Workshop on Radiation Protection Aspects of High Intensity Proton/Deuteron Accelerators

Lutz Moritz Joe McDonald Steve Musolino
A workshop was held at the Italian Nuclear Physics Laboratory at Legnaro (LNL), September 27-29, 1999. Approximately 20 invited speakers made presentations on topics such as: computer codes and data for shielding design, source term evaluation, determination of dose equivalent, handling of radioactive components, activation, internal dose problems, radiation protection at RHIC, microdosimetric measurements and new rem meter designs. Although the title of the workshop suggested an emphasis on radiation protection, some presentations dealt with beam line and target designs, neutron yields and angular distributions, along with descriptions of the applications for the planned accelerator.

The Legnaro National Laboratory is near Padua, and was established in 1960. A series of accelerators have been used for a variety of research including, fundamental nuclear physics, biomedical physics, radiobiology, solid state physics, environmental physics, material technology, cryogenics and superconductivity. A superconducting linear accelerator that uses an existing tandem as an injector has been construction at the laboratory.

The proposal under discussion at the workshop involves building a new superconducting linear accelerator to accelerate both protons and deuterons initially to an energy of 20 MeV with intensities up to 5 mA and eventually to 100 MeV at an intensity of 30 mA. The accelerator is intended to be used for a wide variety of research. The primary mission will be the production of intense radioactive ion beams (RIB) of heavy nuclei. The production of RIB will be via the Isotope Separator On Line (ISOL) method. Unlike previous facilities, the proton beam will not be used to activate the target directly, but rather will be stopped in a Be or graphite target so as to generate a high flux of neutrons. These neutrons will be used to irradiate a UC2 target/ion source that will generate the radioactive ion beams. Hence the need for extremely high proton beam currents.

In addition to nuclear physics research, work is also planned in nuclear astrophysics, tests of the "Standard Model", material science, radioactive waste treatment and possible cancer treatment using boron neutron capture therapy.

Summary of Issues Discussed at Workshop

Global Issues:

* Definition of Radiation Protection Quantities
* Difficulties with Neutron Dosimetry
* Measurement Techniques
* Biological Effects

Accelerator Issues:

* Shielding Calculations (Empirical Recipes vs. Monte Carlo)
* Activation Estimates
* Handling of Radioactive Components
* Environmental Issues (Ground Water Activation, Releases to Atmosphere)

Specific Issues:

Need for Design Criteria (in context of local regulations)

* Chronic/Accidental Dose to Workers
* Chronic/Accidental Dose to Public
* Permissible Ground Water Activation
* Permissible Releases to Atmosphere

Accelerator

* Realistic Estimates of Beam Losses (1-5 W/m ?)
* Acceptable Activation Levels (< 1 mSv/h ?)
* Source Term Evaluation (appears to be in hand)
* Shielding Requirements

Use NCRP51 at energies <20 MeV, M.C. difficult
Irreducible Uncertainties for Very Thick Shields
Options:
1) Measurement (may not be feasable)
2) Allow for Additional Shielding
3) Deliberately Over-shield

Target

* Shielding (as for accelerator)
* Activation (cross-sections are available)
* Handling and Facility Classification will depend on contamination levels and local regulations

Safety Program

* Training
* Monitoring (choice depends on radiation characteristics)
* Contamination Control
* Possible Need for Bioassay
* Environmental Monitoring