Time of Flight Calibration Procedure

 

The Time of Flight (TOF) feature on the Daq measures the time between coincident muon hits on two paddles. For example, this is useful for measuring the speed of the muons or determining whether coincidences are valid. For the TOF measurement to be effective there needs to be a significant separation (> 1 meter) between the paddles. See the lab on “Paddle Separation” for more information.

 

The timing mechanism on the interface board is not perfect and therefore, the paddles will need to be calibrated before you can analyze the TOF data. The calibration run will have the paddles directly on top of each other. You would expect that the TOF between these two paddles to be zero because the muons are traveling at the speed of light. As you will see, this is not the case (you may get readings up to 5 or 6 nanoseconds, which corresponds to a distance of about two meters), so we must calibrate the measurements so it is zero. Also, you may see very large values (30-50 ns), which you should ignore and delete from your data set. These values can be due to a variety of things, but are not true coincidences.

 

To set up the Daq to do TOF measurements:

1.      Choose (7) Configure this program.

2.      Choose (m) muon speed mode.

3.      It will ask you if you want to measure the time of flight between two paddles. The answer is yes, so write “y” and hit enter.

4.      It will then ask you to choose the first paddle. Enter the appropriate channel number.

5.      Then enter the channel number for the second paddle. It does not matter which order you enter the channel numbers, but be consistent and always measure the TOF in the same direction.

6.      To start the run, go back to the main menu (x) and start a coincidence run by hitting (4), just like any other coincidence run.

7.      When your run starts, you will notice that some of the TOF measurements are negative. Do not be worried by this; it means that the muon went the opposite direction from what you expected. If you told the computer to record TOF channel 1 to channel 2, the negative TOF is a muon that went through channel 2, then channel 1, so was coming from the opposite direction. You will see very few of these in an experiment measuring vertical muons and many of them (about half of all hits) in an experiment measuring horizontal muons.

 

To Calibrate the TOF measurements:

1.      Place paddles directly on top of each other, positioned to look at vertical muons. Even if your experiment will be measuring horizontal muons, set up the paddles to measure vertical muons because the count rate is higher.

2.      Run a coincidence run measuring TOF (as explained above) for one hour. This will give you plenty of data to work with.

3.      Import your data into Excel and delete all non-coincidence (noise) points. It is useful to convert your TOF data into “ticks” from nanoseconds. The timer on the board is quantized and can only record 0.75 ns time intervals, which is what we define as a tick. Convert to ticks by dividing your nanosecond data by 0.75.

4.      Make a histogram (see Histogram reference sheet) of the hits using the TOF ticks for the bins.

5.      If the distribution looks Gaussian, you can take the mean of your data and use this for your calibration.

6.      Most likely, your average TOF will be positive. This is your background that will be subtracted from TOF data in future experiments. You need to be sure that your future experiments are set up in the same way as the background run, i.e. channel 1 to channel 2.  If they are not, see Step 7.

7.      If you did your background run one way (1 to 2, for example) and the actual experiment the opposite way (2 to 1), you can still use your data, but with minor adjustment. Muons traveling the opposite direction (2 to 1) from what you assigned have a negative time, so you must make your background average negative to correspond to the direction of your experiment (2 to 1). You are then subtracting a negative background, which is the same as adding the original background reading.