Standing L-R: Yuji Sudo, Hideki Yamazaki. Sitting L- R: Satoru Uozumi, Takashi Maeda.

As part of the Global Large Detector – Calorimetry (GLD-CAL) Collaboration, a group of researchers from the University of Tsukuba spent their summer developing a new photon sensor for the GLD calorimeter. Because the photon sensor that the GLD currently uses is costly and very large in size, GLD-CAL had the task of finding a more optimal photomultiplier for the calorimeter. After a summer of research, the group from the University of Tsukuba believes that a Multi Pixel Photon Counter (MPPC) is the answer.

Three of the four detector concepts for the International Linear Collider use a particle flow algorithm in order to measure the energy of the quarks that emerge from the collisions inside the machine. Using this method, the momenta of the charged particles are measured in a tracking detector inside the calorimeter. In order to make precise measurements however, it is necessary to distinguish the neutral particles from the charged particles. This requirement leads to a very finely segmented calorimeter, meaning that a vast number of photo channels need to be handled for any given particle collision.

Multi Pixel Photon Counters and Silicon Photomultipliers are new compact semi-conductor photon sensors that are currently under development and designed to handle the large number of requirements of an ILC detector. The MPPC consists of an array of APD (Avalanche Photo-Diode) pixels on an area of 1 x 1mm². If a photon injects into a pixel, it will sputter an electron and then induce an avalanche in the strong electric field. Thus the pixel fires the signal. Because each pixel can have only two states - fired or not fired - the MPPC has to have a multi-pixel structure. By counting the number of "fired" pixels, one can know the number of photons injected into the MPPC.

After working on simulations of the new design for the calorimeter over the past two years, the High Energy Physics Lab at the University of Tsukuba focused on MPPC studies this year. The MPPC not only appears to have a very superior performance as a photon sensor, but its very small size and markedly low cost also makes it competitive with the currently used photomultipliers.

"The MPPC is still a developing sensor, but it has a enormous possibilities," said Satoru Uozumi, a postdoctoral fellow, "I think it is worth studying." Takashi Maeda, a second year master's degree student, just began studying photon sensors in April. "I want to play a part in the development of the MPPC," he said. "I think it is very exciting to be present at the scene of the development of a new device."

GLD-CAL plans to make a prototype of the calorimeter, using a MPPC and perform experiments early next year. "I want to show to the world that the GLD calorimeter, which we are now studying with the new photon sensor, has high performance," Uozumi said. "We expect that the MPPC will become the mainstream for photon sensors."

Read more about work on HCal photomultipliers in Europe: /newsline/readmore_20060330_feature1.html

-- Nobuko Kobayashi