Difference between revisions of "Repairs made to TAGH scintillator counters over the summer of 2019"

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(Overview)
(Cleaning)
 
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*a heat gun was used before the exacto knife to allow for easier removal, a problem with this is that the plastic light guide heats up faster and damage to the light-guide may ensue, therefore heat may only be used for very brief periods making sure not to damage the light-guide
 
*a heat gun was used before the exacto knife to allow for easier removal, a problem with this is that the plastic light guide heats up faster and damage to the light-guide may ensue, therefore heat may only be used for very brief periods making sure not to damage the light-guide
 
*a glass tool, seen below, was also used to remove the epoxy, this had the benefit of not damaging the surface as much as the exacto knife
 
*a glass tool, seen below, was also used to remove the epoxy, this had the benefit of not damaging the surface as much as the exacto knife
*once only a thin layer of epoxy was left, a fine file was used to get rid of the remaining bit (in hindsight, '' this is a much more effective technique than the exacto knife and heat gun '', as it does not scar the surface nearly as much)
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*once only a thin layer of epoxy was left, a fine file may be used very lightly to get rid of the remaining bit
 
*an acrylic polish, as seen below, was used extensively on the scratches left by the exacto knife and file on the surface of the light-guide
 
*an acrylic polish, as seen below, was used extensively on the scratches left by the exacto knife and file on the surface of the light-guide
  
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| [[File:Polish.JPG|thumb|Acrylic polish.]]
 
| [[File:Polish.JPG|thumb|Acrylic polish.]]
 
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=== Covering ===
 
=== Covering ===
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To make sure that wires could not be pulled off the circuit board, a plastic insert was made. This insert was attached to the end of mu-metal housing the PMT where the wires came out. A zip-tie fastened the wires to the insert. The idea was that if the wires were pulled, the whole housing would be pulled, so that the wires attached to the circuit board would stay intact.  
 
To make sure that wires could not be pulled off the circuit board, a plastic insert was made. This insert was attached to the end of mu-metal housing the PMT where the wires came out. A zip-tie fastened the wires to the insert. The idea was that if the wires were pulled, the whole housing would be pulled, so that the wires attached to the circuit board would stay intact.  
  
 +
The second group of scintillators were tested the same way and after being repaired all of them worked.
  
  

Latest revision as of 14:55, 10 July 2019

Overview

During the second week of June 2019, 5 counters were removed from the TAGH. These counters all had scintillators that were loosely connected to their respective light-guides and were believed to not be functioning properly. The initial 5 counters were the numbers 113, 116, 125, 144, and 161. These counters were reinstalled on July 2, 2019 and 5 new counters were pulled to be repaired, namely, 119, 187, 196,212, and 228. The second group of counters were then reinstalled on July 10, 2019 and a third group was removed, namely, 83, 84, 92, 123, 126, 138, 184, 229, and 230. The third group had a counter that was surveyed as radioactive.

Excel spreadsheet documenting counter numbers, what was wrong with them, repairs made, and date of removal/re-installation. File:19.xlsx

Initial State of the Counters

Two of the five counters had scintillators that were broken at the base, as is seen in the picture below. The other three counters had scintillators that were loose and no longer held by the epoxy to the light-guide. These damages are thought to be the result people bumping into them and not putting the protective caps over the scintillator part of the counter.

Initial state of the counters. Note how the scintillators, the long thin part wrapped in electrical tape, appear crooked.
Picture showing scintillator broken at the base. The base is still attached to the light guide by epoxy.

Scinillators

Cleaning

Scintillators were wrapped in mylar and then in electrical tape. Numbers 125, 144, and 161 were loose from their respective light-guides, while 113 and 116 had there bases broken off and still in the epoxy.

A white film, which can be seen in the first image below, was formed over the sides of the scintillators in contact with the light-guides. This film was polished off the scintillators using the same technique for polishing the light guides.


Picture showing the white film over one of the scintillators and a polsihed one for comparison.
All five scintiallators. Note the two broken ones.
Overhead view of the two broken scintillators
The three usable, mostly polished scintillators

Reattaching to the Light Guide

Once the scintillator and light-guide were polished, any epoxy that cures in UV light was placed on the light-guide where the scintillator is placed. Safety glasses were worn while doing this. The UV light emitting pen was held near each corner of the scintillator and tinged the epoxy green, while the UV light was present. After 15 seconds per corner of UV light exposure, the epoxy dried and the scintillator was held fest by it.


Scintillator freshly glued to the light-guide.
To replace a scintillator look at the first number on the numbers found on the cable, this number coincides with the size of the scintillator used for the counter.

Covering

Scintillators were wrapped tightly in a reflective material, namely, mylar, so light does not escape from them. After this, they were wrapped in tedlar to block incident outside light. Lastly, there was a light layer of liquid electrical tape applied on the tedlar to minimize light penetration.

Note, the original covering of the scintillators only had a mylar wrapping followed by a wrapping by electrical type. The following was kept in mind while wrapping the scintillators: use as little material as possible when wrapping the scintillator, do not use too much tape, keep it aesthetically pleasing, cover any holes in the wrapping letting light in, and keep the wrapping tight and without too much wrinkles in the mylar.

Mylar wrapped around scintillator, covering all exposed surface area above light-guide.
After mylar, tedlar is wrapped around the scintillator.
Liquid electrical tape applied to the outer wrapping of the tedlar covering the scintillator and top of the light-guide.

Light-Guides

Cleaning

After the electrical tape wrapping and the inner mylar wrapping have been removed off the light guide and the scinillator removed, the epoxy was removed using the following technique:

  • first an exacto knife was employed to cut the epoxy off
  • a heat gun was used before the exacto knife to allow for easier removal, a problem with this is that the plastic light guide heats up faster and damage to the light-guide may ensue, therefore heat may only be used for very brief periods making sure not to damage the light-guide
  • a glass tool, seen below, was also used to remove the epoxy, this had the benefit of not damaging the surface as much as the exacto knife
  • once only a thin layer of epoxy was left, a fine file may be used very lightly to get rid of the remaining bit
  • an acrylic polish, as seen below, was used extensively on the scratches left by the exacto knife and file on the surface of the light-guide
Polishing stages for light-guides
Unpolished, exacto knife and heat gun used.
Just polished.
File and more polish used.
Unbelievable amount of polishing and more filing done.
This is the glass tool used to remove epoxy. This cloth material was used with the polish.
Acrylic polish.

Covering

Covering the light-guides was done with mylar, then tedlar, followed by electrical tape. The light-guides were cleaned with a little alcohol before wrapping them.

Covering Process
The top surface of the light-guide was covered like this so as to minimize wrinkles in the mylar and light escaping.
Mylar wrapped around the light-guide.
Tedlar wrapped around the light-guide.
Electical tape wrap.

Testing Counters

After replacing or reattaching the scintillators and recovering them, the counters were connected to an oscilloscope and a voltage was applied. Counters 116, 125, and 144 showed no pulse on the oscilloscope. Counter 116 and 125 had a clear wire disconnected from its circuit board, while counter 144 did not show clear signs of what the problem was. Upon reexamination of 144 it did work. The loose wires were reconnected to the circuit boards and then both counters worked. Counters 113 and 161 both worked, but showed signs of not completely blocking outside light causing noise on the oscilloscope reading. This light was most likely entering from the tip of the scintillator, where the wrapping is not as tight and difficult to cover. The scintillators were re-wrapped to minimize this problem, as well as applying a light layer of liquid electrical tape over the tedlar.

To make sure that wires could not be pulled off the circuit board, a plastic insert was made. This insert was attached to the end of mu-metal housing the PMT where the wires came out. A zip-tie fastened the wires to the insert. The idea was that if the wires were pulled, the whole housing would be pulled, so that the wires attached to the circuit board would stay intact.

The second group of scintillators were tested the same way and after being repaired all of them worked.


The oscilloscope used.
Counter 125 showing a wire disconnected from the circuit board.
Plastic insert picture.
Plastic insert.
The insert and mu-metal are wrapped in masking tape to secure them to each other.
The insert is attached and then wrapped in electrical tape along with the mu-metal.
The Tagger, where the detectors are from/ were returned to upon being repaired.