Difference between revisions of "Light Guide Design"
| Line 5: | Line 5: | ||
==== Geometrical Situation ==== | ==== Geometrical Situation ==== | ||
# The following picture shows the situation with having 66mm outer diameter readout systems coupled to the end of 60mm high paddles in terms of space as viewed for the end. In this figure [[Media:tof_paddle_setting.pdf|tof_paddle_setting.pdf]] one can see that because of the additional space required by the PMT plus shielding the paddles could not be put on top of each other unless there is a light guide with a bend. Also one sees that staggering the paddles with not solve the problem because of the rather large impact angles tracks can have at the location of the TOF. The red line indicates the largest possible impact angles of tracks. | # The following picture shows the situation with having 66mm outer diameter readout systems coupled to the end of 60mm high paddles in terms of space as viewed for the end. In this figure [[Media:tof_paddle_setting.pdf|tof_paddle_setting.pdf]] one can see that because of the additional space required by the PMT plus shielding the paddles could not be put on top of each other unless there is a light guide with a bend. Also one sees that staggering the paddles with not solve the problem because of the rather large impact angles tracks can have at the location of the TOF. The red line indicates the largest possible impact angles of tracks. | ||
| − | |||
# In the following an estimation is done of what bend angle is required. As an arbitrary number it is assumed that the light guide has a total length of 200mm. The picture [[Media:tof_lightguide_bend1.pdf|tof_lightguide_bend1.pdf]] assumes a bend of 10 degree and shows that this is sufficient to provide enough space for the PMT and shielding on only at the end of the light guide but also at a location more than 50mm toward the paddle which is necessary as the shielding has to extend beyond the PMT entrance window by at least 50mm and additional space for the holding structure is needed as well. As one can see on the bottom right side of the picture a 5 degree bend will not be sufficient given a light guide length of 200mm. | # In the following an estimation is done of what bend angle is required. As an arbitrary number it is assumed that the light guide has a total length of 200mm. The picture [[Media:tof_lightguide_bend1.pdf|tof_lightguide_bend1.pdf]] assumes a bend of 10 degree and shows that this is sufficient to provide enough space for the PMT and shielding on only at the end of the light guide but also at a location more than 50mm toward the paddle which is necessary as the shielding has to extend beyond the PMT entrance window by at least 50mm and additional space for the holding structure is needed as well. As one can see on the bottom right side of the picture a 5 degree bend will not be sufficient given a light guide length of 200mm. | ||
| + | # Given the parameters from above a light guide design based on a total length of 200mm is drawn here with the emphasis to make the tapering from the rectangular cross section of the paddle to the round cross section of the PMT as shallow as possible. This is shown in [[Media:tof_lightguide20cm.pdf|tof_lightguide20cm.pdf]] where the transition from square to round with established over 110mm. This is also the same region where the 10 degree bend needs to happen because once the light guide is in its cylindrical shape no additional bending should be present. | ||
Revision as of 09:17, 18 March 2011
Requirements
Given the dimensions of the individual paddles of 60mm x 2.54mm x 2520mm and the dimensions of the photo multiplier tube (PMT) including necessary magnetic shielding it is necessary to have light guides. Further arguments for the use of light guides are the rectangular shape of the paddle cross section versus the round shape of the PMT entrance window that does not favor a direct coupling of the readout to the scintillator. In addition particles that pass very close to the end of the paddle would generate a disproportionate large signal in the PMT because of the highly non linear light absorption at short distances for short wave lengths (UV). The PMT H10534 is a 10 stage tube with a quoted 2 inch window. Measurements indicate a total diameter of the tube of 50mm. The H10534 is a full assembly including shielding and base as received from the manufacturer. At this point this assembly has an outer diameter of 60mm. Because of the expected high magnetic field in the region of the PMT of the order of 100 Gauss additional iron shielding is necessary. At this point a total outer diameter including such a shielding is estimated to be of the order of 66mm. As a consequence this dimension becomes larger than the paddle height of 60mm requiring the light guide to be bend in order the provide enough space to tack the paddles on top of each other.
Geometrical Situation
- The following picture shows the situation with having 66mm outer diameter readout systems coupled to the end of 60mm high paddles in terms of space as viewed for the end. In this figure tof_paddle_setting.pdf one can see that because of the additional space required by the PMT plus shielding the paddles could not be put on top of each other unless there is a light guide with a bend. Also one sees that staggering the paddles with not solve the problem because of the rather large impact angles tracks can have at the location of the TOF. The red line indicates the largest possible impact angles of tracks.
- In the following an estimation is done of what bend angle is required. As an arbitrary number it is assumed that the light guide has a total length of 200mm. The picture tof_lightguide_bend1.pdf assumes a bend of 10 degree and shows that this is sufficient to provide enough space for the PMT and shielding on only at the end of the light guide but also at a location more than 50mm toward the paddle which is necessary as the shielding has to extend beyond the PMT entrance window by at least 50mm and additional space for the holding structure is needed as well. As one can see on the bottom right side of the picture a 5 degree bend will not be sufficient given a light guide length of 200mm.
- Given the parameters from above a light guide design based on a total length of 200mm is drawn here with the emphasis to make the tapering from the rectangular cross section of the paddle to the round cross section of the PMT as shallow as possible. This is shown in tof_lightguide20cm.pdf where the transition from square to round with established over 110mm. This is also the same region where the 10 degree bend needs to happen because once the light guide is in its cylindrical shape no additional bending should be present.
