Diamond Detectors for Ionizing Radiation - HEPHY

CHAPTER 7. RADIATION HARDNESS 38
7.2 Pumping Eect
A diamond detector that has never been irradiated before is in a virgin state, called
\unpumped". With moderate irradiation uence, the signal output, or charge collection
distance, increases signicantly. The cause for this unique behavior are defects of the
material. There are non-diamond atoms in the bulk, generating energy levels inside
the band gap, which act as charge traps. With irradiation, these traps are lled and
made inactive, thus they do no longer absorb electrons or holes. When all such traps
are passivated, the diamond is called \pumped" and this state is conserved until the
diamond is exposed to UV light. By UV absorption, the trapped charges are released
again, resetting the diamond to its original, or unpumped state. Present understanding
is that this procedure is fully reversible and there is no limitation in the number of
pumping/unpumping cycles.
The pumping transition occurs with all types of particles and needs a radiation uence
of approximately 10 10 particles cm ,2 . With this uence, the collection distance increases
by 30 to 100%, depending on the sample. Fig. 7.1 shows the pumping eect by exposure to
a 90 Sr source. Recent measurements show that the uence needed for complete pumping
increases after intense irradiation, indicating an increased number of traps in the diamond
bulk, as expected. A linear relationship between pumping uence and irradiation uence
has been observed.
ccd d c
[µm]
120
100
80
60
40
20
0
0 100 200 300 400 500
time [min]
Figure 7.1: The pumping eect during exposure of a diamond sample to a 90 Sr source.
In future experiments such as the LHC, diamond detectors will reach the pumped
state within several hours, depending on the luminosity and the distance from the vertex.
As this will be the working condition, all charge collection distance values are given in
the pumped state unless noted otherwise.