Projectiles for Nuclear Reactions

Neutrons are the most frequently used projectiles for nuclear reactions. As they do not carry a positive charge, they do not experience Coulomb repulsion, and even low-energy (thermal and slow) neutrons can easily enter the nuclei. Neutrons with energies of the order of 1 to lOeV (resonance neutrons) exhibit relatively high absorption maxima. Furthemiore, neutrons are available in large quantities in nuclear reactors with fluxes of the order of about 10 ° to 10 ° cni s .  

Reactions with neutrons are used to produce nuclides on the right-hand side of the line of y stability, i.e. nuclides exhibiting decay. 

Charged particles, such as protons, deuterons or ions with higher atomic numbers Z, must have a minimum energy to pass the Coulomb barrier of the nuclei. Approximately, the Coulomb barrier can be calculated from the equation 

If the collision between the projectile and the nucleus is not central, angular momentum is also transmitted to the nucleus, and in addition to the Coulomb barrier a centrifugal barrier has to be taken into account which is given by 

Angular momentum can only be transmitted in multiples / of hjln (/ = 0 for central collisions), n = m + mf) is the reduced mass of the system. 

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ISOLDE at CERN (SC), make it feasible to consider using such secondary ions as projectiles for nuclear reactions. A pressing need for reaction rate data involving radioactive species exists in nuclear astrophysics. This requires having available projectiles (A < 60) in the energy range from about 200 keV/amu to 1.5 MeV/amu. It has been proposed to install an ISOL device at the TRIUMF facility to utilize the available intermediate energy (200-500 MeV), intense (<100 yA) proton beam as the primary production source. The mass analyzed, radioactive beam (RB),... 

Because the cross-sections for nuclear reaction are usually lower than the cross-sections for elastic scattering of projectiles used in RBS or in elastic recoil detection analysis (ERDA), higher currents must be used to obtain comparably high intensity in... 

The stripping process energy dependence. The peculiar excitation functions of deuteron reactions were discovered soon after the first use of deuterons as projectiles in nuclear reactions. Lawrence, McMillan, and Thornton, found, by activation methods, that the rate at which the cross section of the d, p) reaction increased with energy was lower than that expected from the usual formula for penetration of charged particles. For Cu they found measurable activities for energies as low as 2 Mev. These results suggested that the mechanism of the... 

A charged-particle accelerator, as the name implies, can produce only beams of charged particles (such as iH ) as projectiles. In many cases, neutrons are most effective as projectiles for nuclear bombardment. The neutrons required can be generated through a nuclear reaction produced by a charged-particle beam. In the following reaction, iH represents a beam of deuterons, the nucleus of a deuterium atom (actually, iH ), from an accelerator. 

For example, uranium-238 when bombarded with fluorine-19 produced Md-252. Also, certain nuclear reactions carried out by heavy ion projectiles involve stripping reactions in which some protons and neutrons may transfer from the projectiles onto the target nucleus, but the latter might not capture the projectile heavy ion. 

Radioactivation analysis has been used to measure bromine in polymers (37—39) and recently a novel technique for trace oxygen has been reported (40). Any polymer or other material (e.g. metal alkyl) which is miscible with butyl lithium solutions may be analysed since the procedure involves the intermediate production of triton particles by the nuclear reaction 6Li (n, a) t. The tritons then act as nuclear projectiles for the activation of oxygen 0 (t, n) 18F and the radioactivity due to fluorine-18 is measured. A sensitivity of 1 x 10 g in a 0.5 g sample is claimed. 

The kinetic energy carried in by the projectile, rlab, is not fully available to be dissipated in the reaction. Instead, an amount Tcm must be carried away by the center of mass. Thus, the available energy to be dissipated is r,ab — Tcm = T0. The energy available for the nuclear reaction is Q + Tq. To make the reaction go, the sum Q+T0 must be greater than or equal to zero. Thus, rearranging a few terms, the condition for having the reaction occur is that... 

The compound nucleus is a relatively long-lived reaction intermediate that is the result of a complicated set of two-body interactions in which the energy of the projectile is distributed among all the nucleons of the composite system. How long does the compound nucleus live From our definition above, we can say the compound nucleus must live for at least several times the time it would take a nucleon to traverse the nucleus (10-22 s). Thus, the time scale of compound nuclear reactions is of the order of 10 18-10 16 s. Lifetimes as long as 10-14 s have been observed. These relatively long times should be compared to the typical time scale of a direct reaction that takes place in one transit of the nucleus of 10-22 s. 

One of the projectiles most often used in modern times to initiate nuclear reactions is the neutron. To be effective, a neutron does not need as high an energy as an alpha particle or a proton because it is uncharged and can penetrate a nucleus more easily than a positively charged particle can. Some nuclear reactions initiated with neutrons produce more neutrons. For example, the bombardment of with a neutron produces two large nuclei plus two or three new neutrons. The following are examples of the many possible reactions ... 

Bombardment of certain nuclei with small particles, such as alpha particles or neutrons, can lead to artificial nuclear reactions. The splitting of heavy atoms is one such process, called nuclear fission. Two fairly massive products plus some small particles are apt to result from splitting one large nucleus with a projectile particle. For example ... 

Cross sections are defined only for certain nuclear reactions. For example, is the cross section for the (n, y) reaction of the nuclide A, and is the absorption cross section of the nuclide A in which all absorption processes of certain projectiles, in particular neutrons, in A are summarized, independently of the kind of reaction they are inducing. Another quantity that is often used is the total cross section... 

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