Neutron bombardment experiments

Not all of the Pu-239 will fission during the fuel cycle in a nuclear reactor. Some of the plutonium will not experience neutron bombardment sufficient to cause fission. Other plutonium atoms will absorb one or more neutrons and become higher numbered isotopes of plutonium, such as Pu-240, Pu-241, etc. Plutonium comprises just over 1 percent of nuclear reactor spent fuel—the fuel removed from the... 

Hahn confessed his perplexity that Christmas in a letter to his former colleague Lise Meitner, an Austrian physicist whose Jewish heritage had forced her to flee the Nazis for refuge in Stockholm. Meitner had begun the neutron-bombardment experiments with Hahn in 1934, before escaping from Berlin after the Anschluss in 1938. She shared his disbelief, replying ... 

The fission reaction was discovered in 1938 by two German scientists, Otto Hahn (1879-1968) and Eritz Strassmann (1902-1980). They had been doing a series of experiments in which they used neutrons to bombard various elements. If they bombarded copper, for example, a radioactive form of copper was produced. Other elements became radioactive in the same way. When uranium was bombarded with neutrons, however, an entirely different reaction seemed to occur. The uranium nucleus apparently underwent a major disruption. 

With proper safety procedures, radiation can be very useful in many scientific experiments. Neutron activation analysis is used to detect trace amounts of elements present in a sample. Computer chip manufacturers use this technique to analyze the composition of highly purified silicon wafers. In the process, the sample is bombarded with a beam of neutrons from a radioactive source, causing some of the atoms in the sample to become radioactive. The type of radiation emitted by the sample is used to determine the types and quantities of elements present. Neutron activation analysis is a very sensitive measurement technique capable of detecting quantities of less than 1 X 10 9 g. 

In other experiments the ampoule was filled with a pure inert gas at ambient temperature and was bombarded with neutrons for two half-lives of the radionuclide under study. Then, upon heating, the ampoule was flushed for a short time with the carrier containing gas, like in the experiments of the first kind. 

What I should like, Henry Tizard wrote Mark Oliphant after he had studied the Frisch-Peierls memoranda, would be to have quite a small committee to sit soon to advise what ought to be done, who should do it, and where it should be done, and I suggest that you, Thomson, and say Blackett, would form a sufficient nucleus for such a committee. Thomson was G. P. Thomson, J.J. s son, the Imperial College physicist who had ordered up a ton of uranium oxide the previous year to study and felt ashamed at the absurdity. He had concluded after neutron-bombardment experiments that a chain reaction in natural luranium was unlikely and a war project therefore impractical. Tizard, who had been skeptical to begin with and had taken Thomson s conclusions as support for his skepticism, appointed Thomson chairman of the small committee James Chadwick, now at Liverpool, his assistant P. B. Moon and Rutherford prot g John... 

There was one problem with this program. Various experiments suggested that the voids in the neutron (reactor) irradiated materials contained helium. Helium is formed rapidly during neutron irradiation as a decay product. Harkness et had shown that the swelling is a nucleation-controlled process. As the accelerator ion bombardment experiments, intended to mimic the reactor neutron bombardments experiments, do not produce helium within the sample, it is important to ask what effect helium has on the void nucleation problems. 

The radioactive indicator method has been used in just the same way to follow the diffusion of lead ions in lead chloride and lead iodide (32). With the discovery of artificial radioactivity the method seems capable of very wide application indeed. Gold, for example, has been rendered radioactive by neutron bombardment, and then used (33) to measure the selfdiffusion constant of gold in gold and radioactive copper has been used to measure self-diffusion in copper (33a). Some of the substances which may be rendered radioactive by bombardment with neutrons, deuterons, protons, or y-rays, and whose half-life period seems adequately long for the duration of possible diffusion experiments, have been collected in Table 59. 

At the time, the results from the neutron bombardment experiments appeared to have no direct application, until it was discovered that during the fission process, extra neutrons are released — in other words, when the splits, two daughter nuclei are produced with some surplus neutrons as well. This led to the possibility of a chain reaction taking place, in which one fission would give rise to two or three extra neutrons, which could then go on to split two or three more uranium atoms, which would release even more neutrons, and so on. The reaction would increase exponentially. In practice, of course, matters are rather more comphcated. 

If the pattern of yields obtained is similar to that observed through neutron bombardment, then Auger ionization can be eliminated as a necessary mode of excitation. (The reverse experiment is not so easily conceived.) An additional item of flexibility is added in the possible use... 

Three groups had roles in the discovery of nobelium. First, scientists at the Nobel Institute of Physics in Stockholm, Sweden, used a cyclotron to bombard Cu-244 with heavy carbon gC-13 (which is natural carbon-12 with one extra neutron). They reported that they produced an isotope of element 102 that had a half-life of 10 minutes. In 1958 the team at Lawrence Laboratory at Berkeley, which included Albert Ghiorso, Glenn Seaborg, John Walton, and Torbjorn Sikkeland, tried to duplicate this experiment and verify the results of the Nobel Institute but with no success. Instead, they used the Berkeley cyclotron to bombard cerium-... 

But none of the experiments succeeded. Rutherford and Chadwick came close. They probably produced neutrons when they performed an experiment in which they bombarded aluminum with alpha particles. However, they saw nothing. In those days, neutral particles were hard to detect. Protons and electrons, which were electrically charged, could be manipulated with magnetic and electric fields. Neutrons couldn t. 

Rutherford and Chadwick knew that if the Joliots realized that their conclusions were erroneous, they might discover the neutron first. So Chadwick immediately went to work performing new experiments. He soon found that if beryllium was bombarded with alpha particles, a kind of radiation consisting of particles with a mass close to that of the proton were produced. He ruled out the possibility that the radiation consisted of gamma rays by showing that, if it did, the gamma rays would have insufficient energy to produce the effects that were observed. Chadwick had discovered Rutherford s neutron. 

Bowden Singh (Ref 5) found that when crysts of LA were irradiated with,an electron beam of 75KeV and 200 microamps, an explosion took place, but it was suspected that this expln was probably due to bulk-heating of the sample. In order to prove that considerable heat is evolved on irradiation with electrons, crysts of K nitrate (mp 334°) and also some metallic wires were treated in the same manner as LA. During these experiments, the crysts of K nitrate readily melted and as some metallic wires fused, it was proved that there was considerable evolution of heat. In order to avoid this bulk-heating, Bowden Singh bombarded the crysts of expls with slow neutrons (flux 10 8 neutrons per cm 3 per second). Bombardments of Silver Acety-lide and of Pb, Cd and Li Azides produced... 

In a second series of experiments, the crysts of LA were mixed with uranium oxide and irradiated with neutrons so that fission of U-235 took place. Again no expln was produced. Bombardment with slow neutrons of Nla did produce an expln, but this was not due to neutrons, but simply to the well known fact that Nig usually explodes at ordinary temp as soon as ammonia (which is usualLy mixed with. Nig) becomes evaporated... 

The sample being analyzed in this experiment contains fissionable material that had been sealed in a quartz tube and irradiated with neutrons. The material was dissolved in strong mineral acid to prepare it for radiochemical analysis of fission products. You will be informed of the date and time of the end of bombardment (EOB), the type of material, the amount of fissionable material in your sample, and the acid type and strength. You can assume that the person who prepared the sample based the amount of fissile material to be irradiated and the period of irradiation on the available neutron flux and the desired amounts of fission product radionuclides. You should estimate the activities of the major fission products that remain after the time interval between formation and analysis. 

---