One of Berkeley's primary contributions to the KamLAND experiment was the front-end electronics (FEE). Our group designed, built, and installed this key component of the detector. The front-end electronics record the output of the photomultiplier tubes (PMT's.) The PMT's produce short (20 nanosecond) pulses of a few millivolts when they detect single photons. The front-end electronics record and measure these pulses.The KamLAND electronics are based on a chip called the ATWD, for Analog Transient Waveform Digitizer. The ATWD is a custom ASIC (application-specific integrated circuit) developed at LBL. The ATWD allows us to sample the voltage of a pulse as a function of time, rather like an oscilloscope on a chip. The ATWD output is a "waveform" such as the example shown below. The PMT pulses show up as peaks in the plot of voltage versus time. As the picture shows, the waveforms allow us to distinguish multiple PMT pulses that arrive in series.
A sample KamLAND waveform showing two PMT pulses. The horizontal axis is samples taken 1.5 nanoseconds apart. The vertical axis is ADC (Analog-to-Digital Converter) counts; each count is approximately 120 microvolts at the KAMFEE input. The blue points are the raw data. The red points are the pedestal, the ADC values that would be read out with no input signal. The green points are the pedestal-subtracted waveform, the measurement of voltage vs. time. The peaks in the waveform are the PMT pulses.
The KamLAND front-end electronics performance is optimized for our
experiment. The electronics allow timing resolution at the nanosecond
scale, which is critical for reconstructing the position of events in
the detector. The noise performance is very good, so we can set
our single-channel discriminator thresholds well below a millivolt. As a
result, the efficiency to detect single photoelectrons is 95% or better.
The electronics have a large dynamic range, covering PMT pulse amplitudes
from single photoelectrons through thousands. This allows us to measure
high-energy muons as well as low-energy neutrino candidates. Finally,
the electronics have two ATWD's per channel. This minimizes the "dead"
time of each channel, since the second ATWD is available to take data
if the first one is busy.
A 12-channel KAMFEE (KamLAND Front-End Electronics) board.