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Read Ebook: The Kallikak Family: A Study in the Heredity of Feeble-Mindedness by Goddard Henry Herbert
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Deuterons or alpha particles are obtained in a similar fashion using deuterium or helium gas in place of hydrogen.
The 184-inch synchrocyclotron has a single dee instead of the double-dee arrangement described above for illustrative purposes. The accelerating electric field is developed between the dee and a dummy dee which is grounded to the vacuum tank. Using a single dee does not change the principle of operation, yet it offers the advantage of allowing more space for auxiliary equipment inside the vacuum tank. Also, the construction is much simpler. The dummy dee is not essential for operation, but it does improve performance.
Radiofrequency power is supplied to the dee by a vacuum-tube oscillator. The frequency of oscillation must decrease during the acceleration cycle, as indicated above. For protons, the frequency at the start of acceleration is 36 megacycles . At the end of acceleration the frequency is only 18 Mc . This change in frequency is achieved by varying the electrical capacitance in the tuned circuit of the oscillator. This tuned circuit, which is called the cyclotron resonator, is shown in Fig. 6.
Because the frequency must change over such a wide range , the electrical capacitance must be varied by a factor of 20 to 1. This is done by a variable capacitor of unique design. It resembles two giant tuning forks. As the blades of the tuning forks vibrate, the capacitance is alternately increased and decreased by the required amount.
These two tuning forks must be kept in step with great precision. This is to prevent the oscillator from exciting lateral rf resonances. With a cyclotron of this size, this is a problem. These resonances, if excited, would cause loss of beam. The method for keeping the blades moving together is as follows: The blades are made to vibrate at their resonant frequency, which is approximately 64 cycles per second. One set of blades operates at its natural frequency as a tuning-fork oscillator. The second set of blades is driven from an amplified sample of the signal from the first; its natural period is adjusted automatically to equal that of the first. The amplitude of each set is regulated to within 0.003 in.; the phase angle between the blades is regulated to within 1 deg.
Ions are accelerated only when the radiofrequency is decreasing . The remaining portion of the cycle is "dead time." Thus, 64 pulses, each of about 500 microseconds' duration, are obtained every second. The average ion current of a pulsed beam is much less than for a continuous beam, such as that obtained from a conventional cyclotron . This is part of the price paid for higher energies.
The simplest target is one placed inside the vacuum tank where the circulating beam will strike it. The target may be any substance that the physicist or chemist wants to irradiate. The target material is attached to a movable probe. If the experimenter wants to use the full-energy beam, he places the target at the maximum usable radius of the circulating beam . However, if he desires to use ions having less than the maximum energy, he inserts the target further into the cyclotron so that it is intercepted sooner.
TABLE I
Beam intensity -- peak current 120 120 40
Beam intensity -- average current 0.75 0.75 0.25
Beam intensity -- peak current 100 150 100
Other experiments may require an external beam of mesons. A meson beam is obtained in the following way : A movable target such as a block of carbon is placed inside the cyclotron near the end of the outward-spiraling proton beam. When the proton beam hits this target, a shower of mesons is produced. These mesons are bent in various directions by the main magnetic field. Some of them pass through a thin metal window in the vacuum-tank wall and are focused by a magnetic lens into a beam. This meson beam then travels through a hole in the concrete shielding wall into the meson cave. The maximum intensity of this extracted meson beam depends on both the charge and energy desired. Beams of more than 100,000 mesons per second have been obtained through an aperture 4 x 4 in. in the shielding wall.
CYCLOTRON EXPERIMENTS
About 86% of the operating time of the 184-inch synchrocyclotron is devoted to experiments in nuclear physics. Most of the experiments study the production and interaction of mesons. These particles are considered to be essential factors in the intense but short-range forces that bind the nucleus together. The three types of mesons are designated according to their electric charge as ^+, ^0, and ^-. These mesons materialize only in high-energy nuclear collisions.
Of great importance are those experiments that determine the probability of producing each of the three types of mesons in a nuclear collision. This type of experiment is repeated for different beam energies and target elements. Other experiments measure the energy and direction of emission of mesons from a target.
A typical -meson experiment is represented in Fig. 9. The purpose of this experiment was to detect the spin directions of protons as they are knocked out of a liquid hydrogen target by a -meson beam. An extracted proton beam from the cyclotron enters the physics cave from the left, striking a polyethylene target and producing mesons. A beam of these mesons is formed by a series of two bending magnets and three focusing magnets. This beam passes through a carbon absorber to remove unwanted particles. The meson beam then strikes the liquid hydrogen target. A few of the incoming mesons scatter, knocking protons out of the liquid hydrogen. Scintillation counters at A and B record the passage of a proton, thus defining its direction. The scattered mesons are counted by a scintillation counter at C. A few of the protons scatter off the carbon target and are detected by counters at E and D. From the detection of such events, the spin directions of the recoil protons can be analyzed. In this way, more is learned about the fundamental -proton interaction.
Further studies of the interactions of mesons are made in the meson cave. Other experiments performed there are concerned with mesons. The meson is a particle created in the decay of a meson and is the principal constituent of cosmic rays striking the surface of the earth. The muon is unstable, eventually undergoing a radioactive decay into an electron. Although the muon does not experience nuclear forces, it can interact weakly with nuclei. The behavior of the muon is well understood, but its role as one of the elementary particles is unknown. That is, if the muon did not exist, what effect would this have on the structure of matter? The answer to this question, among others, is being sought by physicists using the 184-inch cyclotron.
Experiments in biophysics are conducted in the medical cave. In these the interest lies not in nuclear interactions but in the effect of ionizing radiation on living tissue. High-energy beams of particles can be used for selective destruction of specific areas of the brain. This permits physiological mapping of the functions of the brain in experimental animals. It further offers a therapeutic approach to the treatment of brain tumors. One of the important investigational programs is concerned with the relationship of the pituitary gland to the growth rate of certain cancers and to some endocrine disorders.
For techniques of radiochemistry to be employed successfully, high interaction rates are needed. For this reason, chemistry targets are usually inserted right into the cyclotron so that they can be bombarded directly by the circulating beam. After the bombardment is completed the target is removed from the cyclotron. It is then taken to a chemistry laboratory and subjected to detailed chemical procedures. Individual elements are removed, and the radioactive isotopes of each element are identified by quantitative counting techniques. In some cases a mass spectrometer is used to analyze the products. Many deductions about the nature of the breakup of the target nucleus can be drawn from the pattern of the observed radioactive products. Sometimes the nucleus splits almost in half. This is called fission. More frequently smaller parts of the nucleus are split off. Two general types of reactions, known as spallation and fragmentation, are distinguished. One of the goals of this research is to learn more about the constitution of the nucleus and of the forces which bind the particles in the interior of the nucleus.
FOOTNOTES:
Mesons are elementary particles intermediate in mass between the electron and proton.
It may be interesting to note that the ^0 meson was discovered with this cyclotron in 1950. This was the first particle to be discovered with an accelerator. All particles that had been previously discovered were observed first in cosmic rays or some other form of natural radiation.
BIBLIOGRAPHY
SUMMARY OF SPECIFICATIONS
Size length 25 width 20 height 4
Material: mild steel
Operating pressure 10^-5
Vacuum pumps: six 20-in. oil-diffusion pumps with 8-in. boosters: one Beach-Russ 750-cfm; one Kinney 300-cfm; two Kinney 105-cfm.
Pumping speed of oil-diffusion pumps 20,000
Core diameter
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