IARPE - TRIUMF

Canadian Laboratory for Particle and Nuclear Physics

News from TRIUMF

There has not been a report from TRIUMF to the IARPE Newsletter for some time. This is not because there has been no activity at TRIUMF but rather because everyone has been so busy getting the new radioactive ion beam facility (ISAC) up and running. This facility is housed in a new building and one of the 500 MeV proton beam lines from the TRIUMF cyclotron is transported to this building via an underground tunnel. The proton beam is made to bombard a heavily shielded, thick target that is simultaneously heated to a high temperature. The radioactive atoms generated in the target diffuse into a contiguous ion source which transforms them into a beam of ions. This initially undifferentiated ion beam is then mass analysed and a particular species is selected and transported to the experiments. The radionuclides of interest are usually those with short half-lives, i.e. those far from the line of stability.

Commissioning started last November with a CaO target bombarded by 1 microA of 500 MeV protons. The ions of interest were K-38 and K-39 and new, high-precision measurements of their half-lives were made. During the commissioning period which has just ended a Nb target was bombarded with a proton beam current of up to 10 microA. This made ISAC the most intense radioactive ion beam facility in the world, surpassing the ISOLDE facility at CERN. In addition to measuring the yields of a number of different species from this target, radioactive ions were supplied to three different experiments: a life-time measurement, TRINAT (TRIUMF Neutral Atom Trap), and LTNO (Low Temperature Nuclear Orientation).

The radiation safety group was involved in making measurements verifying shielding estimates, radioactive emissions and induced radioactivity. No major surprises were found although the residual fields encountered made it clear that the extensive preparations for remote handling of the target and front end components were justified. Preparations are now underway for a 100 microA proton beam test run in December.

There were two major failures of components in the high intensity proton beam line of the meson facility which cut short the last operating cycle. One was a leak in the water cooling of a quadrupole magnet downstream of the first meson production target and the other was a water leak from the collimator in front of the beam dump. Repairs are currently progressing; much of the work is done either remotely or with long-handled tools. We have good, detailed records of dose expended for similar jobs in the past and can reasonably confidently predict what the likely dose incurred by the workers will be for these repairs.

For several years the practice at TRIUMF of providing shielding for some proton beam lines for operating losses only (rather than for the maximum possible loss) has been under scrutiny by the Canadian regulatory body (Atomic Energy Control Board). TRIUMF relies on a system of fast 'beamspill' monitors to trip the cyclotron if any excessive beam loss is detected. We have been asked to evaluate or demonstrate the reliability of this system. This is somewhat the same question addressed by CASOG some years ago, but in a different form. The first conclusion we have come to is that it is very expensive to do a detailed probabilistic risk assessment. Instead we have analysed the system to try and identify and eliminate possible common mode failures. In addition we have a second independent system that measures the neutron fields outside the shielding and we will have this system generate an independent trip of the cyclotron if a high field is detected. We feel that with two independent systems, each of which has a historically demonstrated low failure rate, we will be able to detect any excessive beam loss and turn of the cyclotron with high reliability (failure rates of less than 10^-6 per year).