Environmental Engineering Reference
In-Depth Information
Figure 8.4 Sketch of intermediate band quantum dot solar cell device. Adapted from Marti
et al. [103], Figure 7.12, p. 161.
device to measure the energy of a single high-energy gamma ray photon. A pure
single crystal of germanium is treated in a special way with lithium, so that the
mean free paths of electrons and holes are larger than the dimensions of the
device. Metallic electrodes apply a small bias voltage across the germanium, and a
sensitive detector of the charge flow in the circuit is provided. The germanium is
pure and is also cooled to 77 K, the temperature of liquid nitrogen, so that the
numbers of thermally generated free electrons and free holes is low, and only a
small background current
flows. A gamma ray, that is, a photon of high energy
(MeV)
E
c
¼
E
g
, is absorbed, creating initially an electron in the conduction
band and a hole in the valence band, whose kinetic energies add up to
hf
hf
E
g
.The
final result from the single absorbed gamma ray photon, an outcome of internal
processes in the germanium, is
n
electrons and
n
holes
flowing in opposite
direction across the crystal and into an external charge meter, such that the total
measured charge
ð
I
Q
¼
n
2
e
¼
2
e
ð
hf
=
E
g
Þ¼
ð
t
Þ
d
t
:
ð
8
:
1
Þ
From this relation, the gamma ray energy
hf
is deduced from the measured
charge
Q
and the known value of
E
g
, according to
E
c
¼ð
Q
=
2
e
Þ
E
g
:
ð
8
:
2
Þ
