Experimental Setup
|
||||||||||||||||||||||||||||||||||||
|
Preliminary study on the experimental setupNotes about Triggering Circuit Box There are 3 detectors and 1 GPS signal which will be fed into the triggering circuit box. After manipulation, 4 signals will be transmitted to computer through SCSI cable. The triggering circuit will handle the following tasks. Circuit box will modify raw signals to logic favour signals, remove some ambiguous signals (signal pulse is too weak; time between triggering is too close, and those signals will be generated by the same event). Owing to the different response of photomultiplier tude on detectors, before raw signals are fed to logic circuit, a part of them will be transcated in order to maintain a comparable event rate (~1000/s) to logic circuit. This can be achieved by applying discrimination circuit (Transcate the signal which strength is below a preset discrimination level). Circuit box will filter out the stand alone detector signal. Suppose there is signal from A, but not B and C, circuit logic will ignore it and no stand alone signal A will be fed into computer. Circuit box will merge GPS signal to 3 detector signals. What by it means ? Inside your circuit box, there is a GPS receiver circuit responsible to analyse signal from GPS antenna. The receiver circuit will try to figure out how long 1 second is and sent one electronic signal to logic circuit per each accurate second. Such 1 Pulse Per Sec signal will always be merged on 3 detectors signals by simply logical OR. As a result, there will be a exact agreement for such 1PPS between 4 output signals to computer through SCSI port. Notes about Data Acquisition Inside your computer, there should be an interface card adapted at the PCI slot. The SCSI output cable from your circuit box will be connected to such interface card. The interface card is acted as 4 indepentent counters. The time between raising edges for incoming signal are recorded for each counter and be written on suitable text files. The timing unit is based on the crystal in the interface card. The crystal oscillates around 80M times per sec. In your data files, a period with 80M unit corresponds to ~1 sec. Before performing any experiment, it is advised to finish the following exercises and make sure you have a clear understanding on your setup. A. Checking on Your Detectors Step 1. Close up any irrelevant application programs, and then run PulseWidth.exe only. There must be a crash with more than one application willing to allocate the resources of PCI-6602. Step 2. Obtain the event rate of channel A, B, C and GPS by taking a long time measurement (1~5 minutes per each channel). Question 1 : What is the event for GPS? Why? Event rate of A, B, C are larger than GPS? Why? Question 2 : Are there any different on event rate between channel A, B, C? What are the reasons? (The signal rate you measured is already manipulated by your triggerring circuit box. Before the appratus are sent to your school, 3 detector signals are set to carry similar weight of informations.) Step 3. Disconnect BNC connector on circuit box labeled channel A. So signal from detector A cannot be fed into our logic circuit. Step 4. Measure event rate on channel A, B, C and GPS again? Question 3 : What is the event rate for channel A? Is it zero? How about channel B and C? Are they affected by disconnecting channel A? Why? Step 5. Estimate the pulse width for 4 signals in SCSI cable. The order of magnitude of pulse width before feeding to logic circuit and that in SCSI cable (After logic operation) are similar. Question 4: It is known that the signal rate just before logic operation is around 1000/s. What is the chance of 2-coincidence by random error? How about 3-coincidence? Click download page to obtain PulseWidth.exe and learn how to use it. B. Checking you raw data Step 1. Locate the directory C:\GPS_CR\data, there should several folders with number as folder name. Actually, each folder corresponds to one day. Naming conversion is YYMMDD. (YY - last 2 digits for year; MM - 2 digits for Month; DD - 2 digits for day) For example folder 011030 corresponds to 30th October, 2001. You will find data files inside the folder. File naming conversion is MMHHCC.dat (MM - 2 digits for minute; HH - 2 digits for hour; CC - 05 = channel C, 06 = channel B, 07 = channel A, 08 = channel GPS). Open 4 files corresponding 4 channels at the same time. This can be achieved by using simple text editor or EXCEL (packaged in your computer). Question 1. Observe the pattern on GPS channel. What can you discover? (Crystal say that it oscillates 80M times per sec. You know GPS receiver generate 1PPS.) Step 2. Observe the pattern on other channel. Here list the header of 4 sample files
Red - 1 st sec, Green - 2 nd sec, Blue - 3 rd sec, Yellow - 4 th sec, Purple - 5 th sec Question 2. Based on the table listed above. In the first 5 second, how many events for channel A, B, C? Can you see the coincidence of A and B in 1st sec, B and C in 2nd sec, A and C in 5 th sec? Is there 3 coincidence? Question 3. Notice channel C, the 2 nd sec is 74153061+5847657=80000718 units long, 3 rd sec is 80000717 units long. It does not agree with other channels. Explain why? (1unit ~= 12.5ns) Question 4. Notice the coincident events. For A and B, A lags B by 1 unit. For A and C, A lags B by 2 units. In your data, you may find even large dispersion of arrival time for coincident events. Does it make sense? (1unit ~= 12.5ns, speed of light ~= 300000km/s, detector separation ~= 5m) Question 5. Given a list of data files, how can you obtain the interarrival time for A and B, 3 coincident event? |