Production of fast neutrons at nELBE:

Fast neutrons are produced at the neutron facility nELBE at the radiation source ELBE. Bremsstrahlung, which is produced when accelerated electrons are stopped in the lead, undergoes (γ,n) reactions with lead nuclei leading to fast neutrons. nELBE is the only photo neutron source at a superconducting cw-linear accelerator in the world.

The neutron energies that can be measured range from 50 keV up to 10 MeV with an electron beam repetition rate of 0.5 MHz.


Figure: sketch of the liquid lead radiator

Figure: liquid lead radiator in the neutron cave (© A.Junghans)

some properties of nELBE:

  • micropulse charge: 80pC (thermionic injector) / 1nC (SRF Injector)

  • micropulse length: <10ps

  • adjustable repetition rate: 13MHz / 2n

  • average beam power up to 40kW for time of flight measurements

  • flight path: 4-8m

  • neutron flux on target sample: 1.5·107 cm-2s-1

  • neutron energy range: 100keV...10MeV

  • energy resolution: <1%

Figure: neutron spectrum of nELBE (© R.Beyer)

Research at nELBE:

At nELBE investigations of fast neutron induced reactions of relevance for nuclear transmutations and the development of generation IV reactors can be executed. There are basically two types of investigation in the project:

  1. Inelastic neutron scattering on 56Fe, Mo, Pb, 23Na and measurement of the total cross section of Ta, Au, Al, C, H
  2. Investigation of minor actinide targests 239Pu, 242Pu, 235U, 238U

56Fe(n,n'γ) experiment:

For measuring inelastic neutron scattering the ntof setup at nELBE is used. The time of flight of the incoming and outcoming neutron and the scattering angle are detectable.  Hence it is possible to compute from kinematics the exited state on which the neutron was scattered. For detecting the photon (used as starting/end point for the time of flights) a BaF2 array is used. The scattered neutron are detected in plastic scintillators.

left figure: top view of the ntof setup (© R.Beyer)

top figure: ntof setup (© R.Beyer)

Figure: time of flight of incoming vs. outgoing neutron after inelastic scattering on 56Fe

Experiments at PTB Braunschweig:

The contributions of PTB Braunschweig to the joint research project are:

  1. Construction of a standard for the neutron flux density for investigations of the detection probability of fission chambers and neutron detectors
  2. Investigation of the comparability of different methods of measuring the cross section of inelastic neutron scattering by means of the time of flight spectrometer of PTB

As standard for measuring the neutron fluence a TCAP (time correlated associated particle) setup is suggested. A beam of deuterons is shot on a Tritium target. Free neutrons are produced via the reaction T(D,n)α. The produced α particle and the neutron are detected in coincidence (see figure 7). Angular straggling of the α particle in the target material is a problem of this method. Therefor this effect has to be simulated in detail.

Moreover to the construction of the TCAP standard, the comparability of different methods of measuring the cross section of inelastic neutron scattering should be investigated at the time of flight spectrometer of PTB. This should help to explain the deviations in the cross section of inelastic neutron scattering on 206Pb,207Pb and 208Bi between data measured at PTB and GELINA.