Proceedings - Viện Vật lý

_________________________________________________________________________________Những tiến bộ trong Quang học, Quang tử, Quang phổ và Ứng dụng. 8/2006, Cần Thơ, Việt Namwhen the oscilloscope starts to acquire data and display a waveform. When a triggeris set up properly, the oscilloscope converts unstable displays or blank screens intomeaningful waveforms.The monochromator GDM-1000 consisting of two 650lines/mm gratings canoperate in spectrum region of 7500 – 16675 cm -1 (for the first order of the grating)and in the region of 16675 – 28700 cm -1 (for the second order of grating). Wemodified this monochromator with a step motor and coupled it with a computer tocontrol and monitor wave-number. PMT Hamamatsu R928 (Extended red, highsensitivity, diameter of 28mm, 9 stages and side-on) has been used as an opticaldetector. The spectral response range of R928 PMT is 185 – 900 nm. Anodeluminous sensitivity and gain characteristic of PMT R928 are shown in Fig.3a. Thehigh voltage supply for R928 PMT is provided by a DP-type socket (C6270)incorporating a regulated high voltage power supply and an active voltage divider.It enables simple yet stable PMT operations with extended DC output linearity byonly supplying +15Vdc and connecting to a potentiometer or a 0 to +5V for highvoltage adjustments. In this experiment, high voltage supply has been adjusted at -1000V by tuning control voltage of 4V (Fig.3b). At high voltage supply of -1000V,PMT's corresponding gain is 10 7 and corresponding anode luminous sensitivity isabout 10 3 A/lm .(a)(b)Fig. 3: a-Anode Luminous Sensitivity and Gain Characteristic of PMT R928b-High Voltage Controlling Characteristics of socket C6270We used a TDS2014 oscilloscope (Tektronix -USA) for electrical signalacquisition and time-based spectral regeneration. The TDS2014 is a digital storageoscilloscope with 4 channels, 100 MHz, 1.0 Giga-sample per second and LCD colordisplay. This digital storage oscilloscope has three acquisition modes: Sample, PeakDetect, and Average and can capture a single-shot signal. When measuring thefluorescence spectrum excited by Nitrogen laser in pulses with duration of 6ns andrepetition rate of 10Hz, the oscilloscope TDS2014 has been set in average mode ofthree samples in order to reduce experimental error. The other parameters also havebeen adjusted such as vertical scale of 90mV/div, horizontal scale of 50ns/div, andtrigger settings at 2.4mV of rising edge456