14—23
.
SLOW NEUTRONS
The energy of the gamma ray photon is equal to _the' binding energy of the deuteron (1H2), the energy of the slow neutron being
negligibly small. The capture—cross-sections in this and other
.
similar cases are very large.
'
_
The velocity of the slow neutrons which are strongly absorbed
in cadmium has been measured directly with a velocity selectorl3.
_
As
illustrated
in
gure
14—14, a Rn-Be source of
”
-
neutrons is located at x in a
'
,
ê_lj“
'
large cylinder of parain.
f”*
‘
'
This is surrounded by a câd—
‘4
-
G
mium can which prevents
! . '
.
,
the type of neutrons under
_
_teSt
from traveling OUtward
FIG. 14—14.
Direct mea3urement of the
to
the walls of the room
velocity of slow neutrons.
from which they could be
.
_
scattered back to the detector D, However, the front
of the
.
‘
source is open, so that slow neutrons
can pass through two rotating
discs 4.4 and two xed discs BB into the boromlined ionization
chamber D.
Each of the duraluminum discs has on its, surface 50
sectors of sheet cadmiùm.
Each Sector covers 3.7° and each gap
between sectors covers 3.5°. Each pair of discs (JB) serves' as a
,
,
shutter for neutrons absorbable in'cadmiùm-i lîIf 71 is
-
of revolutions per second of the—discs
5077 Spuî‘tS Of neutrons
leave the rst pair of discsyeachsécond.a
:
=If they. travel
to the _n€Xt
_
pair, et distance d, With a velocity a such that an QP€nlng
13 P΀S€nt
between the cadmium
into the detec—
.
tor
On the other hand, they
if
‘
”
-
'
'
v=lOO’
f
(14—43)
…
For- a thermal
22 X105Çm/sec,andad1stance = 54
-.
f
,
'
neutrons
“".83
'
* ‘i‘ïpÿsä and 'wasÏïfoundtofolleWtheMaxwelhandlsmbuwn
‘
!
«