Chapter 5
Aging Studies
It is well known that lead glass is damaged by ionizing radiation that creates color centers
[3] and causes a strong absorption of light in the near ultraviolet, visible and near
infrared spectra. Usually UV light and temperature cycling was used to anneal this radiation
damage in lead glass [5]. A detailed investigation of the radiation damage effects
in CEREN 25 glass was done only by the OPAL collaboration [2]. Two full-size blocks of
CEREN 25 with a length of 520mm were irradiated with a total absorbed dose of 10Gy.
The transmission of each block was studied with a blue light-emitting-diode, LED, and
compared with that of a third block that was not irradiated. To assess the radiation damage,
the relative transmission is defined as the ratio of the optical transmission after
and before the irradiation. Immediately after the irradiation, the relative transmissions
of two blocks was measured to be 29% In addition, the measurements have been performed for
the 25 mm glass sample, approximately one radiation length of CEREN 25, as function of
absorbed dose. Fig. 28 [2]. Fast reduction in transmission with increasing dose measured with
light of wavelength 440 nm was observed.
Therefore, it is important to estimate the full absorbed dose of ECAL modules in the realistic
HADES environment at SIS 100 (Fig. 2.1). The simulation of the absorbed dose and neutron
fluency has been done for Ni+Ni reaction at 4 and 10 AGeV beam energy, 1% target thickness
and 2 month of continues work per year with reaction rate 105 per second. The FLUKA package
has been used for the simulation. Results have been shown on Fig.XX. The maximum expected
absorbed dose will be at the upstream end of the modules placed near the beam axis, but even
here the absorbed dose is less than 0.2 Gy. From OPAL radiation dose investigations it follows
that the light transmission lost will be on the level of 1- few percent (see Fig.28 in [2]).
Neutron fluency at the rear part of the modules will be less than 101 0 neutrons per cm2 measured
during the run and practically will not have any influence on the PMTs and electronics placed
behind modules. The light transmission of lead glass modules can be recovered after irradiation
using combining treatment by heating and UV illuminating. This methods are described in [2] in
details.
Fig XX: Absorbed dose (left) absorbed dose of and neutron fluency (right) across the lead glass
modules in one of HADES ECAL sector.
Fig. XXX shows absorbed dose distributions along 3 ECAL modules placed at 18, 30 and
45 degrees relative the beam axis. Here the dose is simulated again for all particles (including
gammas, electrons, charged hadrons and neutrons) generated by FLUKA.
(NOTE: These figures are practically another type of presentation shown on Fig. XX and we
should decide what figures to show in the TDR).
Relative contribution to the dose from gammas, charged hadron and neutrons is estimated as
A:B:C (Anna promised to send these numbers).
Fig. XXX. Absorbed dose distributions along 3 ECAL modules placed at 18, 30 and 45
degrees relative the beam axis for beam energy 4 AGeV (left) and 10 AGeV (right).
As it is known (??) the most effect on the light transmission is due to irradiation by neutrons
and decreasing of the light transmission due to 1 Gy dose from neutrons is equivalent to
decreasing of the light transmission caused by ?? Gy dose from gammas [??].
As can be seen from figures 28 and XXX, expected decreasing of light transmission during the
run will be on the level of 1 percent ?? and will be controlled by light monitoring system.