Q.E. 0.225 0.2 0.175 0.15 0.125 0.1 0.075 0.05 0.025 0 250 300 350 400 450 500 550 600 650 wavelength (nm) Figure 19: Quantum efficiency of the Hamamatsu tube R5912-02 as a function of wavelength. The maximum is located at 400 nm. A.E. 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 cosΘ Figure 20: Angular sensitivity of a standard Hamamatsu R5912-02 pho<strong>to</strong>multiplier as a function of incidence angle [40]. 32
of 100 V over PMs. The TDC module used has 32 channels that can record at most 16 leading- (or trailing-) edge times each, with 1/2 ns resolution. It is operated in common-s<strong>to</strong>p mode, registering hits occurring within a time-window. The Multiplicity ADDer (MADD) trigger system which takes one of the ’A’ pulses is designed for the AMANDA experiment. It is housed in a 6U-Euro crate and is composed of several MULT20 modules and one ADDER device. The MULT20 makes an ECL/TTL conversion of the signals coming from the 4413 discrimina<strong>to</strong>r, stretching the pulse-length <strong>to</strong> the pre-set trigger window of ¥ . These pulses are then added and converted in<strong>to</strong> a binary output which is sent <strong>to</strong> the ADDER for final adding and comparison with a given threshold. In AMANDA-B4, one MULT20 module is used for each one of the four strings, making the implementation of a majority trigger easy. The adding is made asynchronously in the MULT20 with a tree algorithm. A similar technique is used by the ADDER and the <strong>to</strong>tal conversion time (MULT20 + ADDER) is ¥ 100 ns, with a time-jitter below 10 ns [43]. The trigger produced by the MADD system is then sent <strong>to</strong> the NIM trigger logic, which also handles trigger inputs from AMANDA-A, SPASE-1, SPASE-2 and GASP, making AMANDA-B a slave <strong>to</strong> all these experiments. The <strong>to</strong>tal trigger rate during 1996 was ¥ 26 Hz on average, with fluctuations of a few Hz depending on whether the coincidence experiments were on or off, changes in the electronics setup, etc. The coincidences from other experiments contributed some 4 Hz <strong>to</strong>gether <strong>to</strong> the <strong>to</strong>tal trigger rate. Upon triggering, an ADC gate is formed, a s<strong>to</strong>p signal is sent <strong>to</strong> the TDCs and a readout signal is sent <strong>to</strong> a Hytec LP1341 list processor with different delays. Then a ve<strong>to</strong> lasting several microseconds is issued <strong>to</strong> the trigger, inhibiting further issuing of trigger signals. A signal is also sent out <strong>to</strong> latch a GPS time which will serve <strong>to</strong> identify coincident events with other detec<strong>to</strong>rs. The noise contents of atmospheric muon events can be seen in Fig. 22, showing that the average number of PMs with a noise hit is 0.9 per muon, using a window of 32 ¥ . The simulation of muons was made with the atmospheric shower program Bosiev [44], then they were propagated through the ice with MUDEDX, based on routines from [45] and passed <strong>to</strong> AMASIM [46], which simulates the detec<strong>to</strong>r response. After cleaning up the data, the hits which were not taken ¥ away are usually with a window of 2 ¥ and the noise being flat (see Fig. 23) in the ¥ whole 32 interval is reduced accordingly. The proportion of afterpulses is calculated <strong>to</strong> be 6% [47], which is lower than the figure measured in the labora<strong>to</strong>ry (10%), but the comparison is not easy, since the conditions are very different (at the South Pole, 2000 m of cable were used, the voltages ¡ were set differently and the ¥ ¡ ¡ ¢ temperature is from the deepest <strong>to</strong> the shallowest module). The data acquisition software is written in the KMAX language and is running on a Mac- In<strong>to</strong>sh PowerPC 7200 communicating through a SCSI bus with the CAMAC crate-controller, a Jorway-73A. Since the access time this way is very long, a list processor LP1341 is used, <strong>to</strong> which the KMAX writes a DAQ program when starting the data-taking, and which s<strong>to</strong>res the data in a 32 kB buffer after reading out the modules. This buffer is then read out in single block transfers every 600 ms by the MacIn<strong>to</strong>sh. By fitting the distribution of the time £ between consecutive events with an exponential, and comparing the integral of this fitted function with the integral of the distribution (see Fig. 24), the dead time of the DAQ can be estimated <strong>to</strong> be ¥ 13%. Furthermore, the data is written directly on an NFS mounted disk, inducing further dead time [23]. 33
Change language