Report on the program of 4 K irradiation of insulating materials for the Superconducting Super Collider

Abstract

under Contract No. DE-AC35-89ER40486. e-damage or any difference in damage due to electrons at different energies, t8 The other prevalent type of irradiation is neutron irradiation. Neutrons tend to produce primary knock-on atoms, which tend to be hydrogen atoms selectively, which recoil and cause ionization. Elastic scattering of neutrons can form radicals. Neutrons also react with boron to form 7Li and an alpha particle which causes further damage. 19 There have been several attempts at equivalent damage conversion from neutron fluence to absorbed dose of_, and e-. 1,2,17,20There is always a reported difference between the two, and conversions are inexact. In the Collider, 90% of:he dose is expected to be due to electromagnetic showers (e.g., e-and ¥) and only 10% due to hadronic showers (e.g., n, p).27 As the energy of the incoming particle is transferred to the material, there are several damage mechanisms which can occur. Which reactions occur is a function of chemistry and the conditions of irradiation. These mechanisms are outlined below: Radical Formation When an uncharged species does not have the correct number of bonds, it is called a radical. These highly reactive species are missing an electron to complete their outer shells. Examples of radicals include-H and *CH3 but also include long chain molecules with one site missing. These radicals may be frozen in at low temperatures and cause damage upon warmup. Radicals are formed by the reaction: R+y_>2R* Gas Evolution Bonds between the main chain and small side groups like H, CI, or CH3, which are mobile within the matrix and can combine into stable molecules which are gaseous at room temperature. These molecules may not be mobile at low temperatures, but can combine and diffuse out at higher temperatures. These gasses often coalesce into bubbles and cause swelling. Crosslinking Sometimes, a radical will interact with a nearby chain. The two chains are then chemically linked. This tends to embrittle the material. Crosslinking occurs by: Chain Scission When the bond broken by the radiation is a main chain bond and the chain is broken into two pieces, chain scission is said to have occurred. Chain scission tends to weaken the material. The reaction is: Oxygen The presence of oxygen can complicate the situation. The oxygen molecules can serve in a propagation reaction, such that several radicals are formed from the one created by the irradiation. The reaction is as follows: Initiation Ro + 02 _> ROO* Propagation ROOo + R _> ROOH + Ro ROOH _> ROe + OH* or ROOe + He. These microscopic chemical changes in the molecules of the material induced by the radiation cause changes in the macroscopic properties of the bulk material. If one models a polymer as a knot of yam or spaghetti on the microscopic level, the mechanism by which these changes occur is often easy to grasp. As an example, consider chain scission. According to the knot model, cutting the length of the strands results in fewer entanglements and less strength but higher elongation to break, which is in...