Neutron Scattering

(p,n)- and (d n)-Reactions
Bombarding targets (Be, . . . , U) with protons or deuterons ofmedium energy (Eki, < 50 MeV),
either neutrons are released foira the target nuclei in the case of protons or the neutrons are
stripped from the deuterons during the impact and thereby released. Yields are of the order of
10-2 n/p resp . 10 14 n/s per milli-Ampere .
An interesting special case is the reaction between the two heavy hydrogen isotopes, because
it can be exploited in two différent ways. One variant is thé so-called neutron generator
utilizing thé large reaction cross section of thé D-T-reaction, which peaks already at vely
low deuteron energies (5 barn at 0 .1 MeV) . With this low particle energy thé emitted neutrons
are virtually mono-energetic (E = 14 MeV) and thé émission is isotropie . The target may be
gaseous or Tritium dissolved in adequate metals (Ti, Zr) . The yield for a D-T-neutron generator
with Ek;(d+) = 0 .1 MeV is of thé order of 10 11 neutrons/s per milli-Ampere .
The second variant of exploiting thé D-T-reaction is thé plasma source . In this source both
gases are completely ionized by applying high pressure and temperature forming a homogeneous
plasma, which releases neutrons via thé fusion reaction. In principle, this is thé same
reaction as with thé neutron generator .
Such sources operate in a pulsed mode, because thé
plasma bas to be ignited by repeated compression . Due to thé need for this compression this
spécial kind of a plasma source is also called thé plasma focus . Up to now yields of about
3 x 10 12 neutrons / s have been obtained experimentally . Planned facilities are expected to deliver
10 16 neutrons / s .
Chapter 1 .5 bas already been dedicated in greater detail to (p,n)- or (d,n)-reactions at high
particle energies (> 100 MeV), which lead to spalling of thé target nuclei ("spallation") . At
this point we only want to give a typical number for thé neutron yield for comparison with thé
other reactions quoted in this Appendix:
- yield (for 1 GeV protons on lead) : 25 neutrons / proton
resp . 1 .5 x 10 1 ' neutrons / s per milli-Ampere
- average neutron energy : 3 MeV (evaporation spectrum)
+ cascade neutrons (up to proton energy) .
1.A .2.2 Reactions with neutral "particles"
(yn)-Reactions (photonuclear reactions)
Gamma radiation of radioactive isotopes can release so called photoneutrons, a process,
which is indeed exploited in devices analogous to (a,n)-sources . A typically spherical y-
source of a few centimeters in diameter is enclosed by a shell of target material . Due to the
extremely high y-activities needed, even weakest neutron sources (106 n/s) can only be hand-
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