Nuclear Physics: Prefragmentation


My name is Heather Garland and I am a rising sophomore working with Professor Stephenson on a nuclear physics project where we are working to better understand how nuclei are produced in collisions at relativistic speeds.  We call this “prefragmentation” because the nuclei that are produced quickly break up into charged “fragments” and free neutrons. We are revisiting data analysed first by Michelle Mosby, a PhD student at Michigan State University. We seek to answer more questions about prefragmentation through her original dataset.

While going through the process of sifting through data, I am trying to do my best to learn all about nuclear shell theory and to have a working knowledge of it. I am fairly certain I have pulled every book from the library that even mentions nuclear shell theory! And have since added to my collection through inter library loan! Each book explains nuclear shell theory differently and I’m finally getting a bit of a handle on the concepts.

This picture is from when I first checked out all of these books from the library. They’re not quite this organized any more… and more have been added!


I’ve also been learning about Cathode Readout Drift Chambers or CRDC’s. CRDC’s are large boxes filled with gas. In our experiment this gas happens to be tetrafluromethane or CF4. This gas is used because it has a high stopping power for protons. Our beam of nuclei is directed through this gas box. The box has 128 pads on the top. These pads will detect a negative charge. When a beam comes through, nuclei will collide splitting off into charged particles. These particles cause an electron avalanche. An electron avalanche is when the electron charge causes other particles around it to become charged. This avalanche is drawn to the pads by using an anode wire. The avalanche begins to be recorded when the first charged particle hits the pads, and the pads stop recording when there is no more charge. The time between when the first charged particle hits the pad and when there is no more charged particles hitting the pad gives us an amplitude to use as a y-position for the collision of the nuclei. The x-position is simply measured by where it hit the pads.To see what angle the beam is coming in at we use what is called a mask run. This just means there is a large plate of metal placed in front of the electron detectors with a pattern of holes in it. When the beam is shot through we should see the pattern of holes be presented when we graph it. Here you can see CRDC2’s mask run. If this were CRDC1’s mask run, the image would be flipped horizontally. This is a very good picture of what we are looking for with the clear L, five centre dots, and long line on the edge. However, you can see how the line on the right is slightly bent and a better version would not be curved at all.

mask run

After looking at the mask runs, I studied how the CRDC’s electronics may have drifted over time during the experiment.  (The experiment lasted many days.)  In particular, I have been looking at the output from electronic devices called “TAC’s” — Time to Amplitude Converters.    I can see how the TAC’s operated during the experiment by graphing the number of counts read by each TAC as a function of time which tells us vertical position.  Analyzing these TAC graphs will then give me a mean value, a total centroid value, and centroids of each peak. Ideally, there would be only one peak per graph, but many times this is not the case. These numbers are then compiled and graphed. If the world were a perfect place, this graph would be a straight line. Unfortunately, the graph never is a straight line and so I will be doing corrections to our data shortly to create this straight line. The lack of a straight line means that the CRDC’s had moved during the run and the beam had shifted over different detector pads. This is noted in Michelle Mosby’s log book. This is a picture of a TAC graph:


This picture shows the centroids on our system called ROOT. I am only interested in the centroids at the moment.

TAC Root

Here is a picture of just some of the TAC runs recorded in my log book. There are only three recorded here.

TAC recorded

And this picture shows how much of my logbook the 103 TAC runs took up! I just took this one for fun:

103 TAC's

This is a scatter graph of the means collected from CRDC1:

TAC excel graph

I have not yet done the TAC corrections, but when I am finished, instead of the four sections of line you see here there should be one line straight across if I graphed the CRDC1 means again.

We’ve also been working with the construction happening in Masters 208. They have removed the tiered seating, the back wall, and moved the door to a different place in the hallway. It is all very excited to watch,  but the process is also very loud and on occasion smells highly of paint! It has been exciting to say the least. Something new every day! Here is a picture of the door when they were blocking it up:

new wall

Soon, Professor Stephenson and I will be travelling to Michigan State University to visit the National Superconducting Cyclotron Laboratory to test detectors for an experiment in early September. So stay tuned!


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s