Vacuum tests on Vacuum tests on scintillating fibresscintillating fibres

Table of contentTable of content

• The Aim of the vacuum tests on the scintillating fibre plane

• Equipments used in the vacuum tests

• Outline the experimental methods

• Final Conclusion

Aim of the testsAim of the tests

The fibre plane is constructed in the air environment. It is very likely that the air-molecules could be trapped in the plane during the construction process. The general aim is to test whether the air-molecules cause the structure of the scintillating plane to deform significantly.

Computer controlled stage

Vacuum vessel

Figure 1.0 The set-up of the vacuum tests

High vacuumPump

Valve

Stage controller PC

MicroscopeMonitoring TV

Plastic Glass lid

Figure 1.1 The picture of the main componentsFigure 1.1 The picture of the main components

Pump

Vacuum meter

Microscope

Valve

Light controller of the microscope

Figure 1.2 The high vacuum pump and the computerFigure 1.2 The high vacuum pump and the computer

Pipe

PC

Figure 1.3 The monitoring TV and the stage controllerFigure 1.3 The monitoring TV and the stage controller

Monitoring TV

Stage controller

Figure 1.4 The rest of the componentsFigure 1.4 The rest of the components

Vacuum vessel

Laser pointer

Eyepiece

Y-axis of the stage

X-axis of the stage

The Specification of the main componentsThe Specification of the main components

• The high vacuum pump is in the range of to mbar. • The stage is transverse in both x and y direction. The motor of the x-

axis of the stage is 200 steps per revolution. There are 100 microsteps per one step. Each microstep is equal to 0.0002 mm. The motor of the y-axis of the stage is 400 steps per revolution. Each step has 25 microsteps. Each microstep is equal to 0.0001 mm.

• The stage can be both manual and computer-based control. In the controlling programme, the base unit is microstep in both x and y axis.

• The microscope has lens magnification from 1.8 up to 11. The smallest dimension of the microscope is 2.5 μm in x and 2.64 μm in y.

• The minimum value of the vacuum reader is mbar, the maximum value is 1 atm.

• The pump-down time of the high vacuum pump to get the medium vacuum level is in the range of 3-5 seconds. It takes enormous amount of time to get to higher Vacuum level.

310 710

310

Mylar sheet

Mould

Second layer of fibres

First layer of fibres

Figure 1.5 A reminder of the structure of the scintillating fibre plane

A thin layer of adhesive between

the first and the second layer of

fibre

A thin layer of adhesive between the Mylar sheet and the second layer of

fibre

The fibre plane is cut into many 5cm by 5cm pieces for the tests. The piece of the plane is firmly hold by a aluminium plate, which is designed to give sufficient pressure to the edges of the piece without destroying the structure of the fibre plane. After closing the glass lid on the top of the vacuum vessel, the pump can be turned on and with the valve closed. After few seconds, the pressure of the vessel is in stable and steady. The stage can be moved around to spot any underlying structure of the fibre plane. The level of detail can be easily adjusted using the magnification provided by the microscope. To get back to air, just simply open the valve attached to the pump, the pressure drops back to atmospheric pressure within two seconds.

Outline of the experimental methodOutline of the experimental method

Subtasks of the vacuum testsSubtasks of the vacuum tests

1. Find the location of the air bubbles2. Measure the size of an air bubble3. Understand the reason of the existence

of the air bubbles in different layers4. Observe any distortions caused by the

expansion of the air-molecules in the scintillating fibre plane.

5. Check whether or not the air-molecules grow over time

The air bubbles are frequently found in the gaps between thefibres in the second layer of fibre in the scintillating fibre plane.(See Figure 2.0 and 2.4) They have relative high density closeto the edges of the piece of the plane. The size is in therange of 40 to 140μm with error 5 μm. The expansion of theair-bubbles vary from each other. Some do and some do not, ittotally depends on how many air-molecules are contained. Themaximum expansion is measured to be 20+5 μm. The presenceof those air-molecules is due to two contributions. One is the airmolecules trapped in the second layer construction. The second isthe air molecules forced out by brushing forward and backwardof the adhesive spray (resin) apply to the second layer.

Figure 2.4 Observation of the Mylar side without the microscope Figure 2.4 Observation of the Mylar side without the microscope

Air bubbles

Screws to just hold the piece of the

plane

Aluminium plate to

prevent the piece from any

movements

A clamp to hold the lid

Figure 2.0 the structure of the piece of the plane under 11x magnification

Air-molecules

At the vacuum level of the pump, a single air-molecule is capableof expanding 10132.5 times in volume. The most visible airbubbles are between the Mylar sheet and the second layer offibre. They are be easily seen by naked eyes. The largest onecould be around 15mm in length and 6mm in width. The numberof them is usual around 8-16. Because the largest one hassufficient force to the same but much smaller effect on the otherside of the piece of the plane. These air-bubbles are trapped whenthe Mylar sheet is laid on the top of the second layer of fibre.The surface of the second layer of fibre is not smooth due to nonuniform spray of the adhesive and the brush does not form theadhesive to fully fill the gaps between the fibre in the secondlayer. (See Figure 2.1 and Figure 2.2)

Figure 2.1 1.8x magnification of the piece of the fibre planeFigure 2.1 1.8x magnification of the piece of the fibre plane

One of the boundaries of a

large air-bubble

Figure 2.2 The surface of the adhesive on top of the second layer of fibreFigure 2.2 The surface of the adhesive on top of the second layer of fibre

Still in the gap, but with more of the adhesive,

which makes higher than the rest of the gap

The gap of between two fibres in the second layer

filled by the adhesive.Normal value=118+5μm

A single fibre

In Figure 2.2, the higher adhesive may be formed due to both ofthe deformation of the fibres in the construction process and thenon-uniform brushing of the adhesive. The higher adhesiveregions are responsible to the leakage of the air-bubbles inthe piece of the fibre plane. When the piece is left for 30 hours inthe medium vacuum level. The size of the air bubbles in theMylar side shrinks about 1.5mm in length and 0.5mm in width.An alternative method is to keeping switching between vacuumand air several times in the vessel. The rapid drop of the pressurein the vessel creates a sufficient force to squeeze the air bubblesthrough the paths produced by the higher adhesive regions in thegaps between the fibres. Having done all of those, there is no signof deformation of the fibres in the plane. (See Figure 2.5 and 2.6)

Figure 2.3 The Deformation of the higher adhesive regions Figure 2.3 The Deformation of the higher adhesive regions between the Mylar and the adhesive of the second layer of fibrebetween the Mylar and the adhesive of the second layer of fibre

Deformation of the higher

adhesive region in the Mylar side due to the expansion of the air-bubbles

Figure 2.5 Layout of the side of the plane without Mylar (1.8x magnification )Figure 2.5 Layout of the side of the plane without Mylar (1.8x magnification )

Figure 2.6 Layout of the side of the plane without Mylar (11x magnification)Figure 2.6 Layout of the side of the plane without Mylar (11x magnification)

A single fibre in the first layer of fire

The gap between two adjacentFibres = (58+5μm)

Conclusions

• A large number of the air bubbles are found in between the Mylar sheet and the layer of adhesive on top of the second layer of fibre. The air bubbles of the rest of the fibre plane are insignificant in size compared with the size of fibres, which could be neglected.

• The successive number of switching between air and vacuum does not cause any unwanted deformation to the fire plane. It can eliminate air bubbles between the Mylar sheet and the second layer of fibre. This technique could be applied to another prototype to see whether it can squeeze the air bubbles out of the plane or at least out of the active region of the plane.