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BE-BI-PM Section


Segmented Photomultiplier for Time resolved spectrometry

In the CTF3 linac the beam energy spread must be tuned precisely. The use of fully loaded accelerating structures generates transient effects which result in a strong beam loading. In this context and if no correction is applied, the very first electrons experience a much higher accelerating field than the steady state part of the beam. Moreover, any fluctuations of klystron phase and amplitude must be controlled and adjusted so that the variations on beam energy and energy spread remain small during the whole pulse duration (1.5microseconds).

Spectrometer lines were developed with the aim of measuring beam energy spread with 50MHz bandwidth. Three lines are now routinely used along the linac. Using a dipole magnet, the electrons are deviated by 23į to a dedicated beam line. The beam position and its size are measured using an OTR screen imaged by a CCD camera. Time resolved measurements are obtained using a MAPMT. At the end of the line the electrons are then absorbed by Iron blocks. Because of radiation issues the devices (camera, MAPMT) must be installed on the ground surrounded by an appropriate lead shielding. An optical line composed of a set of lenses is then used to optimize the collection of the OTR photons. An optical beam splitter divides the light intensity in two parts, guiding 70% of the photons onto the MAPMT and the remaining 30% onto the camera. An optical density filter wheel equipped with neutral density filters provides the required attenuation to avoid saturation of the CCD camera.

For electron energies higher than 80MeV, the amount of SR produced in the bending magnet is comparable to the intensity of the OTR light. In order to efficiently stop the SR photons a thin carbon foil was implemented a few cm in front of the OTR screen.

The horizontal beam size, measured at the screen position returns directly the beam energy spread. Time resolved energy spectra are obtained using a 32 channel multi-anode photomultiplier from Hamamatsu (model H7260). The signals are then digitized on 100MSa/s ADCís. The MAPMT has a 0.6ns rise time and the crosstalk between adjacent channels is lower than 3%. Due to the manufacturing process, the anode uniformity may fluctuate by 20% so that all channels need to be calibrated first using a pulsed laser. The signals are then corrected after digitization. The output voltage of the MAPMT cannot exceed some mV per channel. Thus to use the full dynamic range of the ADC (12bits, +/-2V), a 32 channel amplifier has been developed to increase the signal amplitude by 40dB with a 200MHz bandwidth. A MATLAB program reads the ADCís and plots the time resolved energy spectrum. The beam energy is calculated and the beam energy spread can be measured in a user-defined time window by the two black lines. A profile is then extracted and the full width half maximum value of the curve gives the energy spread.

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Page last modified on July 25, 2006, at 05:40 PM EST