Physics and Engineering of Radiation Detection

3.4. Ionization Chambers 179

charge pairs are always produced near the cathode (which in this case is actually
the grid). Hence we can simply modify equation 3.4.4 to get

Vout

0:0≤t≤tn 1

N 0 e

Cdvnt : tn^1 ≤t≤tn^2

N 0 e

C : t≥tn^2.

(3.4.17)

where timetn 1 andtn 2 refer to the times the electrons take to reach the grid and
the anode respectively. The shape of the pulse is shown in Fig.3.4.9.

N 0 e/C

Vout

tn 1 tn 2

tn 2 = d/vn

tn 1 = x/vn

t

Figure 3.4.9: Output pulse

shape of a Frisch grid ionization

chamber. The signal is exclu-

sively due to motion of electrons

between the grid and the anode.

D.2 Boron-linedIonChamber

Standard ionization chambers are almost insensitive to neutron flux due to low
neutron interaction cross sections in the usual filling gases. The neutrons, having
no electrical charge, can hardly ionize atoms and almost exclusively interact with
the nuclei. Therefore to detect neutrons one must use a material with which the
neutrons could interact and produce another particle, which could then ionize the
gas. Boron-10 is one such material, which absorbs a thermal neutron and emits an
α-particle according to
n+B 510 →Li^73 +α. (3.4.18)

Theα-particle thus emitted can ionize the gas and thus a measurable signal can
be produced. The boron-10 can be used either in gaseous form asBF 3 gas or in
solid form. TheBF 3 filled chambers are commonly used for neutron detection and
have the same characteristics as the standard chambers. However the difficulty in
purification of theBF 3 gas and its degradation with time is a major problem with
these chambers. BF 3 counters are generally operated in the proportional region to
obtain better signal to noise ratio. The other possibility is to use boron in solid form,
which is relatively easier to purify and maintain. Such detectors, often calledboron-
lined chambers, are actually preferred overBF 3 chambers for this particular reason.
In such a detector, boron is coated on the inside of the cylindrical chamber, which is