Cindy Kelly: I’m Cindy Kelly, Atomic Heritage Foundation. It is Wednesday, April 25, 2018. I am in Oak Ridge, Tennessee, with Zane Bell. My first question to you is to say your full name and spell it.
Zane Bell: Zane Bell. Zulu, Alpha, November, Echo, Bravo, Echo, Lima, Lima.
Kelly: Okay, good. First, I want to know something about you and your childhood—where you are from, and how you got to be interested in science.
Bell: I was born in New York City. I grew up there, attended the Bronx High School of Science. I was always interested in physical sciences. Attended Rensselaer Polytechnic Institute in upstate New York, and then really got into the physics program there. Eventually went on for a Ph.D. at the University of Illinois in physics, and here I am.
Kelly: Here you are.
Bell: I met a recruiter, and came to Oak Ridge. It seemed nice. I was only going to be here two, three, maybe four years. Now, it’s forty years later, and I’m still here.
Kelly: That’s marvelous. I keep hearing that story again and again. “I was going to be here two years, and now it’s been twenty.” People seem to like it here.
Bell: People do. People are friendly. Climate is nice. Four seasons, and the winter is short.
Kelly: Sounds terrific. Tell me, what is it that you do here?
Bell: I am a senior scientist working in radiation detector development. I design radiation detectors, some of the electronics sometimes. But my real interest is in scintillators and the detection mechanisms.
Kelly: What is a scintillator?
Bell: Scintillator is a material that when struck by ionizing radiation, such as x-rays, gamma rays, charged particles, protons, electrons, they will emit light in response to that. The amount of light is approximately proportional to the energy that’s deposited in the material. Examples are sodium iodide, cesium iodide, cadmium tungstate, plastics that are loaded with phosphors, liquids loaded with phosphors –– those organic materials, organic liquids, xylene and toluene based. Even some gasses scintillate.
Kelly: My son was fascinated by fluorescent minerals as a child. These are some of the minerals you just named.
Bell: Some of them are, indeed. Around the turn of the twentieth century, after the x-ray tube was invented in about 1895, Thomas Edison, who was working in New Jersey, went to the American Museum of Natural History in New York. Case by case, he borrowed — I shouldn’t say took — borrowed their mineral collection, and exposed it to x-rays. Some of them glowed and some of them didn’t.
All scintillators will respond to ultraviolet light and x-rays, but not everything that responds to ultraviolet will scintillate. It’s a physics thing.
The way a scintillator works—crystal is the easiest one to understand. In a crystal, you have a three-dimensional regular lattice of atoms. In the case of sodium iodide, it’s sodium next to iodine next to sodium next to iodine in three dimensions. It’s a cube, a cubic structure.
When ionizing radiation, say a gamma ray or an x-ray, goes into it, it causes electrons to be liberated from the atoms. They can travel around the lattice pretty freely. Eventually, they find a defect of some sort. In the case of sodium iodide crystals, people purposely put thallium metal into it. Thallium iodide is put in with the sodium iodide, at about twentieth of a percent. When the free electrons find these thallium sites, they drop down to a lower energy state and light is emitted by the thallium ion. That’s picked up by a photo sensor, and that tells me that something happened.
Kelly: They always have these defects?
Bell: Some are put in purposely. Thallium would be considered a defect in a pure sodium iodide lattice. Other materials, like cadmium tungstate, don’t require anything like thallium. They naturally do it, because there are always vacancies in the lattice. It’s at a small level, a small fraction of a percent, but those vacancies on the lattice are what when electrons reach them, it looks as if there is an atom there with the wrong amount of charge. The electron interprets that as being an activator, and the same process occurs.
Kelly: Interesting. So this technique works. What application is there? Why are we interested in this?
Bell: We are interested in this for a lot of reasons. There are radioactive materials in the world. Naturally occurring is uranium, thorium, potassium. Bananas are radioactive, a little bit, but not very much. There is very little potassium-40, comparatively speaking. When you go for a dental x-ray, that’s radiation. They don’t put a scintillator in your mouth when they do a bitewing x-ray. But if you go for a CT scan at the hospital or a fluoroscopy, that’s a scintillator that is generating the image. That’s a major application.
Other applications: scintillators are used for research purposes, for accelerators like CERN [European Organization for Nuclear Research] or Fermilab or Brookhaven, any of the particle accelerators. That’s an indication of people studying the physics of some process, and the result of the process is particles, not radioactivity. It’s particles that fly out. These are detected in radiation detectors of various sorts. They are used in home inspections. Basements of homes in Pennsylvania especially, in the U.S., tend to accumulate radon gas. You can detect the presence with scintillators.
Where else would they be used? They are used by NASA in probes orbiting the moon or the earth, looking for radiation coming from the planet. That radiation is typically caused by cosmic neutrons, cosmic generated neutrons, that reach the planet, are captured by material at the planet’s surface, and then there is a gamma ray that comes out when that happens. If you wait long enough, you will see enough of them and you will be able to identify what materials are on the planet’s surface. NASA is interested in such things.
Some places you will see along the highways, you will see at truck stops, you will see radiation detectors, people looking to make sure that what is supposed to be in the truck is in fact in the truck, and not something nefarious. There are homeland security applications.
Kelly: If you have a large truck container, the size of a truck, and you have a detector and you are looking for contraband, radioactive materials. It will show up through the wall of the truck, through the box?
Bell: It depends on what is radioactive in there. Alpha particles and beta particles won’t get through the truck, but the gamma rays will.
An issue, I know several years ago, for the scrap steel industry was, let’s say less than honest––maybe that’s not the right word, but it’s close enough––less than honest people getting rid of radioactive sources. A business goes bankrupt that happens to use a radioactive source for something in their work and wants to get rid of it, but it costs money to dispose of a radioactive source. What has been found from time to time is, some people put it in a junk car and take it to the scrapyard. Steel is fairly inexpensive. It’s like twenty-five dollars—no, twenty-five cents a pound, maybe.
This source will be put in the trunk. The first thing that happens is, we take it to a scrap dealer. It will be crushed, and it will still be in the trunk of the car. The car is taken to the steel mill. The car is shredded at the steel mill. If they are unlucky, that source ended up with the steel gets thrown into a graphite crucible and melted along with the steel. That’s if they are unlucky, because then the vapors coming off that crucible when it’s heated to melt the iron will cause—if it’s not a radioactive material that alloys with iron—it will just go up in the vapors and be caught in their exhaust system.
Most steel plants are not permitted for mixed waste, just hazardous waste. They can throw away the ash, but they can’t throw away radioactive ash, and they can’t store it. Then they end up being shut down for a while until this gets cleaned up, at fairly high expense. Some scrap steel plants like this, it will cost them $300 a minute that they are shut down. They don’t like this. They are very interested in portal monitors that will detect radioactive material buried in the car.
Kelly: That’s fascinating.
Bell: Well, people do this.
Kelly: Yeah, that’s interesting.
Bell: The bottom line is important to a lot of people.
Kelly: It costs a lot of money, then.
Bell: It does when that happens. It’s not particularly dangerous, because a small amount of radioactive material in tons of steel, you just never notice it. It won’t bother anybody at all.
Kelly: But they do it because—
Bell: Because it’s too expensive for them to dispose of it in the correct way.
Kelly: Because of regulations?
Bell: Yeah—well, I won’t say regulations. I mean these regulations are there for a purpose. It’s so we don’t end up with radioactive water in the water supply and radioactive meat, that something gets into a food supply. That’s what it’s for.
But still, when we have radioactive waste to get rid of here, we bury it and we do it by the book, so it doesn’t go anywhere and it stays put. A commercial outfit can pay someone to do the same thing, but it costs them money. If they are already bankrupt and going out of business, they don’t have the money to do it. So, sometimes this happens.
Kelly: That’s fascinating.
Bell: People use radiation detectors to try to avoid that problem.
Kelly: Yeah, fascinating. Tell us more about your work, about looking at new elements.
Bell: Not new elements, new materials.
Kelly: New materials.
Bell: New combinations of polymers and inorganic crystals, to check to see if they have a good response to radiation. For gamma radiation, you are looking for something that is dense and has materials in it like lead or tungsten, things with very high atomic number. The best would be to use something that had uranium in it. But unfortunately, uranium itself is radioactive and the crystal would be lit up all the time. There is no point in doing that.
Commercially, there are tungstate crystals that exist, cadmium tungstate and zinc tungstate, I can buy today. Probably it’s about $10 to $40 per cubic centimeter for the material, which is affordable. You don’t need very much material, because the density of cadmium tungstate is about close to eight, about that of steel. It doesn’t take very much to stop gamma rays and register events. It takes a little bit more with sodium iodide.
Among my interests are high Z materials that will still scintillate. Not all of them do. Plenty of tungstates don’t do anything at all. Yhere are other materials. There are thallium compounds that have been reported to scintillate. I have not worked with them yet.
Those are the materials that are useful for gamma rays and x-rays, because they have the highest stopping power. The more dense they are and the higher the atomic number, the better it is for stopping gamma rays. That’s why lead is a shield, and paper is not so good at it.
On the other hand, if I am looking to detect neutrons, I want something with lots of hydrogen in it. Plastic scintillator exists. It’s commercial. You can buy it. Here is a piece of one, a small piece. It comes pretty much looking like this right out of the mold. It’s cast material. This one happens to glow blue in response to radiation.
You can also make scintillators that are made of silicon rubber, like bathroom caulk. This one is flexible. If I threw it on the floor, it would bounce. This one also, I think, glows blue. Yes, this one also glows blue. This has been around for a while. It’s not quite bathroom caulk, but it’s similar. It’s a silicone rubber.
Kelly: Give us some examples of what that would be used for.
Bell: This kind of thing, this you can cast in large sheets, just like in cast plastic. The advantage of this is, there is no machining of this kind of thing. You can cast it in pretty much any shape you want, whereas plastic is best case in slabs or cylinders.
If I have a device or an instrument of some sort, and I want this device to detect radiation in addition to whatever else it does, it just has some random spaces. I can pour the silicone fluid in it, polymerize it in place, and just take off excess space with this flexible material. I can’t do that with plastics. The starting materials in making plastics will dissolve other plastics. The stuff that is in this material is pretty inert, even before it’s polymerized. It won’t attack other plastics. We cast this in generally a glass vial. I just shatter the vial to take it out.
This does not require an oven. Plastic scintillator has to be polymerized at about sixty degrees C for about two weeks to make it. This I can do in about twenty hours, and cast the sheet pretty much as large as I want. There is no heat generated in the polymerization reaction, which is different. In the case of plastics, there is heat. You have to carefully control the temperature of the polymerizing mass. This is, I would say, really just like setting up bathroom caulk.
Kelly: Is it very expensive?
Bell: No, it’s on the order of a few dollars per cubic centimeter to make this stuff, not horribly expensive. They are far more expensive scintillators that are commercially available. Some of them cost a few hundred dollars per cubic centimeter. They aren’t used in big slabs. They are used in handheld detectors. A few companies make them.
Kelly: Why would they not make that one?
Bell: This would not be very useful for gamma rays, because it’s pretty light. This is little over a gram per cubic centimeter in density. Plastics are also about like that. It doesn’t quite float in water. The scintillators that are used for handheld radioactive identifiers are typically sodium iodide. Those have a density of about three and a half to four and a half, depending on what crystals are actually in it. It’s much better at stopping gamma rays, and that’s the intent. You have to match your scintillator to the application.
120 years ago, if you wanted to work in [Ernest] Rutherford’s laboratory in England –– there is a Rutherford Laboratory, now in England, but this is when the real Rutherford was alive. One of the tests for an applicant was to sit in a dark room for half an hour, and then be presented with a zinc sulfide screen that had a little radioactive source on it. He had to sit there with a mechanical counter, and count the flashes. If he couldn’t do that, he was encouraged to pick a different career.
Zinc sulfide is a very well-known scintillator. It’s been around at least 100 years, maybe more. You know about the radium watch dial painters?
Kelly: Why don’t you tell us about it?
Bell: I don’t know if it was considered a scandal then, about 100 years ago. Watches today of course have LCD displays or LED displays on them. For us it’s not a problem, we have batteries. 120 years ago, batteries didn’t exist like we have today. You couldn’t illuminate your display at night. So how did you know what time it is?
Madame [Marie] Curie had recently figured out the chemistry to separate radium from uranium ore. If you mix that radium with zinc sulfide, it will glow green in the dark. It will glow green all the time. You just can’t see it during the daytime, but at night, it’s very visible. Things like this were used in the Second World War in aircraft and ships, when they had to operate at night and couldn’t turn their lights on for fear that an enemy would see it. But these little glow in the dark displays worked just fine. Even today, you could probably go to an antique store and find a radium watch dial.
The people who were painting these were young women, in their twenties, typically. They were given very fine camel hair brushes. They just painted the numerals with this zinc sulfide radium chloride paint.
Of course, if you have ever done any painting, your brush tip eventually frays a bit. What do you do? Well, they got this stuff all over their fingers, got it all over their faces. Sometimes they would paint their faces purposely, just to amuse people at night, because it would glow in the dark. They would have a little smiley face, whatever you wanted. Every one of those workers died of horrible cancers of the tongue, jaw, skin cancers on the face. They all died.
Kelly: How long did it take to manifest?
Bell: Oh about five to ten years.
Kelly: It was very intense?
Bell: It was really obvious that that was the cause. People at the time, around the World War I era, the people knew there was danger with radiation. Thomas Edison had one of his assistants really –– he basically burned one of his fingers off with an x-ray machine. They didn’t know then, but then they figured out what it was. He died, because he was standing next to an x-ray machine that’s running at high voltage with no concept of health physics at the time. People got sick and died.
These watch dial painters, pretty much the same thing happened. Radium is an alpha particle emitter, which does a lot of damage to cells, but only for a few micrometers, five to ten micrometers deep into the skin. That’s enough to get past the epidermis and the dead cells that are already on your skin, and get into the actual flesh. Then that causes the cancers.
Kelly: Speaking of radium, I wonder if you could explain the causes of Marie Curie’s death, because I think at the time ––
Bell: Same thing. She basically contaminated herself with all the radioactive material that she was working with. Around 1900 or so, people just didn’t know just how dangerous it could be. She was working with tons of uranium ore to extract grams of radium. It’s about a million to one or so in natural uranium ore, about a gram of radium in a ton of ore. She was separating all of the daughter products, most of which are radioactive. Basically, she contaminated herself. She contaminated her laboratory.
Kelly: What else should we talk about? You have other samples there.
Bell: Oh yeah, I brought a few props just in case you all would find them interesting like this piece of silicon rubber.
Kelly: No, this is fascinating. What else do you have here?
Bell: This thing glows when hit by ultraviolet light from a lamp. It’s blue. This is the hard plastic. It’s also blue, a little different color blue. It all depends on the organic dyes that are in it. These are other silicon rubber samples that we made. One of them is greenish, and the other is blue. It’s all the question of what dyes. These are not sensitized for neutrons. These are probably from experiments to see if we can get the rubber to polymerize. Sometimes it doesn’t. We put the wrong things in, and it prevents it from polymerizing completely.
These are things that are not scintillators, but they glow different colors. This one is a glass that has europium in it. This one is a marble I got off eBay. and it has uranium oxide in it. You buy this. It’s perfectly safe to have. It’s a consumer product. But neither of these things scintillate. So the rule of thumb is, if it doesn’t fluoresce, it probably doesn’t scintillate. If it does fluoresce, it still might not, but it’s an easy way to get to select the things that might.
Kelly: That’s fascinating.
Bell: Not all detectors are scintillators. This is a piece of cadmium zinc telluride. It’s a semiconductor. This works much like a gas proportional counter would work. A gamma ray would go into it. The gold, a piece of the gold foils on it are for electrodes. The gamma ray will make electrons, and the electrons leave behind holes in the lattice.
By applying voltage across opposite faces, like where my fingers are, you could separate that charge and you can collect it. From the amount of charge you collect, you know the energy of the gamma ray that interacted. This has no optics at all. It’s a density of about 5.5, but this is not cheap. The primary users of this kind of material is the medical community, for imagers. If a dentist puts a detector in your mouth for a bitewing x-ray, it’s probably cadmium zinc telluride that he is using, or she is using.
I don’t grow these materials myself. I work with other people at universities and other places in the laboratory. We might investigate semiconductors that use high atomic number materials like mercury or lead, bismuth.
Kelly: When you say you don’t grow these materials, how are they manufactured?
Bell: There are people who do grow this stuff.
Kelly: Grow, you are saying grow?
Bell: Yeah, like you grow a crystal.
Kelly: I see. Is that how that works?
Bell: Pretty much. To do this stuff, you take cadmium telluride and zinc telluride and grind them up with a mortar and pestle, put them in a glass ampoule, and seal it. Then heat it up to about 900 centigrade. It will react and crystallize as this material.
Kelly: It will look just like that?
Bell: It won’t have the gold foils on them. It is a dark material. That’s how you grow that.
Sodium iodide is grown from a melt also. You take sodium iodide powder and valium iodide powder, mix them up in the right proportion, put them in a crucible and heat them, generally with a an RF [Redstone Furnace] furnace. Heat them, then you take a small crystal of sodium iodide, touch it to the top of the melt, and start withdrawing it at a millimeter or two per hour. This material will stick to that, and crystalize in the same crystal orientation as your seed crystal. You just pull it up.
Sodium iodide is grown by the ton in the U.S. each year. Cadmium tungstate, zinc tungstate are also grown like that. Lithium—not lithium, lutetium oxyorthosilicate is grown like that. That’s done actually very close to us here. That’s done by Siemens, they grow the stuff. The furnaces are out somewhere near the airport. They run twenty-four hours a day, seven days a week, they run the furnaces.
Kelly: They are mostly servicing the medical industry?
Bell: Yeah, lutetium orthosilicate also called LSO, it’s primary purpose is the medical community, PET scanners.
Kelly: Can you just since you say PET, can you spell that out?
Bell: Positron emission tomography systems.
Kelly: Say that again.
Bell: Positron emission tomography systems.
Kelly: Can you explain how that works?
Bell: That’s magic. No, it’s magic. You can make radioactive isotopes that emit positrons –– sodium-22 does it, but the ones of interest for the medical community is fluorine-18 and carbon-11.
You can make a sugar where some of the hydrogen is substituted by fluorine-18. The half-life is about two hours. You don’t have much time to make it, but the process is known. You will make the fluorine-18 with a cyclotron onsite. Then you will make the sugar with the fluorine-18. Then you will inject it in the patient.
The place where you have a tumor will generally have a higher metabolism than the rest of your body, because that’s why it’s a tumor. It’s rapidly dividing. It will take up the sugar with the fluorine in it faster than the rest of your body does. But it can’t metabolize that fluorinated sugar, so it just sits there.
In the meantime, you can look at when the positrons come out of the fluorine. They don’t get very far before they slow down and are captured and annihilate with an electron that’s somewhere nearby. That causes the emission of two gamma rays that come at 180 degrees apart.
You have an array of detectors. You get coincidences between detectors on opposite sides of the patient. By analyzing the order of things, where things are being detected, which pairs of detectors are firing and when, you can reconstruct a picture of the distribution of the radioactive material in the body, with something like a millimeter or so resolution.
With the PET scan, you find the regions of the body that are biologically active. If you use a different molecule than that, you can tailor it to whatever part of the body, I suppose, takes up that particular molecule. If you did it with a radioactive iodine compound, you would image the thyroid really well.
Kelly: Usually they are used for diagnostics? Can you do you therapy at the same time?
Bell: That’s diagnostic only, no. The dose to the patient is fairly small. The dose to a cancer patient, you want to kill a tumor. People do use x-rays for that, because the tumor is so full of rapidly dividing cells, radiation will kill those a lot faster than it will kill the rest of the cells in your body. The tumor is more susceptible.
That’s why you treat brain tumors with radiation, because brain cells don’t divide rapidly on their own. You have got your brain. That’s pretty much it. That’s as good as it’s going to get. But the tumor is atypical. It’s growing rapidly, so the radiation interrupts the cell division process and the cells die. Most mutations in cells kill the cell.
Of course, even in radiotherapy, you have to have detectors around to know what dose you are giving the patient. Scintillators are used for this from time to time.
Kelly: You also have applications for this for material sciences?
Bell: Sure.
Kelly: Wear and tear in a reactor?
Bell: There is that. That was one of the primary missions at ORNL [Oak Ridge National Laboratory] years ago. It still is to some extent, but we don’t build too many reactors anymore. When I first came here, I was measuring neutron cross sections for that purpose, structural materials. When you expose materials to high flux neutrons, the neutrons can be captured, and the material transmutes.
The philosopher’s stone problem of 1,000 years ago was finding something that would turn lead into gold. Well, I can turn gold into mercury. I know how to do that. I unfortunately don’t know how to turn lead into gold. It’s’ the wrong way in the periodic table to do it. But that’s what happens. Iron in rebar or in I beams in a reactor, exposed to neutrons, will eventually turn into cobalt.
If you have a high enough flux for a long enough time, and reactors typically have a lot of time like twenty years or more. HFIR [High Flux Isotope Reactor] has been around more than twenty years. It’s been here ever since I have been here, and that was forty years ago. So, exposed long enough and to enough of a neutron flux, you will change the mechanical properties of these materials.
People measure neutron cross sections to know how fast is damage is going to occur. People measure samples that have been irradiated to see what happens. To measure the cross sections, you have to detect neutrons and gamma rays. That’s where these radiation detectors come in.
My personal interests are understanding the physics and chemistry of materials for radiation detection. In the course of doing that, I have built radiation detectors for specific applications. I write proposals. I sometimes get funded, and I go do it. It’s something I have been doing now for—I guess if count my graduate career, something on the order of forty-five years.
It’s all a bunch of steps. I started doing photo neutron measurements in graduate school, irradiating materials and causing neutrons to be emitted by these materials. This was basic physics, understanding how gamma rays interact with nuclei and the structure of nuclei.
Then I came here, and since I already knew how to do neutron detection and build detectors, I got into neutron cross sections. I was recruited to work with the accelerator, which has now been decommissioned, the ORELA, the electron lin acc that we had [Oak Ridge Electron Linear Accelerator].
From that, it got into, “How do I actually make the scintillators that I am buying now. Can I do better?” It’s just a curious mind kind of thing. One thing leads to another. In some cases, I have made scintillators didn’t otherwise exist. Not commercial successes, but it’s certainly interesting that I could do it. I’ve made things with them. It leads sometimes to scintillators, or it leads to semiconductors. The physics is not that different, but there are some differences. I have been doing it long enough now that –– it still interests me. So I haven’t retired yet.
Kelly: That’s great. Are there young people coming along that are going to pick up or have engaged in this?
Bell: There are. Most of them are now in nuclear engineering, come out of nuclear engineering curricula rather than physics curricula. They are certainly capable, most of the ones I have seen. Some of them, like the Rutherford’s applicants, maybe they should seek a different profession. But for the most part, they want to be good instrumentation engineers. Most of them are not really looking at basic physics.
To some extent, I think that a lot of people in the world think that we know all the physics we need to know about scintillators, and we don’t really have to do much research. Let’s just try them now, try different things. Then the question is, “How do you know what materials to try?”
I know zinc tungstate is a scintillator. I know cadmium tungstate is a scintillator. What about mercuric tungstate? It’s the next one down in the periodic table. Well, it, too, is a scintillator, but it turns out you can’t make it. You can’t melt the material. If you make the compound and try to melt it, it simply decomposes, which is not the desired result. It’s actually very hard to grow, although we did try it. We did make a few tiny little crystals, millimeter sized, but it was not what I would call an astounding success.
The silicone rubber was much more of a success in the fact that it worked really well. Not only could I make it just with silicone rubber and a phosphor, but I could also put in materials to make it very sensitive to neutrons. I could put boron in there. I could put gadolinium in there. We never did try to put lithium in, but it probably can be done.
You have to understand the physics of these solids in order to say, “If I change this out, what do I expect to happen?” That’s the way people do it now most of the time. We do calculations to decide what materials to put together, what has a chance. Versus the Edisonian approach, which is just line them up and try them.
I tried to make mercuric tungstate. “Well, it didn’t work. All right, well go on the next one. We need a plus two cation. All right, well zinc and cadmium are like that. What about calcium? It’s also a plus two ion. It does work, too.” Actually, Edison found that. It was one of the first things he found, that calcium tungstate scintillates.
What about magnesium? That’s the one above calcium on the periodic table. As far as I know, it doesn’t. What about lithium tungstate? That’s two lithium ions and the tungstate ion. Well, it might. We are not sure. We haven’t tried to grow the crystal yet, but we have done the calculations. There is a chance it will do it. That’s one of the ones that maybe, it will work.
But then right next to it, among the alkalines, there’s magnesium to try. Calcium we know. Strontium tungstate would be the next one to try, barium tungstate. The one after that would be radium tungstate, but there is probably no point in trying radium tungstate since radium is radioactive. The crystal would be lit up all the time. There is really no point in that one, not for radiation detection. There may be other things to do with it, but not that.
It’s just a matter of curiosity to keep on, to do this stuff.
Kelly: I guess that’s a key element of being a scientist.
Bell: Probably so. I don’t know if I am a better scientist than I am an engineer or not. I like to make things work, or figure out why they work or how they work. Sometimes I am astounded that something does work. But more often than not, it doesn’t work. It’s hours and hours of boredom, followed by a few moments of elation. That’s pretty much it.
Kelly: That’s wonderful. That’s wisdom that people have to understand.
Bell: Yeah, and what I have seen over my career—well I’ve only just had a long career, but at one point I was an editor of a technical journal. It has never ceased to amaze me how the wheel is reinvented about every twenty years. It seems these days that if a researcher doesn’t find the reference with Google or some other search engine within five to ten minutes, that person will assume that it’s never been done before and will file for a patent, or will write an article saying, “What I’ve done is the best thing since sliced bread.”
Then some old geezer like me looks at the paper and says, “You know, I remember in this book published in 1964 that I read, and I have a copy of it, there is a section on this very topic.” Then I write my review of the paper and I say, “That’s very nice, but you failed to mention this book from forty-five years ago.” The response would be “Oh.”
You’ve got to do your homework, but that’s just the way the world is these days. “If I can’t find it with my mobile phone, it probably doesn’t exist.”
I have had lots of good patentable ideas that I was just like 100 years too late. Someone beat me to the patent office with it. I do have seven patents to my name, where I did beat people to the patent office.
What I am holding here is a cadmium tungstate crystal.
It’s got cadmium and it’s got tungsten. It’s a very dense material and it glows blue-green when it’s hit by radiation. I don’t have a radioactive source here, but it’s sitting on an ultraviolet light, which will excite it, as soon as we turn the lights off.
Kelly: Can you hit the light switch over there?
Bell: Right now, the light is on. We see the blue-green glow. The crystal is transparent. If I turn the light off, we are in total darkness. Now it’s back on.
A photo multiplier would sit here or here and view the crystal, and look for the flashes of blue light and generate an electrical pulse every time that happens.
What I’ve got in this box is a marble that has uranium oxide in it and a piece of glass that has europium in it and indium also, but the indium I know does not glow. The indium is there so that if neutrons strike this, the indium captures the neutrons and emits electrons and gamma rays.
Since this is really not a scintillator, what happens is there is something called the Cherenkov Effect, where charged particles like electrons traveling faster than the speed of light in the material cause the emission of blue light, much like a sonic boom happens when you exceed the speed of light in air.
Without putting this up to a gamma ray source, pretty much nothing happens. Put it up to neutrons, and you can see the Cherenkov light from after the capture. These are examples of things that will fluoresce green or, in this case, the europium is what’s fluorescing red, under a blacklight or UV lamp. If I hit this with radiation, pretty much nothing happens. Not all things that fluoresce will scintillate. But all things that scintillate do fluoresce.
What I am holding here is a set of scintillators embedded in these plastic tubes and connected by glass pipe. What happens is, this is a position sensitive detector. Here is a glowing scintillator. You put a lamp here, and nothing really glows. You put a lamp here, and you see it. Put it over there, you see it. Now, I am going to hold it so that it’s facing the camera.
Now if I do it, you will see the very bright glow out the end of the glass pipe. This is an example of total internal reflection. The light from the scintillator bounces around the glass and bounces at the glass air interface. Because the index of refraction of air is smaller than that of the glass, the light is reflected from the surface and travels down the pipe until it gets to the end, and you see that blue glow.
As I move this across, you see that the blue glow is brighter now. It gets dimmer when I go further away from it, and even more dim at the last scintillator. If I put a photocoupler tube at each end of these glass rods, and look at the amount of light that’s generated at each photo multiplier, I can get an idea of where in this chain the event happened, whether it’s this scintillator, this scintillator, or this scintillator.
These are the silicone rubber scintillators, so this whole thing is flexible. I could wrap this around a drum if I wanted to, or wrap it around a person. This is just an example of a position sensitive scintillator, a scintillation detector.