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Gordon Knobeloch’s Interview

Manhattan Project Locations:

Gordon Knobeloch worked on the RaLa Experiment at Los Alamos, which was crucial to developing the spherical implosion necessary for the plutonium bomb. He recalls arriving by train in Lamy, the low average age of the scientists at Los Alamos, and presents a defense of the use of the atomic bomb in World War II.

Date of Interview:
November 16, 2006
Location of the Interview:


Gordon Knobeloch: Okay, it’s Gordon Knobeloch, G-O-R-D-O-N, and the last name is K-N-O-B-E-L-O-C-H.

Kelly: Great. Okay, why don’t you start with how you got to—

Knobeloch: Okay. Well, everybody who came here had their own particular path and mine wasn’t as spectacular as some of them, but it was interesting to me, and I guess it started with good ol’ Pearl Harbor day.  

That was within a month of the time I graduated from high school and within a month of my birthday, my eighteenth birthday.  And the eighteenth is significant because, you might not know, but there was something called the Selective Service System, the draft. 

So on my eighteenth birthday I was called into the draft board, and they asked me what my plans were when I graduated from high school. And I said I intended to go to Washington University in St. Louis and study science. And they said—well, I came from a neighborhood that had a lot of young men who didn’t really know what they were going to do when they graduated, and he said he thought they could spare me from the Army, or the draft, for two years. If I could do my undergraduate work in two years, they’d lay off of me.

They wanted to keep track of my grades, but they’d lay off of me for two years if the conduct of the war permitted it. Well, thanks to good old Washington University, they were very cooperative in setting up a program where I’d go year ‘round.

I’d come out of my first class sometime and the sun wouldn’t be up yet and I’d get kicked out of laboratory at midnight. But after a pretty intensive two years I did indeed graduate. And they were true to their word. They snapped me up and I found myself in an infantry replacement training camp right after I graduated.

This was, I think, a fifteen week training cycle and, early in the cycle, I saw a little bulletin on the battalion bulletin board that said anybody who had a degree, a college degree, would be free to take some examinations if we—if I chose to. And, if the conduct of the war permitted it, at the end of my training cycle, I might be assigned to something that was more in line with my training.

So, sure, I went and took the exams. They were given in the evening. And I kind of forgot about them but, sure enough, by the end of the fifteenth cycle, fifteen week cycle, even though the war in Europe was going at a high rate, apparently they thought they could spare me from the infantry, and I was told to come out to Santa Fe. 

But before that they sent me to Ohio State University for just a very brief period. I think—I did take some courses while I was there, but I think, principally, it was to put me in a place where they could get a hold of me again when my clearance was completed.

And shortly after that I was told to report to Santa Fe and I got on a train. And I remember getting as far as Kansas City, and I guess I found the station master and asked him what was the best way for me to get to Santa Fe; I had military orders. 

And the Santa Fe railroad most days didn’t run many trains. I think they ran three of them a week or something like that, but he said, “There’s a tube train about to leave that’s going out to the West Coast. If you get on that you can get off at Santa Fe.” So I rushed around—there were about three of us together, I wasn’t alone.

And we got on the tube train and it was full of a bunch of G.I.s who’d just left the North African campaign and were being sent to the Pacific Theater. And they were a little upset because they hadn’t had any time off to visit anybody on the way through the country.

But, at any rate, I found the only civilian on the train. He was a conductor, and I told him I wasn’t supposed to go to the coast but I wanted to get off at Santa Fe. He said sure, he’d tell me when, but he said, “You’d better find me.” He said, “It’s going to be more than a day, but you’d better find me sometime tomorrow.” 

Well, I guess tomorrow came around and I hadn’t seen him, and I looked around the train and finally found him. He was in a poker game with some of these boys. And they’d been passing the bottle around. And I wasn’t sure he remembered me but he said, “Yeah.” He said, “In about three hours,” he said, “Get your things and stand in the vestibule of the train, the area between two cars.” And he said, “Be ready to get off. It’s not a regular stop but the engineer said he’d come to a full stop and let you off.” So he said, “As soon as the train stops, get yourself off.” 

Well, the other two guys and I stood in this vestibule. And we couldn’t look out the right hand side because it was stacked to the ceiling with the barracks bags of all the other G.I.s on the train, but we could look out the left window and could see nothing but sage brush and open country. 

Train slowed down pretty much a couple times. I think, in retrospect, we were probably going over the Glorieta Pass. But finally the train did grind to a halt and we got ourselves off that train and looked around, couldn’t see on the right hand side until the train went by. And, sure enough, there was a station, but it was all locked up because nobody was anticipating a train that day.

And the sign didn’t say “Santa Fe,” it said “Lamy,” and I was just sure something went wrong. But there was a general store at Lamy—are you familiar with Lamy, New Mexico at all? Not many buildings there, but there were even fewer in those days, but one of them was a general store and he had a telephone, and he assured me that this was indeed the stop for Santa Fe because the train didn’t go through Santa Fe. And I was naïve enough to think that the Atchison, Topeka and Santa Fe Railroad probably went through Atchison, Topeka, and Santa Fe, but it didn’t do that.

But anyway, he had this old crank phone, old style phone, and I did have a telephone number for Dorothy McKibbin in Santa Fe. And she assured me that she’d send a car down. She said, “We didn’t know when you were coming in but I’ll send a car down.” And she did, and I finally got up to Los Alamos. She provided me with a gate pass when I got to Santa Fe.

And that was my pathway here. I was surprised and pleased to be at Los Alamos, not really knowing what I was doing there until a few days later. I can talk a little bit about the town but you probably aren’t interested in that so much—I mean, my impressions of the town. 

Kelly: Yes, because we’re talking about the buildings and how the whole tech area was just like every other World War II construction. But, you know, why don’t you describe that? So was it all green? People haven’t talked what color it was.

Knobeloch: Say again?

Kelly: Was it all painted green?

Knobeloch: Yes.

Kelly: They can’t hear my question. You’re going to have to make a sentence of that.

Knobeloch: Okay. Yes, well, everything looked green. The buildings were certainly temporary buildings. Housing was limited. As a matter of fact, the first house I was assigned was a plywood house about eight or ten feet square with no windows. But it had screens and plywood flaps that could fold down over the screens when the weather was cold. And it got cold.

We had a potbelly stove in the middle of it, and I think there were perhaps six of us who shared that building for a while. But it turned out we didn’t spend much time there anyway. We spent much more time at work, and the free time I had I would spend up at the lab or at the Big House, which was the old dormitory left over from the Ranch School days. And it had comfortable chairs and a small library, and I’d spend some time there.

The only other housing in town, of course, was Bathtub Row, which was left over from the Ranch School days, and the Big House, I already mentioned. The majority of the civilians lived in Sundt apartments. These were usually four unit—four family units per building. There were some trailers, there were some dormitories for single civilians, and a few McKee houses, but that’s about it.

But the interesting thing about the town, of course, was the fact that everybody was so young. I believe the average age was somewhere between twenty-five and thirty. Niels Bohr was probably the oldest guy I ever saw, and he seemed ancient, but he must have been about fifty-five. Oppenheimer himself, I think, was only thirty-seven.

So anyway, it was a sort of vibrant community, a lot of young people. There wasn’t a great deal to do except work, and we did a lot of work. I guess for recreation we did form a softball team but I don’t think we played that until after the war. We called ourselves the “Downtown Athletic Club,” sort of ironic because there was no downtown and there was no athletic club.

Hiking was popular. I wasn’t a skier, but Sawyer’s Hill was available. Movie theaters—there were two of them as I recall. Most of the people entertained at home and, while I didn’t have a home, I was often asked out to homes of cohorts of mine.

I can remember, too, acting as a babysitter a couple times for my boss, which gave me a chance to get out of the barracks and sit in a nice homey atmosphere.

Kelly: You know what? That’s good. Why don’t you talk about your job now?

Knobeloch: Okay. I’ll have to give a little preface to what I did. You know, most people on the tours I’ve given have said, you know, “What was your job?” And you can’t come right out and try to describe with detail without giving a little bit of background. 

So I think I would say that, in order—to start at the very basics, the main reason for the laboratory was to produce an atomic bomb. And you do this by taking two subcritical masses, separated, and bring them together quickly so you form a prompt critical device. 

And, well, how do you do that? You can’t do it by hand, obviously. And mechanical means like springs are too slow. It probably gives assembly in fractions of a second, which isn’t very good. You need something faster.

Well, everybody’s heard of ol’ Superman, who’s faster than a speeding bullet. So, somewhere along the line, certainly before I came to the project, somebody had the idea of firing a bullet of uranium-235, the subcritical mass, into a target of uranium-235, which was subcritical. But the two of them combined would form a prompt critical device.

And, sure enough, that’s what the Hiroshima bomb was. They simply dropped a cannon, if you will, out of the airplane. And, when it approached the ground, the cannon went off and assembled it and formed a prompt critical assembly that produced twelve or thirteen kilotons of energy.

Okay, that was one way of making a bomb. Also early on, in order to ensure success, the laboratory pursued both a uranium weapon and a plutonium weapon. And the intent was to make a plutonium gun weapon, which would have been the simplest thing.

Jeff (Cameraman): We’re just going to pause because there’s an announcement they make periodically.

Knobeloch: When you’re ready? Okay. Well, as I say, the intent then was to make a plutonium gun weapon, but unfortunately the laboratory was misled because the first plutonium we got—there was a great need to get some in order to learn what you could about it, you know, both metallurgically and chemically and so forth. So there was a big need for a small amount in a hurry.

And that amount came to us, and the metallurgists and us chemists did our predictions and calculations based on that. But you see, in order to make that plutonium, it had to be put into a reactor and subjected to neutron capture. And in order to get it out in a hurry, they didn’t leave the uranium in there to produce the plutonium very long. 

When they wanted to produce more plutonium, it was necessary to leave the uranium in the reactor and capture more and more neutrons to make more and more plutonium-239.  But unfortunately plutonium-239 itself had a fairly good capture cross-section for an extra neutron to make plutonium-240. And there was very, very, very little plutonium-240 in that first sample we got, but in the larger masses needed to make a bomb its presence was considerably higher.

And that’s bad because plutonium-240 has the unfortunate capability of undergoing spontaneous fission. It doesn’t need a neutron to hit it to start it off. It spontaneously can undergo fission and produce two or three neutrons in the process. And the presence of these early neutrons would have caused the gun-type of plutonium to fizzle, pre-detonate, and not give a proper yield.

So they needed a faster method of assembly rather than the gun. And I’m not sure who it was who first suggested a shaped charge, but, when I was in the infantry, I was firing bazookas which have—rocket-propelled—but it has a shaped high-explosive charge in the nose. So when it hit the wall of a tank, the explosive would go off and squirt out a jet of energy which could melt a hole in the tank and kill the tank. 

So shaped charges might be a way to go. And if you could make a shaped charge to produce a jet of energy, maybe you could make a shaped charge to produce a symmetrical squeeze, and squeeze a shell of plutonium, or a small grapefruit-sized ball of plutonium, to a density such that it becomes supercritical. And that was called the implosion weapon.

[Tape switch.]

Knobeloch: Okay. Somewhere back there I said the laboratory, in order to ensure success, wanted to pursue every avenue available. And, of course, they considered a hydrogen—I mean, a uranium bomb as well as a plutonium bomb. And I’ve described the implosion of the plutonium bomb.

So how do you test that?  I mean you have to—the high explosive experts have to form and shape and mix different kinds of high explosives together in order to get a good compression wave. And, once they had a system, how do they see if it’s any good? Well, you could put it around a ball of plutonium and, if you blow up your town, you’re a success. But that’s not very practical.

Another way of doing it was thought up by Luis Alvarez, who was a Nobel physicist up here. And he suggested taking a mock-up of the bomb, not using plutonium, but any other metal, any other heavy metal like tungsten, perhaps, or uranium-238, which doesn’t undergo fission that well, or cadmium, tungsten, and putting a small hole in the center of it, and putting a very intense radiation source right in that spot. And then measure the signal that comes from that radioactive source as you implode the bomb.

Well, we chose lanthanum-140, which has a very strong gamma ray, easily measured.  It’s got a reasonable half-life, like forty hours, so if you do one test and you wait two or three days, or a couple half-lives, you can get back into there and work there again.

Well, if you were to do this, and watch the signal of radiation coming out of that device as it implodes, and plot the intensity as a function of time, and if you surround it with several ionization chambers, which detect the irradiation, and compare the results, you can see whether or not you have a symmetrical bomb.

I don’t have a chalkboard here but I can wave my finger in the air. So here’s this mock-up of the bomb with a very intense lanthanum-140 radiation source in the middle and, say, four ionization chambers around it. Before you fire it, the signal from the radiation that each chamber sees is a constant, and when you implode it, the device gets denser and denser and becomes a better and better shield for the radiation coming from the lanthanum-140, so your signal will drop down.

When it’s fully imploded, it’ll start dispersing itself, and the detectors will see—if it’s a symmetrical implosion, each detector will see the same amount of radiation. If it’s not a good symmetrical implosion, some of the guts of the thing, including lanathum-140, were going to squirt out of one side of it, and whatever chamber is on that side will see a high signal at the expense of the other chambers on the other side.

Now, when I was waving my hands in the air here, I went very quickly over the bottom part. Before it starts dispersing, there’s a little flat spot in the bottom, which was a bonus that nobody really expected, but it gave the theoreticians the chance to see how long it was at what they called the sitting time. And if you could enhance that sitting time, you could enhance the number of generations of fissions that take place and increase the yield of the whole device and give a pretty good idea what it might be.

So that’s what the system was, and my part in it was working in a small group that produced this very high, intense radiation source. As one of my bosses pointed out, nobody had ever worked with intensities like this before. And, if you’ll let me quote from him, this is Rod [Roderick] Spence who said this. He said, “In this period, chemists were familiar with working with curie quantities of material.” A curie quantity of material is the radiation given off by a gram of radium, and it produces an exposure of one roentgen per hour at a distance of one meter. 

So let me get back to what Rod says: “Chemists were familiar with working with curie quantities of material, but to work with a hundred curies concentrated in a point source was unprecedented.” Los Alamos chemist Rod Spence remarked recently, “No one ever worked with radiation levels like these before ever, anywhere in the world. Even radium people normally deal with fractions of grams, fractions of a curie.”

But we were producing not just curies or hundreds of curies. We were producing thousands of curies, kilocurie sources. And that made the work a little tedious. If you were to stand a meter away from a curie of material, as I say, it’d produce a body exposure of one roentgen per hour. If you stood a meter away from a thousand curie source, you would produce a thousand R [Roentgen] per hour on the body, and the mean lethal dose is something like 600 R. So that means if you worked a meter away from a kilocurie source for six-tenths of an hour, you’d get half—a probability of fifty percent of living or dying. And we were working with multi-kilocurie sources.

Well how did we do that? Well, we did it with shielding where we could, lead shielding. We did it by distance as much as we could. But, even so, radiation exposures were pretty severe, and in the beginning we didn’t have film badges. We had what we called pocket dosimeters that could measure up to a hundred milli-R per hour, and then they’d discharge.

But there was another way—they took blood samples from us. And radiation tends to kill off the white cells in blood. So if the blood count dropped below 5,000 we were supposed to lay off of work for a while. If you had a head cold you were in good shape because, when you get a head cold, your body makes extra white cells and the effect isn’t as well known.

But anyway, it was interesting work. It was exciting work. Even though it wasn’t a pleasure to be exposed to these radiation sources, it was a lot better than if I’d stayed with my infantry friends, who, at that time, were getting killed off in the Battle of the Bulge in Europe. 

So I felt proud and happy to participate in an experiment which that official book describes—the official history of Project Y describes the RaLa experiment as being the single most important experiment to lead to the final design of the implosion weapon.  

Kelly: Great. That’s terrific. Were you at Trinity?

Knobeloch: No, I was not, for several reasons, the most important of which—I was the peon, and even my bosses and my RaLa group were not invited. But there’s another reason, too, they didn’t go. I never would have gone under any circumstances. 

But there was another method of trying to measure the symmetry of implosion gadget called the pin method, wherein small wires would be studded throughout the assembly, and scientists could look at the time at which these wires were crushed and see whether or not the signal was giving a smooth implosion.

That pin method was used on an experiment, I don’t know, about a week before Trinity was planned, and those people said, “Uh-oh, something’s wrong. Our experiment says it’s not going to be a symmetrical implosion.” Hans Bethe took the data, I learned later, and looked at it and said, “No, I don’t believe the pin method,” but it was decided anyway to do a last minute RaLa experiment.

And we did one shortly before Trinity, I mean a couple of days before, and, again, got good results. And, to quote Oppenheimer again, I found this interesting—I’m not sure I have the proper thing here. Oh yes. Very early on from our experiments, Rossi, who was one of the physicists associated with our experiment, quoted Oppenheimer as saying, when he saw the RaLa results, “Now we have our bomb.”

So, in spite of his poor measurement of the pin method, and Hans Bethe’s encouraging words of—the RaLa method gave proof that it worked when they went ahead and fired Trinity.

Kelly: That’s great. All right, what about—would you like to talk to morality? We have a few minutes. You can talk about—

Knobeloch: I can talk about that. Would you be more interested in that or some funny stories?

Kelly: Funny stories? Okay. [Laughter.] Jeff, you be the judge.

Jeff: You know me, I like funny stories.

Kelly: Funny stories, let’s go with the funny stories.

Knobeloch: Funny stories, okay. I described these copper or brass ionization chambers that were used to measure—and, by the way, I should have mentioned, you know, our signal, our radiation signal from the RaLa, moves with the speed of light, like 186 thousand miles a second, whereas the shock wave from this thing only goes with roughly the speed of sound, like 1100 feet per second. So there’s plenty of time for the radiation signal to be captured in these ionization chambers before they get blown up by the blast wave, but they do get blown up.

Well, I didn’t participate in this story but later on one of the guys who worked at the firing site told me what I thought was a funny story. The machinist who made these copper tanks was a perfectionist. I mean, he wanted everything just to be perfect. If he saw a smudge on the side of his thing, why, he’d whip out his handkerchief, “Hah,” breathe on it, and polish it off. He wanted it perfect. 

And one day he said to the fellows who came to pick him up from one of our RaLa shots, “What do you guys do with my chambers? You know, what are these for?” Well they figured they could take him down and show him and not have to tell him all the, at that time classified, details.

But they took him down to the firing site and set up the chambers and showed them to him, and described a little bit about—they have a detonation and these ionization chambers detect the signal, which is very important. And he said, “You blow them up?” He said, “I make these things, and you just blow them up?!” [Laughter.] He was crushed. He was crushed, but I thought it was kind of funny. But that’s my funny story.

Kelly: That’s a very good story.

Knobeloch: As far as morality is concerned, you know, everybody in Los Alamos—except two people that I heard of were against using it on the Japanese—most other people, I guess, felt like I did. We were tired of a war that had been responsible for forty million—forty-five million people, according to the World Almanac, being killed.

And a demonstration is—some people like Leo Szilard, who, interestingly enough, is one of the fellows who went to Einstein to get him to sign the letter to get this country going. But Leo Szilard and Ralph [misspoke: Phil] Morrison, another fellow who I had a great deal of respect for, both suggested that maybe instead of dropping this on people, we should demonstrate to the Japanese what a powerful weapon it was.

Well, there are lots of sides to that, one of which is, they [the Japanese] were so dedicated that I doubt if it would have given them any pause. And if the demonstration failed, it would only enhance their feelings in not wanting to surrender. And what do you do? Put it on a desert island and just blow up a couple of palm trees? That’s not very impressive. Dropping it on a city, I think, impressed them. 

And, as far as the morality is concerned, we’re all human beings, you know, Americans too. And, from the dawn of civilization, whether it was cavemen or scientists, if they were threatened, they picked up the biggest rock, or the biggest stick, or the biggest bomb, and used it. And I think that’s what we did.

And it was the scientists who made the bomb, but it wasn’t our decision to drop it. That was up to the military or the president, Harry Truman. And Truman truly had no option.  I mean, what if he had decided not to use it and gone ahead with a bloody invasion of Japan, which might have failed, but, in any event, that would have killed a lot of people. 

What would he have said to the widows of—whose husbands were killed, or what would he say to the mothers who lost their sons? If they knew he had a weapon that would have prevented it, and didn’t use it, they would have strung him up by the thumbs, or other parts of the anatomy.

So I think it was the right decision, and if people ask me, am I sorry for my part, I’m sorry for the people were killed in Japan. I’m sorry for all the people all over the world who were killed. The Jewish people were exterminated, people were killed in Nanking, if you heard about the Rape of Nanking. It was a sorry thing, but war is a sorry thing to begin with, and I’m proud to have had a part in doing something that might have helped end it a little sooner.

Kelly: Do you have an admonition to the young?

Knobeloch: I have an admonition to the young. When I’ve given talks like over at the historical museum, I’ve talked with a lot of young people. And I was happy when they expressed their feelings, even though sometimes their feelings were such that I was a murderer.

I remembered what Hans Bethe said one time when he was being heckled by a large audience of young people. He said, “Look, we did what we thought was best with the problems we were faced with, and with the tools we had in hand, we did our best. And now it’s up to you people, young people. You’re going to have your turn. You do what you think is best with the problems you face, with the solutions at hand.” And that’s all I can say too.

You’ve got to do things in the context of the times, and you’re human beings. You—all humans have certain innate needs. They want to fill their bellies. They want to procreate their species. They want to survive. And I’ll repeat, sometimes if you’re threatened to survive you pick up the best weapon you’ve got, if it’s a rock, a stick, or a stone, or a bomb. That’s what we did.

Jeff (Cameraman): You said before that the facility you worked in no longer is around? The facility you worked in is not here anymore?

Knobeloch: No, it’s not around. It was a temporary building. It was set off in a canyon, in Bayo Canyon, because we were doing, you know, intense radiation sources that are harmful. Plus we were blowing up six hundred pounds of He, which has to be removed too. 

Jeff (Cameraman): What about the other buildings that might not be around? Is it—

Knobeloch: I had a laboratory and a small office in U building, which is not around. All those Tech Area 1 buildings were about where the Los Alamos Inn is now located, or, what do they call it, the Quality Inn. And, well after the end of the war, most of the Tech Area buildings were moved over to South Mesa, where they are now.

The only buildings in town that I know of that still exist, is the Lodge, the Bathtub Row houses, what we called East Cafeteria is now the performing arts center, and I don’t know of any other buildings from the war era that are still around. The Big House was a nice building that was left over from the Ranch School days, but it was torn down, I guess, about 1947 or so.


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