The Photon Beam Dump at the Nuclear Physics Setup (NP-CBG.01)
When directing the electron beam at ELBE onto a thin (20 bis 100 μm) metallic foil (Al, Ta, Au, etc.) energetic electromagnetic radiation (bremsstrahlung) is generated. By means of a collimator made from pure aluminum (99.5%) polarized photons are selected. The photon beam dump should effectively reduce the radiation background from photon backscattering and from neutron production in the experimental hall (40/109).
Requirements for the Beam DumpBy using selected materials and optimizing the geometrical setup of the beam dump photon backscattering of photons should be reduced. If the production of neutrons is unavoidable the geometrical setup should prevent the neutrons from reaching the detectors.
Vacuum RequirementsThe photon beam will travel inside the evacuated beam pipe until it reaches the beam dump. The vaccum flange sits inside the beam dump and it is made from polyethylene. Consequently, there are no vacuum requirements.
ConceptThe photon beam dump consists of a primary absorber fullfilling the following requirements:
- High energy threshold for neutron production via (γ,n)-reactions.
- Small cross sections for (γ,n)-reactions.
- Small atomic number for minimization of electron/positron pair production (γ + Z -> e+ + e- + Z). Positron annihilation produces two photons of 511 keV, each.
- High density of electrons for widening the beam by Compton-scattering.
|Dichte (g cm-3)||0.95||1.0||2.2||11.34|
|n*Z (mol cm-3)||0.54||0.56||1.1||4.5|
|nmax*Zmax2 (mol cm-3)||2.44||3.55||6.6||368|
These requirements are fullfilled by a light absorber made from polyethylene. Photons which are scattered once or several times are absorbed in a surrounding absorber. This secondary absorber has to accomodate for the following requirements:
- High absorption for secondary photons and electrons.
- High matter density for effective use of the (limited) space.
- Low activation cross section for neutron capture.
These reuirements are fullfilled by a heavy absorber made from lead.
For the optimization of the described requirements simulation calculations based on the code system GEANT  were accomplished. The necessary dimensions and materials for the beam dump resulted from these calculations. Here the following alternatives were compared:
- The photon beam pipe is closed by a lead wall with a thickness of 20 cm.
- The beam dump consists of an inner block of water with a length of 70 cm, a height of 30 cm and a width of 30 cm. This inner block is followed by a quader of triangular basis with edges of 30 cm length and it has a height of 30 cm. Both absorbers are surrounded by a lead shielding of 10 cm perpendicular to and in the beam direction and 20 cm towards the detector location. A hole of 10 cm diameter in the front wall allows the photon beam to enetr the beam dump.
- same, but water subsituted by graphite.
- same, but water subsituted by polyethylene.
The directional flux of backscattered photons from the beam dump towards the detectros has been determined. In the right figure the energy dependent flux of backscattered photons of an energy Eγ is displayed. The flux of photons is determined across a plane which is tilted by an angle of π/4 against the incident beam causing the asymmetry in the distributions. All fluxes are normalized with respect to the first alternative. The calculations have been done for 107 incident photons with an energy of 20 MeV. Obviously, the material with the lowest atomic number (PE) gives the lowest amount of backscattered photons. Furthermore, carbon and hydrogen have the highest neutron production thresholds.
Construction of the Beam Dump
The beam dump consists of an inner absorber made from polyethylene surrounded by a lead shielding (see figure on the right). The widening of the incident beam by Compton scattering and pair production is seen clearly but it requires a rather deep inner absorber length of 70 cm in total. The large mass absorption coefficient of lead of 20 g/cm2 in the energy range of interest accounts for a suppression by a factor of 3.5 * 10-3 for a thickness of 10 cm and of 1.2 * 10-5 for a thickness of 20 cm. The hydrogen content of polyethylene acts as an effective moderator for neutrons generated in the structure materials. At the inside of the lead shielding cadmium sheets of 0.5 mm thickness are mounted as neutron absorbers.
The assembly scheme and the cutting of lead bricks are described elsewhere.
With the photon beam dump accting as an additional absorber for particles emitted into the experimental hall no negative effect either the local or global dose is expected. Rather, the high-energy photon flux impinging onto the concrete wall is reduced effectively.
 GEANT Detector Description and Simulation Tool, CERN Application Software Group, Geneva