons that were definitively from the laser pointer. The picture was being exposed by something however, as small spots getting more numerous with the longer exposures were visible, but it can be difinitively stated because of their dispersion that they were not from the laser pointer.

These initial tests while unsuccessful have shown me two things.

1. My “dark box” is imperfect. I am confident that no light from outside was getting it, but their was a point of weakness in that the laser’s power supply was included on the inside of the box, and it had an LED that was lit when it was on. I tried putting it in another box and covering it with a trash bag, but it is clear it was not enough. I will need to try to somehow keep it outside the box entirely to prevent exposure from anything other than the laser.

2. I did not have a good macro lens for the camera, which means that I could not focus very well on the laser which sat 12-18 inches away from the front of the lens. I didn’t realize it at first, but the fact that it was not focusing well could have been causing the individual photons to be too poorly defined to be identified. If the above point does not fix the problem I will have to wait until I have access to a better macro lens before I can continue this experiment.

To summarize, these past few weeks spent on this project have, while not being successful, shown me weaknesses in my apparatus and have shown me how I will have to proceed with the experiment. Keep an eye out for updates. I should have an update in a couple weeks or so, which will tell whether I will be able to get good results soon, or if I will have to wait longer for a better lens.

Wine bottle CRT

So some time ago I was browsing youtube when I cam across the most wonderful video I had seen up to that point. Someone had constructed an old school cold cathode electron accelerator (called CRTs or discharge tubes) out of a wine bottle and some stuff from a local hardware store. I made a mental note that I wanted to try it, but when I saw the cost of the necessary vacuum pump I kind of lost interest. a few months later or so I was looking at instructables, when I saw that Daniel Kramnik had posted instructions on how to make it. This rekindled my interest, but I still didn’t have the money to buy a pump. That summer I saved a little money and at last I had one. Also that summer I had gone to a local neon sign shop and picked up a small neon sign transformer for $20! For the vacuum port on the bottle (also acts as the cathode) both of my sources for making this recommended using a concave surface which helps focus the beam. The original video said that they had used a drawer handle and the instructable said to use a mini doorknob. I couldn’t find either at my local hardware store, so I made my own design for the vacuum port. I had previously tried to setup some kind of system for connecting my vacuum pump that could act as the standard for all of my future projects. When I bought the pump it was claimed to have both a 1/4″ and 1/2″ port on the pump. When I got it, it looked more like a 1/4″ and 1/8″ ports. This was not much of a setback however. More frustrating was that I could not find a single flared hose barb fitting at any of my local hardware stores. I have found some that will work on belljar.net, but they don’t come cheap. It’s about $10 dollars for one of them. After a while I resolved to just tighten the hose I was going to use directly onto one of the ports threads. I had a length of vinyl tubing sitting in my garage that was the right diameter, but its walls were too thin and it collapsed under vacuum. To combat this I found some 1/4″ fuel line that was strong enough to withstand the vacuum, and used a small piece of the vinyl tubing to connect the vacuum pumps port to the fuel line. Hose clamps were used on both connections and lots of teflon tape on the pumps port. For most of my experiments I simply connected the bottle to that one length of tube directly to the pump and it worked surprisingly well. I am currently working on trying to connect a thermocouple gauge tube to get vacuum readings, but am having some problems with leaks  on new connections. For most purposes the single hose connection will be fine. Image This shows how I made the connection with the hose. A flared hose barb will be used in the future. Based on my system for connecting to the pump (using the 1/4″ fuel line) I decided to just buy a whole bunch of hose barbs that fit the fuel line to connect the chamber to the hose. The hose barb size ended up being 1/8″ pipe thread to 1/4″ barb I think. To make the vacuum port on the wine bottle I used one of the hose barbs described above, a 1/8″ pipe nipple, two washers with holes that roughly fit the nipple, and some epoxy (I have tried both five minute epoxy and JB weld successfully). I started by gluing together the two washers. Once that dried I tightly threaded the hose barb onto the nipple (teflon tape is not needed because epoxy will seal it). I then put the nipple through the washers, and glued the bottom of the hose barb down taking care to leave no gaps in the bead of epoxy. I then aded more epoxy around the edges of the hose barb to ensure a good seal. once the assembly was complete I used a rather large amount of epoxy to secure the washers to the top of the bottle once again put more epoxy on the sides to fill in any gaps and get a good seal.ImageImage The vacuum port also acts as the cathode so we will not have to worry about making at least one of the electrodes. The anode however does need to be added. I found a 3/16 (I think) glass/tile spade bit at my local hardware store which was perfect for drilling a hole in the side of the bottle to allow for a wire to be put through to act as the anode. Drilling the hole is pretty strait forward. You just put the bit in your drill apply a little pressure and start drilling slowly and carefully. I found the hole can be a little difficult to start, but putting a piece of masking tape on the spot you want to drill helps keep the bit from wondering away from where you want to drill. You should probably do this before you attach the cathode/ vacuum port, because if the bottle cracks when you are drilling it is trashed and you can’t use it. Once the hole is drilled You just need to slip the wire for your anode in the hole (the shape doesn’t really matter. I’ve put it in a loop and just bent it downwards and it doesn’t really affect performance), and epoxy it in place. I used a thick layer of epoxy on top of the initial seal to help keep the anode wire from breaking from being bent around a lot (my original CRT had to be decommissioned because the anode wire broke off). Image It’s really messy, but it provides a good seal and good strain relief for the wire. The next step is to make a power supply. As a rule of thumb you want a DC power supply that can provide around 10kv. You can use higher but beyond 30kv x-ray production starts to become a concern, so you don’t want to exceed it. You can also use a lower voltage (as I did). I’m not sure what the minimum voltage required to get it to run is, but I would bet you need at least 2kv. I made my power supply by using a 6kv 30ma neon sign transformer with a microwave oven diode to rectify the AC. To find the DC voltage that is output by rectified AC you use the folling formula: Vin (1.414) = Vout. To wire up the CRT I connected one HV terminal of the NST to the cathode of the CRT, and connected the other terminal to the anode of the microwave oven diode. The cathode end of the diode connects the anode of the CRT. The low voltage terminals of the NST get connected to the hot and neutral or mains. Pay attention to the stripe on the diode which indicates the diode’s cathode when wiring up your CRT and always use wire rated for the voltages you are working with. ImageImage All that’s left to do now is fire up your pump, turn on the high voltage and watch the plasma!ImageImageImageImage Nothing happens at atmosphereic pressures because the free mean path is too small, but once your pump has been running for a while plasma begins to form as the accelerated electrons excite the gas (nitrogen) inside the tube. You will notice several features that occur with in the tube which deserve explaining. One of the first features of the plama you will experience is what’s known as a Faraday’s space, and it is the gap in plasma that starts to form at relatively low vacuum, where the collum of plama begin to separate from the cathode. This occurs because there is a higher concentration of electrons near the anode (which electrons are attracted to) than the cathode resulting in more plasma. After pump runs a little longer you will notice the plasma forming stripes. These are called striations and they occur because there are going to differences in the glass (and any other material in the tube) from one part of the tube to another resulting in a higher concentration of electrons, and therefore plasma, in some parts of the tube. These descriptions are probably over simplified, but they get the idea across. If your pump is good enough and your bottle sealed well enough, then after the striations form, the faraday’s space and striations disappear and the bottle gets filled with a uniform light blue plasma. After this happens you may notice another very interesting feature. Starting from the cathode the plasma will disappear, and be replaced with emptiness within the tube. The glass around the emptiness however will fluoresce under the bombardment of electrons, and as the vacuum increases so does this empty space until the amount of gas inside the tube is too low to facilitate cold cathode emission any longer and the tube goes dark (it is unlikely you will reach this low of vacuum with your pump). This is called the Crookes dark space and to me it is one of the most fascinating features that occurs in discharge tubes. This project was a lot of fun, and now I can tell my friends that I have built a particle accelerator! I am far from finished with CRT projects though. In the future I intend to make a CRT oscilloscope from scratch using an electron gun assembly. I also want to try an experiment that uses an old 2 inch CRT along with some other equipment to determine the charge mass ratio of the electron, just like Thomson’s third major CRT experiment. Also for those who are interested I am currently working with some professors from DU to make a more serious and more powerful particle accelerator (atom smasher) to perform experiments I hope to enter in the ISEF. Well that just about does it for this project. I encourage you to try this and have fun with it, but be very careful when dealing with high voltage and vacuum, and make sure you are not producing x-rays! Also You should probably limit the amount of time you leave this running to a minute or so to decrease the chances of something bad happening.

Seeing alpha particles

A few months ago I deccided it was about time for me to actually do an experiment, which I hadn’t done in a while. There was one that I had heard about from many different sources and wanted to try.

Invent Geek as well as numerous videos on Youtube claimed that with just a smoke detector and a webcam you could indirectly see alpha particles. All you had to do was extract the americium 241 from the smoke detector and place it close to the sensor of the camera and you had yourself a device which could detect alpha particles.

Aside from sounding too good to be true, Americium is technically not an exempt source since its proton count exceeds that of uranium. However after seeing the price of other exempt sources online, and seeing how many people bend the rules, I decided it would not be a big deal.

Extraction of the americium from the smoke detector is relatively simple. There were no screws holding the plastic case together so I just pryed it apart with a couple of flat head screw drivers. From there I removed the PCB. The americium is housed an a bulky aluminum case which is held to the board with two solder joints. After desoldering those contacts the case was removed, exposing a plastic base with metal plate in the middle connecting to one pin of an IC. After desoldering the IC pin I broke of the tabs holding the plastic to the board, and carefully pushed the metal button out of the middle of the plastic base. This button has a thin foil of americium 241 on one side of it and that button is what I was after.013 021

Next was the webcam portion. Alpha particles (or a high energy helium nucleus) are very positively charged, and consequently have a habit of ionizing every atom they come across which takes a lot of energy. This means that not only do they have a range of only a few cm in Earth’s atmosphere, but they are also stopped by just about anything. Because of that, they are not able to go through the lens of the camera.

The lens on the camera i used just screwed right off (I used a $5 PC cam I found on amazon), and allowed me easy access to the cmos sensor to place the americium in front of. In fact, the back of the lens acted as a perfect place to mount the button. By simply gluing the americium to the back of the lens, I killed two birds with one stone. I got the sample close to the cam’s sensor with no barriers, and I blocked all the light that would have otherwise come through the lens.

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To put the finishing touches on the device I painted an old plastic container black and added that classic identifying symbol for all things radioactive.025

And now of course the most important part, the results. I plugged in the webcam to my computer and watched the screen. I was amazed and excited to be greeted by small flashes of light which are a result of the alpha particles hitting the sensor. I thought for a while the flashes may be the sensor detecting x-rays resulting from Bremsstrahlung, but more likely the particles are pulling electrons from the sensor resulting in an electron hole which causes a flash to occur on the screen. Either way it was a fun and pretty simple project.

I will try to post a video soon, but until then you can see more detailed instructions and a video on an instructable I made about this (I got first prize in their Hack it contest with this). Just go to instructables.com and search “how to see alpha particles.”