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 (82 inches). 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 ============================================================= Comparison of external-beam energy and current for a synchrocyclotron and a conventional cyclotron ------------------------------------------------------------- 184-Inch Synchrocyclotron ------------------------- Protons Deuterons Alpha particles ------- --------- --------------- Beam energy -- maximum (Mev) 730 460 910 Beam intensity -- peak current ([mu]a)[6] 120 120 40 Beam intensity -- average current ([mu]a) 0.75 0.75 0.25 60-Inch Cyclotron ----------------- Beam energy -- maximum (Mev) 12 24 48 Beam intensity -- peak current ([mu]a) 100 150 100 Beam intensity -- average current ([mu]a) 70 80 60 ------------------------------------------------------------- [6] [mu]a = microampere ============================================================= [Illustration: Fig. 7. Plan view of the cyclotron, showing the method for obtaining an external beam of protons, deuterons, or alpha particles.] Some experiments require an external beam of protons, deuterons, or alpha particles. A beam of this type can be brought out of the machine by means of a LeCouteur regenerator (Fig. 7). The construction of the regenerator is very simple. It is made of a number of steel laminations of various sizes. What the regenerator does is perturb the magnetic field of the cyclotron at one radial position. Each time the beam passes through the regenerator it receives a kick. With each kick the beam builds up its radial amplitude, until finally it enters a magnetic channel. This channel focuses the beam and steers it outside the main magnetic field. Once outside, the beam travels through an evacuated tube, which is integral with the main vacuum tank. By means of a steering magnet, the beam can be sent into either the physics cave or the medical cave. (These experimental areas are called "caves" because they are rooms inside the massive concrete shielding wall.)