Wednesday, May 7, 2014

Inaugural Lab Book: Gamma detectors and Fiberglass Dewars!

Lab Book 2014_05_07  Hamilton Carter

Table of the available De Lorenci articles we work with on the rotational QFT project.
V. A. De Lorenci 1, 2 and N. F. Svaiter 1
Received 1997
Foundations of Physics, Vol . 29, No. 8, 1999
Received 1998
International Journal of Modern Physics A, Vol. 14, No. 5 (1999) 717–729
V A De Lorenci†, R D M De Paola‡ and N F Svaiter‡
2000 Class. Quantum Grav. 17 4241
V A De Lorenci et al
"Classical and Quantum Gravity Volume 18 Number 1

Dr. DeSilva of UMD passed along the simulation code!!!  It’s written in IDL, so I’ll need to figure out what language that is and how to use it.

Glass bending
I went over to the glass shop and to get the vacuum outlet for the glass Dewar bent into a right angle.  It was awesome to watch!  First the glass blower attached a longer tube to the existing outlet so he'd have a handle.  Then, after heating the tube, he bent it into a complete U before bending it back to 90 degrees  while bending, he blows on the end of the tube to keep a positive pressure inside to keep the tube from kinking or collapsing.   At the end, he detaches the sample tube which leaves the original tube closed.  He heats the end of the original tube and then blows on the opposite end creating a bubble that pops.  The bubble gets broken off using another glass rod over the trash can.  The final step was to put a small flare in the end of the tube using a metal file so that the tube would grip and seal more easily in the red rubber vacuum tubing.  It was an art!

Hose Bracing
I found a brace that will work to relieve stress from the red rubber vacuum hose from affecting the glass joint on the liquid helium Dewar.  Using a drill borrowed from the machine shop, (I found our drill later), I made a small pilot hole for the hose brace. I shielded the Dewar first to make sure it wasn't damaged during drilling.

Once the pilot hole was drilled, the brace was installed and the hose was test fitted.  Everything is working fine and ready to go.

IDL Simulation code
There’s a freeware version of IDL available called GNU Data Language.  IDL stands for interactive data language. 

Scintillator Gamma Ray Detector Testing
I ran the scintillator/PMT detector today and got a few nice traces recorded.  The scintillator is a sodium iodide crystal,(NaI), that produces small flashes of light when gamma rays stop inside it depositing their energy.  These small flashes of light are converted into amplified electricalisgnals by the photomultiplier tube,(PMT).  For the moment, I’m using the oscilloscope to watch the flashes, but ultimately I’ll automate the process using something called an integrating mutli-channel analyzer.  This device automatically keeps track of how many gamma rays of each energy have entered the NaI detector crystal.  The detector is shown below.  The crystal is in the larger portion of the metal tube whose surface is in contact with the lab bench..  The PMT is mounted directly above it in the smaller tube.  The red wire delivers the negative 1700 Volts that biases the PMT and provides the amplification of the light signal.  The black cable connects directly to the oscilloscope so that signals can be viewed.

The negative bias voltage is supplied by an HP high voltage power supply shown here.  Testing was done with a number of bias voltages.  The power supply is shown set to 2000V.

The following two pictures capture the ‘scope settings used.

Video of Cobalt 60 Signals
Each negative pulse is a single gamma ray from the Co 60 source.

Fiberglass Dewar Inspection
We liberated one of the fiberglass Dewars from storage this morning and inspected it.  It looks like it may work with some modification.  There are a few issues though.  First, there’s a fiberglass cup inside that needs to be removed to make way for the superconducting sample holder.  Second, it would be really nice, although not necessary, to be able to access the larger chamber of the Dewar directly to installing a larger superconducting sample.  Right now, access is only available through the 3 1/8” neck opening at the top of the Dewar.  The interior chamber is at least 8” in diameter.
We disassembled the Dewar to determine if there was a way into the larger chamber.  The Dewar has a very simple construction.  There is an inner chamber also made of fiberglass attached to a short neck that is attached to the lid.  Sadly, all attachments are made with glue.  In other words, short of cutting it, we can’t take it apart to access the larger chamber.  The entire inner chamber is surrounded with Mylar super-insulation.  This basically amounts to layer after layer of thin Mylar wrapped around the chamber.  The picture below is the view of the interior of the Dewar through the neck.  The two following pictures are of the exterior of the Dewar.

background to the h-ray experiment if you're new to the game:

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