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2.3 Prototype Tests

2.3.2 Results using Setup II

The setup of fig. 1-1 was rotated by 900 in order to avoid the incidence of charged particles on the WLS plane. The total thickness of aerogel tranversed by a particle, at normal incidence, was 5 cm. The aerogel blocks were wrapped in a white PTFE box, and PMP was dipped in PTFE foils, which were inserted between the blocks in order to wavelength shift the produced Cerenkov light to . The observed mean number of photoelectrons using this setup was:

photoelectrons collected
and, the empty box (i.e. after removal of the aerogel blocks) setup gave:

photoelectrons collected
Although the result was positive, one may remark that the conditions of the test was particularly favorable:

The same setup was used in order to study the performance of a BC408 Bicron scintillator[2]. The advantage of this scintillator is that it absorbs the light at shorter wavelengths (where Cerenkov production is higher) and shifts it to a range were the phototube efficiency is higher (absorbs in the range of 330-400 nm, instead of the range 400-450 nm that the previous WLS was absorbing). However, since PMP also absorbs the light 300 nm and emits at 430 nm, we have not used PMP as incompatible. Since BC408 is also a scintillator one cannot use the "direct" method readout (setup I). The aerogel blocks were wrapped in PTFE boxes.

The mean number of photoelectrons per incident track, were:

while, the empty box measurement gave:

photoelectrons collected per incident track.

Further tests of a bigger setup, based on the same principle (fig. 2-4), using two RTC-3462 phototubes directly coupled to the BC408 WLS, and a 9x9 cm2 aerogel wrapped in a white diffusing box gave:

photoelectrons
using a 9x5x5 cm3 aerogel prototype:

photoelectrons
and, by using light guides covered by aluminized mylar the collection yield improves to:

photoelectrons
which indicates that once the WLS length becomes big, the losses of light transmission in the WLS become important.

In summary, the following factors (table 2-1)must be taken into account when one goes from a small size setup to a medium size geometry:
Attenuation factors between a small and a medium size geometry
ItemSmall SetupMedium SetupExpected factorComment
Aerogel volume5x5x5 cm35x9x90.85MC Simulation
Dimensions of BC40812x6.7 cm2 + reflector59x6.7 no reflector0.32Measurement
Number of PMT's122.0 
TOTAL FACTOR0.54 

The observed attenuation factor is 0.33, which is much lower than the one estimated in table 2-1. Perhaps, in a small geometry part of the direct Cerenkov cone created in the aerogel, reaches the PMT due to the close proximity.

Figure 2-4 Schematic of the medium-size setup.

Taking into account the experimental results obtained a small and a medium size prototype, we estimate the potential photoelectron yield losses, for a real-size aerogel detector using WLS in "lateral" readout mode. The results of the study are summarized in table 2-2.
Attenuation factors between a medium size and an AMS real-size detector
ItemMedium setupReal size setupExpected factorComments
Change aerogel volume5x9x9 cm35x10x5 cm31.21MC Simulation
Dimensions of BC408 59x6.7 cm2 120x6.7 cm2 0.79Measurement
Twisted parts of BC408NoYes0.7-0.6Measurement
Optical couplingGreaseBicron silicoid1.0Measurement
PMT Quantum efficiencyRTC-3462R5900U0.64-0.76data sheets
Number of PMT's221.0 
TOTAL FACTOR0.37-0.51 

From this extrapolation we estimate that for 5 cm of aerogel transversed, we expect:

photoelectrons collected

per incident track, which is clearly not sufficient.

[2] suggested to us by Prof.A.Onuchin, Novosibirsk
Issue: Draft - Revision: 04 - Last Modified: 20 April 1997