What Is a Cyclotron?

Cyclotron English: Cyclotron It is a device that uses a magnetic field and an electric field to make charged particles rotate in a circular motion and is repeatedly accelerated by a high-frequency electric field during the motion. It is an important instrument in high-energy physics.

Cyclotron Basic Information

Early accelerators could only accelerate charged particles once in a high-voltage electric field, so the energy the particles could reach

Cyclotron Limited by high voltage technology. For this reason, some pioneers of accelerators, such as R. Wider & ouml; e, in the 1920s, explored the use of the same voltage to accelerate charged particles multiple times, and successfully demonstrated a straight line that accelerates sodium and potassium ions twice with the same high-frequency voltage. Device, and pointed out that the reuse of this method can in principle accelerate the ions to reach an arbitrarily high energy (in fact, due to the special theory of relativity, it can only accelerate to 25-30 MeV). However, due to the limitation of high-frequency technology, such a device is too large and too expensive, and it is not suitable for accelerating light ions such as protons, deuterons, etc. for nuclear research. As a result, it has not been developed and applied.

Cyclotron related theory

In 1930 Ernest Lawrence put forward the theory of cyclotron ^[1] , which was successfully developed for the first time in 1932. Its main structure is that two semi-circular metal flat boxes (D-shaped boxes) are placed opposite to each other in a vacuum chamber between magnetic poles. An alternating voltage is applied to the D-shaped box, and an alternating electric field is generated at the gap. The particle source placed at the center emits charged particles and is accelerated by the electric field. It is not affected by the electric field force in the D-shaped box. The time to go around a half circle is m / qB, where q is the particle charge, m is the mass of the particle, and B is the magnetic induction of the magnetic field. If the frequency of the alternating voltage applied to the D-shaped box is exactly equal to the frequency of the particles making a circular motion in the magnetic field, the particles will catch up with the voltage direction change on the D-shaped box after half a circle, and the particles are still accelerating. Because the time for the particle to make a half-circle is independent of the speed of the particle, the particle is accelerated every half-circle and the radius of the particle increases. After many accelerations, the particles are drawn from the edge of the D-shaped box along a spiral orbit, and the energy can reach tens of megaelectron volts (MeV). The energy of the cyclotron is subject to the relativistic effect with increasing particle velocity, the mass of the particles increases, and the particle detour period becomes longer, which gradually deviates from the acceleration state of the alternating electric field. A further improvement is the synchrocyclotron.

Cyclotron effect

(1) Effect of magnetic field

When a charged particle enters a uniform magnetic field vertically at a certain speed, it only performs a uniform circular motion under the action of Lorentz force, where the period has nothing to do with the speed and radius, so that each time the charged particle enters the D-shaped box, it can move for the same time ( After half a period), it accelerates into the electric field parallel to the direction of the electric field.

(2) The role of the electric field

There is a uniformly changing electric field in the narrow gap region between the two D-boxes of the cyclotron that is perpendicular to the diameter of the two D-boxes. Acceleration is done in this region.

(3) Alternating voltage

In order to ensure that the charged particles are accelerated each time they pass through the slit, so that their energy is continuously increased, an alternating voltage consistent with the period of the particle motion is added to the slit. ^[2]

Cyclotron related applications

In 1995, the cyc-30 cyclotron jointly developed by the Chinese Academy of Atomic Energy and IBA in Belgium was put into use to produce various medical isotopes.

People's Daily report on China's first cyclotron On June 23, 2006, China's first Siemens eclipse HP / RD medical cyclotron was officially put into clinical operation at the Positron Drug Development Center located in the General Hospital of Guangzhou Military Region.

eclipse HP / RD uses a number of cutting-edge technologies such as deep valley technology, target body and target system technology, and full self-shielding. It has the advantages of high performance, low consumption and high stability.

Cyclotron is a device that generates positron radioactive drugs. After the drug is injected into the human body as a tracer, doctors can observe the physiological and pathological functions of the patient's brain, heart, other organs and tumor tissues through PET / CT imaging and Metabolic situation. Therefore, PET / CT relies on different imaging drugs produced by cyclotrons to specifically image various tumors to achieve early monitoring and prevention of diseases.

Cyclotron History

Early

Early accelerators could only accelerate charged particles once in a high-voltage electric field, so the energy that particles can reach is limited by high-voltage technology

Cyclotron theory

In 1930, Earnest O. Lawrence put forward the theory of cyclotrons, he envisioned the use of a magnetic field to charge

Cyclotron The particles rotate along an arc-shaped orbit, and repeatedly pass through a high-frequency acceleration electric field until reaching high energy. In 1931, he and his student MS Livingston developed the world's first cyclotron. This accelerator has a magnetic pole diameter of only 10 cm, an acceleration voltage of 2 kV, and can accelerate deuterium ions to an energy of 80 keV. It was confirmed to them that the cyclotron principle was proposed. Subsequently, with the funding of M. Stanley Livingston, a larger cyclotron with a diameter of 25 cm was constructed, and the energy of the accelerated particles could reach 1 MeV. The brilliant achievement of the cyclotron is not only that it created the energy record of artificially accelerated charged particles at that time, but more importantly, the cyclotron resonance acceleration method it showed laid the foundation for the development of various high-energy particle accelerators.

Two important stages

Since the 1930s, the development of cyclotrons has gone through two important stages. In the first 20 years, people built a batch of so-called classic cyclotrons according to Lawrence's principle, the largest of which can produce 44MeV alpha particles or 22MeV protons. However, due to the contradictions and limitations caused by the relativity effect, the energy of the classical cyclotron is difficult to exceed the energy range of more than 20 MeV per nucleus. Later, a new cyclotron was developed based on a proposal made by LH Thomas in 1938. Therefore, the synchrocyclotron developed in 1945 solved the influence of relativity by changing the frequency of the acceleration voltage. With this accelerator, the energy of the accelerated particles can reach 700 MeV. With a variable frequency, the cyclotron does not need to use high voltage for a long time, and it can also obtain the maximum energy after a few cycles. The most typical acceleration voltage in a synchrocyclotron is 10kV, and the maximum energy of particles can be limited by changing the size of the acceleration chamber (such as radius, magnetic field).

Isochronous cyclotron

After the 1960s, the climax of the development of isochronous cyclotrons set off worldwide. Isochronous cyclotron

Cyclotron chronous cyclotron) is a cyclotron composed of 3 compact-pole 3 sectors, with variable energy, accelerating positive ions in the first and third chirp modes. In the first chirp, the protons are accelerated to 6 MeV ~ 30 MeV, the deuteron is at 12,5 MeV ~ 25 MeV, the alpha particles are at 25 MeV ~ 50 MeV, and He3 +2 ions are at 18 MeV ~ 62 MeV. The change of the magnetic field is completed by 9 pairs of circular adjusting coils, and the ratio of the gradient of the magnetic field to the radius is (4,5-3,5) × 10-3 T / cm. The magnetic field azimuth is corrected by six pairs of chirped coils. The RF system consists of two Dees at 180 ° with an operating voltage of 80kV. The RF oscillator is a typical 6-stage oscillator with a frequency range of 8,5-19 MHz. Usually the typical ion source is radial and can be remotely controlled by the control system. There is a movable slit in the center area for phase adjustment and center positioning. Using an electrostatic deflector with a non-uniform electric field and a magnetic field shielded channel for beam extraction, the maximum potential on the deflector can reach 70 kV. For 30 MeV, the intensity of 15 mA protons in the radial and axial directions (Emittance) is 16p mm.mrad. The energy diffusion is 0.6%, the brightness is high, and the beam current in the target can reach several hundred mA. Different probes are used to measure the beam intensity. These probes include common TV visibility probes; thin-layer scanning probes and non-interceptive beam diagnostic devices. The system's sensitivity to beam current is 1mA, and the flight time is accurate to 0,2 ns. Beam

Cyclotron main coil (2 photos)

To teleport to six targets, 100% teleport can be completed. The cyclotron was first built by INP in 1972. It can accelerate protons to 1 MeV and beam intensity of several hundred mA. It is mainly used in the research of cyclotron systems (ion sources, magnetic fields, etc.).

Since the 1970s, in order to meet the needs of heavy ion physics research, successfully developed all-ion, variable-energy isochronous cyclotrons that can accelerate all elements on the periodic table, which greatly improves the use efficiency of each accelerator. In addition, isochronous cyclotrons have been developed for superconducting magnets. The application of superconducting technology opens up new fields for the development of accelerators in terms of reducing the size of the accelerator, expanding the energy range, and reducing operating costs. The synchrotron can produce a small beam of pencil-thin, and its ion energy can reach 100,000 times the natural radiation energy. The fringe magnetic field is designed to change the ion orbit radius of each stage of the accelerating tube. The largest proton synchrotron is the Main Ring (500GeV) and Tevatron (1TeV) in Fermi National Accelerator Laboratory Chicago; the higher-level proton synchrotron is SPS (Super Proton Synchrotron) installed and applied in Geneva's European Laboratory for Particle Physics (CERN) , 450 GeV.

Lawrence (EOLawrence, 1901-1958) thus won the 1939 Nobel Prize in Physics.

Cyclotron related calculations

Letter introduction: period T frequency f charge quantity q magnetic field strength B mass m maximum speed Vmax voltage U electric field width ignore time t

f = 1 / T

Cyclotron

Maximum radius Rmax = mVmax / Bq

Maximum kinetic energy Ek = (1/2) m (Vmax) ^ 2 = (BqRmax) ^ 2 / 2m

The energy added per revolution of the ion is 2qU, and the number of revolutions is N when it is increased to Ek.

N = Ek / 2qU = q (BR) ^ 2 / 4mU

The magnetic field movement time is t magnetic = NT = PiB (Rmax) ^ 2 / 2U

Negligible time to move in an electric field

(The following is the expanded content)

Electric field width d

Human education version of high school physics-cyclotron (2 photos)

Without ignoring the electric field width

Ions are regarded as uniformly accelerated linear motion with Vo = 0

Nd = (1/2) (Uq / dm) t Electric ^ 2

telectric = dBRmax / U

ttotal = telectric + tmagnetic = [BRmax (2d + PiRmax)] / 2U

Can a cyclotron accelerate both positive and negative ions with the same mass and charge?

It is theoretically possible because they can move in both directions, and because the specific charge is the same, they accelerate at the same time. It's also a D-box with a space structure, so don't worry about collision.

Cyclotron Domestic Development

The China Experimental Fast Reactor, as a major project of the national "863" plan, has been studied in advance since the "Seventh Five-Year Plan". The fast neutron reactor (referred to as "fast reactor") is the main type of the fourth-generation advanced nuclear energy system, and is a key link of the advanced fuel cycle.