# References

1. E.H. Ballamy et al, Nucl. Inst. and Meth. A 339 (1994)468-476

# Fiber Test Stand

The purpose of the fiber test stand at JLAB is to determine the number of photo electrons produced by a minimum ionizing particle in a scintillating fiber at a distance of 200cm from the fiber end. The photon detector is a standard photo multiplier, in this case an XP2020. In addition the attenuation length of the scintillator can be determined measuring the light output at several positions along the fiber.

## Concept

One end of the scintillation fiber is coupled to the photo cathode of a PMT (XP2020). For monitoring purposes a second fiber (light guide) is also coupled to the same PMT providing light from a blue LED. At a given distance from the PMT surface a collimated Strontium (Sr90) source is mounted above the scintillation fiber. Below the fiber and the Sr90 source a small trigger scintillation detector (TSD) is mounted to trigger on electrons from the Sr90 source that pass through the fiber into the trigger scintillator. The data acquisition is triggered by TSD and the analog signals from both the XP2020 and the TSD are digitized as well as their timing. Simultaneously a trigger is generated for the LED light source to monitor the XP2020 PMT during the measurement.

## Electronic layout

A basic sketch of the NIM electronics layout is shown here.
The signal from the XP2020 PMT is first amplified by a factor of 10 and then connected to a linear FAN In/Out. The DC offset of the linear FAN In/Out is adjusted such that the true pedestal is above the built in pedestal of the V792N VME ADC, which is about 5pC total charge. This corresponds to about 15mV of negative DC offset for the used gate width. The analog signal is then connected to the ADC adjusted for timing. A second output of the linear FAN In/Out is connected to a discriminator the output of which is then connected to F1TDC. The trigger scintillator is also connected to a linear FAN In/Out. Also here the DC offset is adjusted to move the true pedestal of the signal above threshold in the ADC. The analog signal is also discriminated and one output connected to the trigger OR which is linked to the trigger interface TI in VME of the DAQ system. The second trigger of the system is the trigger for the blue LED for monitoring purposes. The rate of this can be adjusted using two Gate/Delay generators and is set to about 10 Hz during measurements. The trigger rate from the TSD caused by the electrons of the Sr90 source is also in the order of 10 Hz.

ADC V792: 0-400pC range, 0.1pC/channel 4096 channels.

## Dark Box

The detectors, scintillators and Sr90 source are currently located in an improvised dark box to protect the system for ambient light. Picture of the setup can be seen here as well and the fiber coupling to the PMT and the Sr90 source collimation:

### Sr90 collimation

The Sr90 source is collimated by a piece of copper shown in the above picture. A schematic view is shown below. The scintillation fiber lies in a 1mm x 1mm groove of the copper collimator thereby being fixed in position. At the center of the copper piece a 0.5mm wide and 5mm long slit provides the collimation. The slit is in the center and parallel to the groove where the fiber is located.

To illustrate the setup of the trigger scintillator with the collimated Sr90 source some pictures are shown here. First the full assembly with indicators to explain the individual components. The Trigger scintillator is located below the fiber and is readout by a Hamamatsu photo multiplier H3165-10. Above the trigger scintillator is small Aluminum platform for positioning the collimator. The collimator is a 5mm thick Copper piece with a 1mm groove to hold the fiber. The collimation of the electrons from the Sr90 is done by a 0.5mm wide and 5mm long slit at the center allowing the passage of electrons through the central part of the fiber.

The second picture shows the front part of the XP2020 housing. There are two feed-through for the fibers. The left black tapped one is the fiber from the blue LED. The right is the fiber to be tested and has no black tape or other protective clading. The feed through itself is a screw with a whole drilled through its length to hold the fiber in place. THE FIBER IS PUSHED UP AGAINST THE PMT ENTRANCE WINDOW. THE COUPLING IS AIR ONLY. NO OPTICAL GREASE IS USED! The last two pictures show the collimator assembly and Sr90 source. A detailed description is given below.

## XP2020 PMT responses to blue LED

The response of the XP2020 on the LED light is shown for two different LED intensities (Low Voltage settings) in the form of the ADC spectra. The voltage divider used here for the XP2020 is a VD124K/T from Photonic (Type C divider). The fit is a Poisson distribution weighted sum of Gaussian distributions that are convoluted with the pedestal width and an exponential background.

The HV of the XP2020 PMT is set at -2200V. The corresponding single photo electron peak position (fit parameter P1) is about 41 ADC channels corresponding to a gain of the PMT of 2.6*10^6. The fit parameter mu is the mean number of photo electrons from the Poisson distribution.

## Trigger Scintillator response

The ADC spectrum of the trigger scintillator using a H3165-10 PMT and a 10cm long, 1cm thick and 8mm wide scintillator bar is shown here

# The Fit Function

$F(x)=A\cdot \sum _{{n=0}}^{{N}}{\frac {\mu ^{n}e^{-}\mu }{n!}}\cdot ((1-w)G_{n}(x)+w\theta (x)I_{{G_{n}\otimes E}}(x))$
where $A$ is the number of events, $G_{n}(x)$ is the nth Gaussian convoluted with the pedestal-Gaussian and $I_{{G_{n}\otimes E}}(x)$ is the nth Gaussian convoluted with and exponential background with $w$ the probability of such an exponential background event occuring. The function $\theta (x)$ is a step function being 0 for $x and 1 for $x>P_{n}$.

$G_{n}(x)={\frac {1}{\sigma _{n}{\sqrt {2\pi )}}}}e^{{{\frac {-(x-P_{n})^{2}}{2\sigma ^{2}}}}}$ with $P_{n}=P_{0}+n\cdot P_{1}$ the ADC position of the nth Gaussian with width $\sigma _{n}={\sqrt {\sigma _{0}^{2}+n\cdot \sigma _{1}^{2}}}$. The index 0 refers to the pedestal and n=1,2,... is the number of photo-electrons.

$I_{{G_{n}\otimes E}}(x)={\frac {\alpha }{2}}e^{{-\alpha (x-P_{n}-\alpha \sigma _{n}^{2})}}\left[erf({\frac {|P_{0}-P_{n}-\alpha \sigma _{n}^{2}|}{\sigma _{n}{\sqrt {2}}}})+sign(x-P_{n}-\alpha \sigma _{n}^{2})\cdot erf({\frac {|x-P_{n}-\alpha \sigma _{n}^{2}|}{\sigma _{n}{\sqrt {2}}}})\right]$ is the convolution of the nth Gaussian with the exponential background leading to a tail for each Gaussian on the right side of the peak.

The Fit parameters are:

1. $P_{0}$ : pedestal position
2. $\sigma _{0}$ : pedestal width (FIXED from fit to pedestal only)
3. $P_{1}$ : single photo electron peak position
4. $\sigma _{1}$ : single photo electron peak width
5. $\alpha$ : exponential background
6. $w$ : probability of exponential background
7. $\mu$ : MEAN NUMBER OF PHOTO ELECTRONS

# Fiber types

The following fibers have been tested:

• Blue 81 MC lot:0801/103
Results:
The following plots show the PMT response to the Sr90 source positioned at 65cm, 165cm and 194cm from the photo cathode. In the pannels the value labled "mu" is the mean number of photo electrons detected by the PMT.
• Blue 78 MC lot:0801/104
Results:
Similar the following plots show the PMT response to the Sr90 source positioned at 61cm, 94cm and 194cm from the photo cathode. Again as above the value labled "mu" in the pannel is the mean number of photo electrons detected by the PMT.
• Green 3HF MC lot:0801/105
Results:
As above, plots for the green fiber the Sr90 source positioned at 65cm, 165cm.
• Saint Gobain (aka Bicron)
• GREEN BCF20 #5 Set2 1/8
Results:
This green fiber is about 200cm long and has been received in the latest batch in May 2008. The Sr90 source is put at 192cm and 100cm from the photo cathode of the PMT. The adc spectra are shown below.
• GREEN BCF20 #7 Set2 3/8
Results:
This green fiber is also about 200cm long and was received in May 2008. The fiber is not trait and actually rather wobbly after about 100cm till the end. The Sr source was positioned at 56cm, 100cm and 193cm from the end of the fiber touching the photo cathode of the XP2020 tube.
• GREEN BCF20 #620-MC-37 101796/18580 12/07/07<
Results:
This green fiber is also about 200cm long and was received in Dec. 2007 batch of green and blue fibers. The Sr source was positioned at 59cm, 100cm and 193cm from the end of the fiber touching the photo cathode of the XP2020 tube.
• BLUE BCF12 #612-MC-5 20535 12/07/07
Results:
This is a blue fiber about 180 cm long and was received in December 2007. The Sr source was positioned at 56cm, 101.5cm and 176cm away from the PMT photo cathode.
• GREEN BCF60 #660-MC-16 10854 12/07/07 line30
Results:
This is a green fiber of about 180cm length. It was received from Saint Gobain in December 2007. The Sr90 source was positioned at 56cm, 96cm and 196cm from the photo cathode.

# Long term stability

Run 46 3.1 million event were taken over a period of 2 days. The data are analyzed and fitted in junks of 500000 events. The spectra are shown below with a list of the obtained value for $\mu$.
During the same period the response to the LED was measured and the mean number of photo electrons extracted. The bias voltage on the LED was constant at -7.91 Volt. The trigger rate for the LED was about 10 Hz.

   Data Set         Sr90 $\mu$                LED $\mu$
--------------------------------------------------
1         2.62  +/- 0.02        1.89 +/- 0.02
2         2.63  +/- 0.02        1.87 +/- 0.01
3         2.62  +/- 0.03        1.86 +/- 0.02
4         2.57  +/- 0.03        1.88 +/- 0.04
5         2.60  +/- 0.02        1.88 +/- 0.01
6         2.65  +/- 0.03        1.82 +/- 0.01


This demonstrates that the data taking and resulting fit values are very stable with time.