B. Patel, R. Rusack, P. Vikas
(University of Minnesota, Minneapolis)

Y. Musienko, S. Nicol, S. Reucroft, J. D. Swain
(Northeastern Univeristy, Boston)

K. Deiters, D. Renker, T. Sakhelashvili
(Paul Scherrer Institute, Villigen)
!!
Avalanche photodiodes will be used as photodetectors for the first time 
in a larger scale HEP experiment in the CMS electromagnetic
calorimeter The CMS ECAL will be made of lead tungstate (PbWO$^{4}$) 
crystals. This high resolution calorimeter will optimize the chances to 
discover the Higgs boson by the search for the 
$H \rightarrow \gamma \gamma$ decay besides studies of a large variety 
of Standard Model and other possible new physics processes. PbWO$^{4}$ 
is a fast compact scintillating crystal limited by a reduced light yield, 
of the order of 100 photons per MeV for crystals of suitable dimensions, 
which peaks around 420-450 nm. The choice of the photodetectors for the 
CMS ECAL was motivated to match the properties of the crystals and that 
they have to operate in a hostile radiation environment and a strong 4 T 
magnetic field. Avalanche photodiodes (APD's) were the chosen devices 
since they are fast, insensitive to magnetic field, have a high gain and 
a reduced sensitivity to ionizing particles.

Possibilities to manufacture APD's suitable for this project were
explored with two vendors, EG&G in Canada and Hamamatsu in Japan. The
goals of this R&D programme were to reduce the APD capacitance, the
dark current and the excess noise factor to minimize the 
contributions to the noise, to improve the quantum efficiency for 
wavelengths of 400-500 nm and to improve the stability of the gain.
In addition, the devices had to be able to withstand the radiation
doses expected in the LHC. A choice between the two for further 
development work and the final production was made in July 1998 after 
extensive measurements and evaluating their performance in the test 
beam. This paper reports on the immense progress that has been made 
since in the APD's expressly developed for this project in 
collaboration with the chosen vendor, Hamamatsu. Demonstrations of the 
radiation hardness of these devices will also be described. In addition, 
the plans for QA/QC, including radiation hardness tests of the APD's, 
prior to their incorporation in the CMS, ECAL during the production 
phase will also be addressed.
!!
Avalanche photodiodes will be used as photodetectors in the CMS
barrel electromagnetic crystal calorimeter, for high precision energy
measurements in a hostile radiation environment. These large 
surface APD's (5X5 mm^2) are particularly suitable for scintillation
detection due to their high gain, insensitivity to the magnetic field,
a good match of spectral sensitivity to the emission wavelength of
the crystals, low coherent noise and radiation hardness.
Recent progress in the performance of 
these devices will be presented, together with demonstrations of radiation 
hardness and plans for QA/QC during the production phase.
!!