|
|
|
Radiation Levels in the NSRL Target Room
We have studied the radiation levels in the NSRL target room that are out of the primary beam. We categorize the radiation into ionizing radiation that can be observed with a scintillation counter, and non-ionizing radiation that scintillators are mostly insensitive to, For ionizing radiation, we use a plastic scintillator of dimensions 5 x 5 cm2 and 2 mm thick. It is highly efficient for charged particles, with reduced efficiency for photons and almost no efficiency for neutrons. We also employ a Boron Fluoride thermal neutron counter as a measure of the non-ionizing radiation. It is very efficient for neutrons of energy less than ~1 eV, dropping like 1/v as the neutron energy increases, where v is the velocity of the neutron. NeutronsNeutrons are produced primarily when the beam strikes the beam dump. Energetic neutrons range out in the dump, ultimately producing many low energy and thermal neutrons that scatter back into the target room. The thermal neutrons fill the target room nearly uniformly, being about twice as abundant near the beam dump as they are at the upstream end of the room. Measurements of the thermal neutron flux throughout the room show this trend, seen in Figure 1.
We measured the thermal neutrons produced by beams of various ions, from protons, Carbon, Silicon, and Iron. As expected, the neutron production scales approximately with the number of nucleons in the beam, Abeam, as shown in Figure 2. The Abeam dependence assumes neutrons are produced in the beam dump. If high-Z targets or collimators are inserted into the beam, details of the beam and target can modify the simple Abeam scaling to Abeam + Atarget.
The energy dependence of the neutron flux fits an exponential F(E) = F0 e2.8E.where E is the kinetic energy per nucleon of the beam in GeV, as is Figure 3. This is characteristic of shower development in a cascade of steps. When running a proton beam at 1 GeV the thermal neutron flux measurement gives 1.5 x 10-9 per square centimeter per proton at a location 1 meter off the center of the beam line perpendicular to the nominal target location. Using the formulae for energy dependence and ion species, and the flux map of the target room, it is possible to obtain the thermal neutron flux for any situation at NSRL. For a typical NSRL exposure using a large 20 x 20 cm2 beam of 1 GeV protons delivering a dose of 1 Gray to a biological sample located on the target stand, this produces a flux of 3250 thermal neutrons per square centimeter at the location 1 meter off the beamline. Delivering the same dose with Fe ions of 1000 MeV/nucleon produces only 270 cm-2.
A small number of high energy neutrons are produced when the beam interacts with material in the beam line. These neutrons predominantly remain within the primary beam, and don’t contribute significantly to the radiation levels at the location 1 meter from the beam.
Ionizing RadiationCharged particles produced when the beam stops in the dump will remain within the dump. A much smaller number of charged particles are produced in interactions with material in the beamline such as the aluminum vacuum window, ionization chambers, and air. Like the high energy neutrons, these remain mostly in the beam. Some low energy scattered particles travel at large enough angles from the beam to get 1 meter off the beamline. These particles are counted by the scintillator.
In addition to the charged particles, many high energy photons or gamma rays travel with the beam. They can ionize molecules in the air which give signals in the scintillator. Other sources of ionizing radiation include material that is activated by the beam such as targets and air. These activated materials often decay almost immediately by the emission of gamma rays which trigger the scintillator. Photon fluence from activation in the beamline will be more or less isotropic near the beam, dropping off approximately like 1/R at large distances from the beam, R.
When running a proton beam at 1 GeV the charged track flux measurement gives 1.8 x 10-7 per square centimeter per proton at a location 1 meter off the center of the beam line perpendicular to the nominal target location. For a typical NSRL exposure using a large 20 x 20 cm2 beam of 1 GeV protons delivering a dose of 1 Gray to a biological sample located on the target stand, this produces a flux of 400,000 ionizing tracks per square centimeter at the location 1 meter off the beamline. Delivering the same dose with Fe ions of 1000 MeV/nucleon produces only 30,000 cm-2.
The rates given above do not account for penetrating power or particle species. A more detailed study would require better calorimetry and tracking with dedicated beam time.
SummaryA measurement has been made of the thermal neutron and charged particle fluences in the neighborhood of the NSRL target room. Copious production of low energy neutrons and photons in the beam dump and along the beamline gives rise to a large flux of particles off the beamline. We have measured those fluences at a point adjacent to the nominal target position but 1 meter to the side of the beamline. Prescriptions are given for calculating the fluences as a function of ion species, beam energy, and location within the room. For a 1 GeV proton beam delivering 1 Gray in a large beam, there would be 3250 thermal neutrons and 400,000 ionizing tracks per square centimeter. Fluences would be reduced for higher Z ions and for lower beam energies.
Figure 1 shows a schematic of the NSRL target room with indiction of the location of the nominal target position and the relative thermal neutron flux at eight locations within the room.
Figure 2 shows the energy dependence of the neutron flux as measured for silicon ions of kinetic energy 300, 600, and 850 MeV/nucleon. The line is the result of a fit to the exponential F(E) = F0 e2.8E.
Figure 3 shows the Abeam dependence of the neutron flux. The line is the result of a linear fit.
Last Modified: February 1, 2008 |
|
|
