Vacuum tests on scintillating fibres

Table 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 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 vacuum Pump Valve Stage controller PC Microscope Monitoring TV Plastic Glass lid

Figure 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 computer Pipe PC

Figure 1.3 The monitoring TV and the stage controller Monitoring TV Stage controller

Figure 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 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 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 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.

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 method

Subtasks of the vacuum tests 1.Find the location of the air bubbles 2.Measure the size of an air bubble 3.Understand the reason of the existence of the air bubbles in different layers 4.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 the fibres in the second layer of fibre in the scintillating fibre plane. (See Figure 2.0 and 2.4) They have relative high density close to the edges of the piece of the plane. The size is in the range of 40 to 140μm with error 5 μm. The expansion of the air-bubbles vary from each other. Some do and some do not, it totally depends on how many air-molecules are contained. The maximum expansion is measured to be 20+5 μm. The presence of those air-molecules is due to two contributions. One is the air molecules trapped in the second layer construction. The second is the air molecules forced out by brushing forward and backward of the adhesive spray (resin) apply to the second layer.

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 capable of expanding times in volume. The most visible air bubbles are between the Mylar sheet and the second layer of fibre. They are be easily seen by naked eyes. The largest one could be around 15mm in length and 6mm in width. The number of them is usual around Because the largest one has sufficient force to the same but much smaller effect on the other side of the piece of the plane. These air-bubbles are trapped when the 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 non uniform spray of the adhesive and the brush does not form the adhesive to fully fill the gaps between the fibre in the second layer. (See Figure 2.1 and Figure 2.2)

Figure x 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 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 of the deformation of the fibres in the construction process and the non-uniform brushing of the adhesive. The higher adhesive regions are responsible to the leakage of the air-bubbles in the piece of the fibre plane. When the piece is left for 30 hours in the medium vacuum level. The size of the air bubbles in the Mylar side shrinks about 1.5mm in length and 0.5mm in width. An alternative method is to keeping switching between vacuum and air several times in the vessel. The rapid drop of the pressure in the vessel creates a sufficient force to squeeze the air bubbles through the paths produced by the higher adhesive regions in the gaps between the fibres. Having done all of those, there is no sign of deformation of the fibres in the plane. (See Figure 2.5 and 2.6)

Figure 2.3 The Deformation of the higher adhesive regions between 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 2.6 Layout of the side of the plane without Mylar (11x magnification) A single fibre in the first layer of fire The gap between two adjacent Fibres = (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.