Repairs made to TAGH scintillator counters over the summer of 2019
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 was reinstalled on July 17, 2019 and a fourth group was then pulled, namely, numbers 98, 101, 110, 170, and 231. The fourth groups was reinstalled on July 26, 2019.
A more detailed technical report can be found here.
Initial State of the Counters
Two counters had scintillators that were broken at the base, as is seen in the picture below. All counters had scintllators no longer secured to their respective light-guides. 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.
Scintillators were wrapped in mylar and then in electrical tape.
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.
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.
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.
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
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.
After replacing or reattaching the scintillators and re-covering them, the counters were connected to an oscilloscope and a voltage was applied. At this stage if one or both channels did not work the wires were checked to see if they made a proper connection with the circuit board. In all three cases where a channel was not working, the counter had a dettached wire from the bottom circuit board. Loose wires were reconnected to the circuit boards and then the three counters worked.
One problem faced experienced when testing counters was noise from outside light. 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. For counters that were placed in the middle row, a smaller sized plastic piece was used.