Instructions on how to fabricate polystyrene scintillators

This is an overview and summery of everything we need to know to synthesize and polymerize the non-gadolinium loaded plastic scintillators currently fabricated by Jake Brutman (brutm003@umn.edu) in Chemistry. These plans are for a ~20g polystyrene plastic scintillator doped with 0.1 wt% PPO and 0.01 wt% POPOP fluorescent dyes by weight. The polymerization catalyst benzoyl peroxide is used in 0.8 wt%.

Chemicals

- styrene (~ 20g fills half a small vial)

- alumina (more than 2 mL for 20g styrene)

- PPO (1% of styrene by weight)

- POPOP (0.1% of styrene by weight)

- purified benzoyl peroxide (0.8% of styrene by weight)

Chemistry Stockroom Supplies

- large syringe

- vial with teflon cap

- cotton

- pipette and bulb

- weight boats / paper

- metal and plastic scoops

- ring stand

- ring clamp

- rubber tubing

- small syringes (NO LUBRICANT)

- large beaker for water bath

- balloons

Lab Safety Requirements

- lab glasses

- lab coats

- polyethylene glove liners

- secondary containment

- Solid waste container

- Liquid waste container(s?)

- NOTE: Before we can start working with wet chemistry in our lab, there are probably a couple safety courses required by DEHS.

Large / Expensive Equipment

- analytical balance

- sonicator

- laboratory refrigerator

- water traps for nitrogen line

1. Purify Styrene

In order to remove excess water from the styrene, it needs to be purified with alumina. When not in use, the styrene should be kept in a refrigerator. Before it can be purified, it needs to warm up to room temperature. More than 2 mL of alumina should be placed into a large syringe that has cotton in the tip to prevent the alumina from falling into the purified styrene. With the cap-less vial placed beneath the syringe, slowly pipette the styrene over the alumina making sure not to overflow the syringe. Allow the styrene to run through the alumina into the vial until the vial is about halfway full. This should be about 20g of styrene.

The used alumina should turn yellow. After the alumina has dried out it may be put into solid waste. It should take 2 to 3 days for the alumina to dry.

2. Add fluorescent dyes and catalyst to styrene

When adding the fluors and the catalyst, in order to get an accurate measure of the amount added, the vial containing the styrene should be weighted before and after addition as well as weighing out the amount to be added. The PPO is a white powder and should be added at 1 wt%. It should dissolve easily in the styrene. The POPOP appears fluffy and should be added at 0.1 wt%. Because it is much less dense than the PPO, the amount added will look similar to the amount of PPO added. The POPOP is much harder to dissolve, and so will require sonication in a later step. The benzoyl peroxide should be added with 0.8 wt%. The benzoyl peroxide requires a plastic scoop as it will oxidize a metal scoop. The benzoyl peroxide should be purified (see instructions below), and after purification, the vial containing the benzoyl peroxide needs to be wrapped in aluminium foil to block out light.

After all chemicals have been added to the styrene, the mixture should be sonicated for about 5 minutes.

3. De-Gas styrene mixture

The cap of the vial contains a teflon piece that will allow a syringe needle to pass through without letting air in. This cap should be taped to the vial with electrical tape to make a secure, airtight connection. The rubber tubing should connect the nitrogen gas to a syringe with a need tip. This connection should go through a trap of some sort to remove water from the gas. Because the pressure of the gas coming from the wall is very high, a regular should be used to set the pressure to around 2 psig. The needle connected to the nitrogen should be pushed through the cap down into the bottom of the styrene mixture. Bubbles will not form until a second needle is pushed through the teflon into the air above the styrene mixture. The mixture should be allowed to bubble for 30 minutes to an hour.

If an over-pressurized system is required without bubbles, a balloon may be placed over the exit needle. After the balloon has filled with some nitrogen, the needle connected to the nitrogen system may be removed from the vial. The balloon should keep the system consistently over-pressurized.

4. Allow styrene to polymerize

The balloon system mentioned above may be used to keep positive pressure on the vial while heating. The styrene mixture should set for 4 to 5 days in a heat bath of ~ 57°C.

Equipment

- 250 mL Erlenmeyer flask

- magnetic teflon stir bar

- magnetic stir plate

- vacuum flask

- Büchner funnel / funnel paper

- vacuum desiccator

- vacuum aspirator

- scroll pump

Chemicals

- Unpurified benzoyl peroxide

- methanol or petroleum ether

- chloroform

1. Dissolve impurities

This procedure should be done for small amounts of benzoyl peroxide only as it can be explosive. Make sure this is done in the fume hood with the glass as low as feasibly possible.

In order to dissolve the benzoyl peroxide in chloroform, add small amounts of chloroform to an Erlenmeyer flask until the benzoyl peroxide in the flask dissolves. While adding the chloroform, the Erlenmeyer should be placed on the magnetic stir plate with a stir rod inside. Precipitate out the purified benzoyl peroxide by adding in the exact same volume of methanol or petroleum ether. Now the impurities in the benzoyl peroxide have been dissolved out, and only the pure benzoyl peroxide is left in the precipitate. The stir rod can either be removed now magnetically or during during the vacuum filtration.

2. Remove precipitate from liquid

In order to remove the precipitate from the liquid, a vacuum filtration setup must be used. The rubber line from the vacuum aspirator must be attached to the hose barb on the vacuum flask. A hose clamp might be necessary to insure a tight fit, but be careful not to break the glass! The Büchner funnel is placed over the opening of the vacuum flask with funnel paper placed over the top to block the holes. The mixture containing the benzoyl peroxide precipitate should be poured slowly into the funnel. After all visible liquid has been pulled through the funnel, transfer the purified benzoyl peroxide from the funnel paper to a clean watch glass. The liquid in the vacuum flask is organic waste.

3. Dry under vacuum

The watch glass should be placed in the vacuum desiccator and connected to a vacuum pump in room temperature for 24 hours. The vacuum pump is the scroll pump found in the Cryo-lab (PAN 43). After 24 hours, the dried and purified benzoyl peroxide should be placed in a vial that has been wrapped in aluminum foil to block all light and placed in the refrigerator in Noireaux's lab (PAN 484).

We eventually need to cast a prototype of the gadolinium loaded scintillator as well as cast gadolinium sheets for a commercial scintillator. The size casts we are aiming for is 24“ x 13” x 4.75“. The scintillator blocks will be grooved for wavelength-shifting fibers to be read out by (four?) SiPMs. The final prototype will be made from 5 pieces of scintillator such that each slice is only 0.95” thick.

After we successfully fabricate a 20 g cylindrical scintillator, we will need to create a small square sample using an aluminum cast. The aluminum casts will also use teflon liners to keep the scintillator from sticking to the mold. This sample should be the same height and thickness of the intended final cast (13“ x 0.95”) but should be much less wide. The final prototype will be made “on its side” so that the vertical component is 13“ tall. The prototype will be rotated from this so that it is 13” long.

Increasing the size of the prototype from 20g will not be trivial. Here are a list of things that need to be figured out before we can proceed.

1) The process for purifying the styrene will need to be modified.

2) A way of sonicating the mixture so that the POPOP dissolves will need to be decided upon.

3) A large aluminum cast with a teflon cap will need to be designed and manufactured.

4) A new system of syringes will need to be designed for bubbling the styrene.