Neutron Scattering

For uranium, which releases neutrons by fission as well, b = 0 .02 GeV. In Figure 1 .3 the yield
for lead and uranium is plotted and related to the energy released per neutron . The latter has
important consequences as already discussed in section 1 .3 .
1 .5 .2 Technical details
A spallation neutron source consists of three important components, the accelerator, the target
and the moderators . For reasons discussed in sections 1 .3 and 1 .4 the planned European
Spallation Source (ESS) will be pulsed .
1 .5.2.1 The accelerator
The concept of the ESS envisages a pulsed linear accelerator (linac), which will supply the
full beam power, and two subsequent storage rings for compressing the pulses from the linac .
The ESS design parameters are :
linac proton energy
1 .33 GeV,
average curr ent 3 .75 mA
average beam power 5 MW
linac peak current 0 .1 A
ring peak current 100 A
repetition rate 50 S-1
linac pulse duration 1 ms
pulse duration after compression 1 ps
300 m long (superconducting cavities)
ring diameter : 52 m
It is worthwhile to point out that we need a rather complex machine to accelerate particles
from rest to kinetic energies of 1 GeV or above and extract them in pulses of only 1 ps duration
. For the case of the ESS we need five stages of acceleration and compression such as
- electrostatic acceleration to 50 keV
- radio frequency quadrupole (RFQ) acceleration from 50 keV to 5 MeV
- drift tube linac (Alvarez-type) from 5 MeV to 70 MeV
- superconducting multiple cavity linac from 70 MeV to 1330 MeV
- two (!) compressor rings (space charge !) .
1 .5.2.2 The target- solid or liquid?
According to relation (1 .2), heavy elements (large mass number A) are favored as target candidate
materials, in particular the refractory metals tantalum, tungsten or rhenium, but also
lead, bismuth or even uranium . Whatever material is selected, it will be subject to heavy multiple
loads . Firstly, about 60% of the 5 MW average beam power is dissipated within the tar-