A decisive influence on his scientific activity was exercised by Prof.L. Mandelstam, under whose guidance he worked a number of years and with whom he was closely associated since 1920, when they met for the first time, and up to the death of Prof. Mandelstam in 1944. Tamm is an outstanding theoretical physicist, and his early researches were devoted to crystallo-optics and the quantum theory of diffused light in solid bodies. He turned his attention to the theory of relativity and quantum mechanics and he evolved a method for interpreting the interaction of nuclear particles. Together with I.M. Frank, he developed the theoretical interpretation of the radiation of electrons moving through matter faster than the speed of light (the Cerenkov effect), and the theory of showers in cosmic rays. He has also contributed towards methods for the control of thermonuclear reactions. Resulting from his original researches, Tamm has written two important books, Relativistic Interaction of Elementary Particles (1935) and On the Magnetic Moment of the Neutron (1938).
I. Tamm was elected Corresponding Member of the U.S.S.R. Academy of Sciences in 1933, and in 1953 he became an Academician. He shared the 1946 State Prize with Vavilov, Cerenkov, and Frank, and is a Hero of Socialist Labour. He is also a member of the Polish Academy of Sciences, the American Academy of Arts and Sciences and the Swedish Physical Society.
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Cosmic Ray Tests of a Prototype TOF Counter
To optimize the performance of TOF counters of the geometry we plan to use in the CDT location we have begun testing prototype counters at Fermilab. The first three prototype counters are 40 40 mm in cross section and approximately 300 cm long. The scintillator for two counters was Bicron BC408, for the other Bicron BC404, all were cut and polished at the factory. The counters were wrapped in foil and black paper. We first tested both the BC404 and the BC408 with R2490-05 Hamamatsu PMT glued on each end. This tube is a 2" diameter version of a fine mesh dynode tube similar to that which we plan to use in the final system. Its performance both in and out of a magnetic field has been well studied and documented[9]. The photocathode diameter of the R2490 is 38 mm which means that it accepted 70% of the light produced in the bar. We also tested the BC408 bar with two Hamamatsu R5946 PMT's. This tube is 1.5 inches in diameter with a 27 mm diameter photocathode and is small enough to be used in the CDT location. Finally, we tested one BC408 bar with CPC light concentrators (more on this later) and two Hamamatsu R5946 PMT's. The test setup is shown schematically in Figure 19. Cosmic ray trigger counters above and below the TOF counter were mounted on a movable track so they could be positioned along the length of the TOF counter. For most of the results discussed here the top trigger counter used a 40 20 50 mm piece of BC408 scintillator and was equipped with an RCA8575. The scintillator was oriented with the 20 mm dimension along the length of the 3 m counter and the PMT was horizontal. The lower trigger counter used a 40 40 50 mm piece of BC408 scintillator read out with a Hamamatsu R2490-05 PMT. The bottom counter was oriented such that the 50 mm dimension and the PMT were vertical. The anode signals from all four PMTs were split with a resistive divider. One split signal was connected to an Ortec model 934 constant fraction discriminator, the other to a LeCroy 2248 CAMAC ADC. The signals from the constant fraction discriminator were passed through 64 ns of delay and then were connected to the stop inputs of a LeCroy 2228A CAMAC TDC (50 ps least count). The coincidence of the trigger counters provided the common start to the 2228A. The relative timing of the trigger counters was adjusted so that the counter with the R2490-05 always determined the timing of the coincidence. Figure 20 shows a typical pulse height distribution from one of the PMT's from a cosmic ray run with 4000 triggers. A run with this many triggers takes about 20 hours. The top two plots corresponds to the two trigger counters, ADC C2 the bottom and ADC 1 the top. Clear minimum ionizing peaks are observable. The bottom two plots show pulse height distributions for PMT's on the 3 m counter, the left tube (ADCL) and the right tube (TDCR). Again, minimum ionizing peaks are apparent. To measure the resolution we use only those events in which the ADC values are within the minimum ionizing peaks. For this run, we required ``MIP'' cuts of 400 < ADC C1 < 800, 400 < ADC C2 < 800, 100 < ADCR < 400 and 90< ADCL < 390 counts.
When the time measurement is limited by the photoelectron statistics, we expect that the time resolution () is related to the pulse height (ADC) in the PMT by . By weighting each time measurement by the pulse height in that PMT we should be able to minimize the width of the resulting combined time distribution. The bottom right plot of Figure 21 shows the distribution of the average of the times measured by the PMT's on each end of the 3 m counter weighted by their pulse heights. The RMS width is 2.4 counts, which is nearly identical to that of the simple average. This is not unexpected for a run with the trigger counters at the middle of the 3 m counter because two tubes have almost the same pulse heights at this location. We note however that this technique does improve the resolution over the simple average when the cosmics pass through locations away from the middle of the counter. Superimposed in the plot is the result of a Gaussian fit, which gives an RMS width of 2.4 counts (120 ps). Figure 22, and Figure 23 show these time resolutions measured at several locations along 3 m counters fabricated from BC408 and BC404 respectively and instrumented with R2490 PMT's. Figure 24 is a similar plot for the same BC408 bar but instrumented with the smaller R5946 tube. For these figures we used data taken with a small top trigger counter 20 40 50 mm to minimize trigger counter width effects. We observe that the difference between the trigger counter time and the pulse height weighted average time of the two PMTs exhibits a time resolution of less than 110 ps nearly independent of the position of the trigger counters for both the BC408 and BC404 bars readout with R2490 PMTS. We find no significant difference in performance between BC404 and BC408. The resolution of the BC408 bar instrumented with the R5946 is about 135 ps. In this case we expect the R5946 results to be worse than the results with the R2490 since this tube has a smaller photocathode and collects only about half as much light as the R2490. The fact that the resolution with the R5946 is not worse than the R2490 indicates that other significant factors in our test setup other than photostatistics contribute to measured resolution.
