# Test of a Light Emitting Diode Flasher with the Fermilab Water Tank.

**François Montanet^2^, Hal Spinka^1^, Derek Strom^2^**

1)  Argonne National Lab. , IL , USA

2)  Fermilab, Batavia, IL, USA

October 12, 2001

# Abstract

We report on measurements made on the Fermilab surface detector
prototype (FATII) of a Light Emitting Diode (LED) system. The setup was
similar to future LED system, intended for the monitoring and
calibration of the pre-production surface detectors. The LED was
installed on one of the available water fill ports i.e. in a very much
off centered position. Despite this, it was possible to show that the
light scattered back by the liner was yielding well-balanced signals on
the three PMT's. We conclude from this study that it is not necessary to
aim at installing the LED ports at the very center of the future liners
but that an off centered position is acceptable.

## Introduction

It is planned to install Light Emitting Diodes (LED) flashers in each
tank of the pre-production Surface Detectors (SD). Although they may not
be required for a successful calibration, it was considered that they
would ease the control and monitoring of the array (mainly fast
functional tests and linearity measurements. They would also provide a
simple first order check of the entire SD station (detector+electronics)
before deployment (in the assembly building, before transport and water
filling). The LED system must be simple, reliable and add a minimal load
to the rest of the project. Recent discussions have led to two a priori
decisions:

-   The LED system should consist of 2 independent flashers in order to
    test linearity.

-   The LED's should be installed in a dedicated port equipped with a
    window (to preserve the water sealed). Further more, it was
    advocated that in order to keep the light yield naturally balanced
    on the 3 PMTs, this port should be in a central position, on the top
    of the liner.

Installing the LEDs in a central position is not without drawbacks: this
point is not accessible after the liner is installed within the tank.
Therefore, it is necessary to:

-   Find a procedure to install the flashers as an integral part of the
    liner, before the liner is placed in the tank: the flasher, the
    cables connecting it and the supporting port have to be compact,
    flexible and robust enough to survive the folding, the installation
    and the inflation of the bag. It should also be made such as not to
    damage the bag. Although this is not considered impossible, it is
    not a very easy task.

```{=html}
<!-- -->
```
-   When the bag is in place and inflated or filled with water, it
    cannot be maintained. This raises the question of the long term
    reliability of the LED flashers. We plan to use blue LEDs because
    their emission wavelength (470 nm) is a good match to the mean
    wavelength of detected Cherenkov photons. These ultra-bright blue
    LEDs, which were still cutting edge technology not very long ago,
    are nowadays used in general-purpose lighting applications and
    progress in their manufacturing has led to highly reliable and long
    lifetime products. Part of the flasher electronics is the LED driver
    itself, which is a high voltage, high current, and very short time
    pulser. To avoid time smearing and EM noise emission, this tiny
    circuit has to be placed as close as possible to the LED itself i.e
    inside the dedicated port. If installed in a central inaccessible
    position, it would also become a non-maintainable electronic device
    of probably poorly know MTBF. One could live with this, assuming the
    LED flashers as "consumable" devices only useful during detector
    installation and commissioning but we will probably wish to ask more
    for our money!

An alternative solution to the two above problems would be to install
the LEDs still in a dedicated port as central as possible but positioned
in a way as to remain accessible via the largest hatch of the tank. The
radius of the tank is 1.8 m, the PMT's and the hatches are 1.2 m away
from the centre. The large hatch is 56 cm in diameter and is 30 cm above
the normal water level. My estimate is that a small LED port that would
be situated about half way (i.e. \~60 cm) between the centre of the bag
and the PMT, would still be easily accessible by a normally constituted
human being (see Figure 1).

Obviously, a question is raised by this proposal: is the sharing of the
light yield between the three PMTs affected by the off centred LED
position? Is the unbalancing acceptable? Using data taken in September
2001 in the Fermilab water tank using a LED pulser, we have tried to
answer this question.

Figure 1: On this schematic, one sees that a LED port located half way
between the PMT and the center of the bag is still accessible, within a
45° angle and a distance of less than 50 cm of the hatch.

## The LED setup in the Fermilab Water Tank FATII

### The Fermilab Water Tank FATII

The Fermilab water tank called hereafter FATII, is a full size surface
detector prototype identical in size to the southern observatory tanks.
It is composed of a cylindrical polyethylene tank of full dimensions,
10 m^2^ surface area and about 1.2 m height. The tank interior is
accessible through 3 hatches, above the location of the PMTs. The water
is contained in a liner made of a laminate of polyethylene and Tyvek^®^
identical to those used in the Engineering Array. This liner is equipped
with dome kits with floppy windows intended for 8" PMTs. This liner was
installed in the tank in March and filled with high resistivity
(\~18 MΩ) purified and filtered water. The total volume of water
enclosed is about 12,000 liters. The tank is linked to the control room
via 30m long cables for providing the High Voltage supply and for
transporting the signals back to the DAQ system. The readout and trigger
electronic is base on standard NIM and CAMAC modules as well as on a 4
channel digital oscilloscope Tektronics TDS3054. The CAMAC and the scope
are controlled and readout using a GPIB bus via a PCI GPIB interface
card from National Instruments. The data acquisition program runs on
Windows NT, it is written in C++, and uses Root. The data are saved on
local disks as Root histograms and trees.

### Installing an LED in a water fill port

Obviously, the liner we are using in the Fermilab prototype was produced
well before a special LED port was envisaged. We will have to use
existing ports i.e. water fill ports. The liner presently used in FATII
has the following particularity: the kits were assembled in China such
that the 3 water fill ports are all positioned on the radius of their
associated PMT and in between the PMT and the center (see Figure 2). The
PMTs are 120 cm away from the center and the fill ports are 92 cm from
the center. The distance from the port to the axis of the nearby tube is
\~ 28 cm. The distance to the two other tubes is \~ 184 cm. PMTs are
numbered 1, 2, and 3. An LED is installed in the fill port near PMT 2.

![](media/image1.png){width="0.5118110236220472in"
height="0.544488188976378in"}![](media/image2.png){width="0.7251968503937007in"
height="0.7145669291338582in"}![](media/image3.png){width="0.7251968503937007in"
height="0.7145669291338582in"}

Figure 2: A drawing of the liner used in FATII. Note that the water fill
ports are positioned on the radius in between the PMT and the center.
The distance from the center to the fill port is 92 cm. The distance
from the fill port and the nearby PMT axis and is \~28 cm. It is
\~184 cm to each one of the two other PMTs.

We did not received in time a specially designed "plug" to host the LED
and its driver inside the water fill port. We had instead to do a quick
and dirty installation by perforating one of the spare port caps
available at Fermilab with a hole of 5 mm in diameter. The LED head was
forced into that hole, making the assembly reasonably watertight (see
Figure 3). The LED is thus directly in contact with the water, although
we cannot exclude that a small air bubble is trapped in the port. It is
also important to note that as the fill port is made of white
polyethylene and as there is no collimation of the LED light, the fill
port itself acts as a light diffuser therefore scattered light that can
go directly to the nearby PMT.

![](media/image4.png){width="3.0362204724409447in"
height="1.580708661417323in"}Figure 3: The water fill port and
perforated cap. The LED is inserted lightly into the 5 mm hole in the
cap. It is in direct contact with the water. The fill ports are made of
white polyethylene.

## The Sheffield LED pulser

A complete driver and its interface to the SD Front End electronics is
currently being developed by our colleagues from the Moscow State
Engineering Physics Institute. In order to get an answer quickly, we
used an already existing pulser, which was developed for the ANTARES
collaboration by people from the Univ. of Sheffield UK \[2\] , from an
earlier design by J.S.Kapustinsky et al \[1\].

![](media/image5.png){width="2.425196850393701in"
height="2.0244094488188975in"}![](media/image6.png){width="1.8507874015748031in"
height="2.043700787401575in"}This is a very compact, robust and simple
driver, with a very small time jitter. Associated with ultra bright blue
LEDs (for example Agilent HLMP CB30 or Nichia NSPB300A), it provides
light pulses which are fast (FWHM \<5 ns) and powerful (up to 40pJ or
9.5×107 photons/pulse at 465nm). The driver is triggered on the leading
edge of a logic (TTL or NIM) pulse sent on a simple twisted pair cable.
The pulse energy can be adjusted from 0 to 40pJ via a DC bias
superimposed on the same pair (Figure 5). A firm in the UK is currently
commercially producing these drivers as a small (13×27 mm) SMC board
(see Figure 4).

Figure 4: The Sheffield LED driver.

![](media/image7.png){width="4.360416666666667in"
height="2.8979166666666667in"}Figure 5: The energy per pulse delivered
by 4 different Agilent HLMP CB30 diodes as a function of the bias
voltage applied to the pulser (taken from ref \[2\]).

## LED versus Muons

It is worth comparing the light yield delivered by a LED pulser of the
kind we plan to use with the Cherenkov photon yield from a muon or from
a shower.

As we already said, the energy per pulse from the LED can be adjusted
from 0 to 40pJ, which is close to 10^8^ photons/pulse. Note that these
are 465 nm wavelength photons (the spectral width of the LED is
\<30 nm).

A vertical ultra-relativistic muon traverses L = 120 cm of water during
which it produces

$L\ \left( \alpha^{2}/r_{e}m_{e}c^{2} \right)\ \text{sin}^{2}\theta_{c}\ \text{photons/eV}$,

with
$\alpha^{2}/r_{e}m_{e}c^{2} = \text{370}\text{  eV}^{\text{-1}}\text{cm}^{\text{-1}}$
and $\ \text{sin}^{2}\theta_{c}\  \approx 0\text{.}\text{43}$. This
corresponds roughly to $2\text{.}8 \times \text{10}^{4}\text{ photons}$,
assuming an effective spectral width of 1.5 eV. The number of
photoelectrons detected by a PMT is given by integrating over the
spectrum:

$$N_{p\text{.}e\text{.}} = L\frac{\alpha^{2}}{r_{e}m_{e}c^{2}}\int_{}^{}\varepsilon_{\text{coll}}(E)\ \varepsilon_{\text{det}}(E)\ \text{sin}^{2}\theta_{c}(E)\ \text{dE}$$

where $\varepsilon_{\text{coll}}$is the collection efficiency and
$\varepsilon_{\text{det}}$is the detection efficiency (quantum
efficiency). For bialkali cathode PMTs like the one we are using,

$\int_{}^{}{\varepsilon_{\text{det}}(E)}\ \text{dE} \approx 0\text{.}\text{27}$.

The average number of photoelectrons measured in good running conditions
with tanks prototypes using well-defined vertical muons is about 40 to
50 p.e. per PMT. One can therefore infer that the collection efficiency
is of the order of:

$$\int_{}^{}\varepsilon_{\text{coll}}(E)\ \text{dE} \approx 1\text{\%}$$

and the total detection efficiency is:

$\int_{}^{}{\varepsilon_{\text{coll}}(E)\ \varepsilon_{\text{det}}}(E)\ \text{dE} \approx 3 \times \text{10}^{- 3}$.

Assuming that the collection efficiency is comparable for Cherenkov
photons produced across the tank volume and for mono-chromatic photons
produced by a LED at the top of the volume, one can convert the LED
energy/pulse to photoelectrons per PMT:

$N_{p\text{.}e\text{.}} \approx \text{10}^{8} \times \int_{}^{}{\varepsilon_{\text{coll}}(E)\ \varepsilon_{\text{det}}(E)\ \text{dE}} \approx 3 \times \text{10}^{5}$p.e.

and to VEM units:

$$N_{\text{VEM}\text{.}} \approx \text{6000}\text{VEM}$$

This is more than enough to test the PMTs plus electronics linearity and
almost comparable to the light yield induced by a shower at some
distance from its core.

## Studying the light yield balance

The Fermilab prototype is currently equipped with three Hamamatsu R5912,
Ø200 mm, 10 stages tubes. These tubes are the same used in part of the
EA tanks, they are high gain tubes and are therefore not suitable for
high illumination. They are highly non-linear and saturate for anode
current \~ 40 mA at a gain of 10^6^. We were thus not able to explore
the LED at high values of the energy/pulse. Instead we set the LED
driver bias voltage such as to produce muon like pulses. Observing the
PMT signals on a oscilloscope, we lowered the bias voltage until the LED
induced signal was very much similar in amplitude and charge as the
signal from a few VEMs. This corresponds to bias voltage values just
above the threshold in Figure 5.

The gain versus HV curves of all three PMTs were measured prior to their
installation in the tank, using fits to single photoelectron spectra.
The HV of the PMTs was then set to correspond to a gain of 6.4×10^6^.
This gain was chosen because it is close to the one of the high gain
channel (amplified dynode) when low gain tubes will be run at 2×10^5^.
It is also convenient to note that for a gain = 6.4×10^6^, the charge
corresponding to 1 p.e. is 1pC. The VEM calibration is established using
a dataset of muon events that pass a minimum bias trigger (2-fold
coincidence of the tank's PMTs) and a further selection requiring some
balance of the charges seen by the three PMTs. The ADC spectra
corresponding to this internal trigger are fitted to obtain the number
of p.e. per VEM and per tube (). The fit function is the sum of a
gaussian for the muon peak and of an ad hoc function describing the
underlying background. The fits to these spectra give mean values of
43.5, 61.1 and 40.6 p.e./VEM for PMT1, PMT2 and PMT3 respectively.

To avoid possible bias in the gain measurements and to take into account
possible differences in the quantum efficiency and in the optical
coupling of the PMT to the water, we will express the measured charges
in units of VEM using the above individual PMT calibration.

The LED ADC spectra are shown in Figure 7. The fits to these spectra
give mean values 123.2, 259.9 and 98.69 pC for PMT1, PMT2 and PMT3
respectively. When normalized to VEM, this reads **2.63 VEM**,
**4.58 VEM**, and **2.49 VEM** respectively for PMT1, PMT2 and PMT3. We
recall that the LED is very close to PMT2. The solid angle ratio for
direct light (i.e. the ratio of the distance square to PMT1&3 to the
distance square to PMT2) is \~ 43. The observed light yield ratio
between PMT2 and PMT1 or PMT3 is \~ 1.8. One can thus already conclude
that the amount of direct light seen by the PMTs is small compared to
the isotropized light scattered by the liner.

![](media/image8.png){width="5.990972222222222in"
height="2.0145833333333334in"}Figure 6: ADC spectra obtained from muons
calibration (internal trigger) for the three PMTs. The abscissa is in pC
(\~ equivalent to photoelectrons units for the gain of 6.4×10^6^ used
here). The gaussian fits gives mean values of 43.5, 61.1 and 40.6
p.e./VEM for PMT1, PMT2 and PMT3 respectively.

![](media/image9.png){width="6.0in" height="2.157638888888889in"}Figure
7: ADC charge spectra obtained flashing the LED. The abscissa is in pC.
The gaussian fits gives mean values of 123.2, 259.9 and 98.7 pC for
PMT1, PMT2 and PMT3 respectively.

### Pulse shape analysis.

One way to disentangle between the direct light (i.e. the light
traveling directly from the LED port to the PMT) and the light scattered
back by the liner, is to analyze the PMTs pulse shapes using the digital
scope traces. As in the case of Cherenkov light, the LED light pulse is
very short (few ns) but the light is scattered by the liner and bounces
back and forth, producing a signal with an exponential decay with a
characteristic time of the order of 100 ns. On the other hand, if part
of the light goes directly from the LED to one of the PMTs, this should
show up as a few ns spike in front of the signal.

The vertical muon average pulse shape is shown in Figure 8. The
exponential fits give a lifetime of the light in the tank of roughly
100 ns. After-pulsing in the PMTs dominates the waveform for time larger
than 500 ns after the pulse start. This tends to increase artificially
the fitted lifetime. The fit is therefore restricted to the unbiased
region. One can have a feeling of the remaining systematic error on the
lifetime measurement by comparing the fit results on the individual PMT
signals: they should a priori give the same value. From the measured
differences, we estimated a systematic error of ± 5 %. One should note
that two kind of data set were used and compared for this lifetime
analysis: vertical muons as defined coincidence of scintillation
counters (external trigger), and minimum bias muons asking for a 2-fold
coincidence of the tanks PMTs (internal trigger). Both data samples gave
very similar results in terms of lifetime (consistent within ± 2 ns) as
can be seen in table Table 1.

![](media/image10.png){width="6.927083333333333in"
height="2.1333333333333333in"}

Figure 8: Waveforms from PMT1, PMT2 and PMT3 averaged over 600 vertical
muon events (external trigger). The time abscissa is in ns. The
exponential fits gives 108 ns, 103 ns, and 96 ns. The effect of
after-pulses is clearly seen for times larger than 500 ns after the
pulse start.

Table 1: The light exponential decay time as fitted from averaged
waveforms from different datasets:\
*external trigger* events are central vertical through going muons
defined by scintillator paddles (fits from Figure 8), *internal trigger*
are events from a minimum bias trigger obtained by asking for a 2-fold
coincidence on the tank PMTs and a further selection requiring some
balance between the 3 measured ADC charges, and *LED pulses* are flasher
events.

  --------------- ------------------ ------------------- ------------------
                  PMT1               PMT2                PMT3

  External        108.37 ± 0.41 ns   102.96 ± 0.30 ns    96.53 ± 0.40 ns
  trigger                                                

  Internal        108.09 ± 0.40 ns   100.95 ± 0.281 ns   94.93 ± 0.39 ns
  trigger                                                

  LED pulses      106.51 ± 0.05 ns   94.31 ± 0.03 ns     99.26 ± 0.06 ns
  --------------- ------------------ ------------------- ------------------

The same analysis was then repeated but using LED pulse events. The
waveforms averaged over 1000 flashes are shown in Figure 9. Again, the
basis shape is an exponential decay with a time constant very close to
the one obtained with muons. It is also clear from Figure 9 that a
different structure shows up at the beginning of the waveform from PMT2.
This spike is interpreted as being produced by "direct" light. If one
looks closely at the pulse rise time, one finds out that the pulse from
PMT2 starts also \~ 8 ns earlier than those of PMT1 and PMT3, as indeed
we expect from the distance difference to the LED (1.56 m × 5 ns/m).
This time difference is more clearly seen in Figure 10 in which the
three traces are super-imposed (after normalization discussed below).

### Light yield balance.

The waveform of PMT2 is thus the sum of a "diffused" contribution
analogous in shape to the waveforms of PMT1 and PMT3 (i.e. exponential),
and of a peak of width \~ 8 ns of "direct" light coming in front of the
diffused component. To try visualizing this, to quantify the fraction of
the "direct" and to study the balance between the "diffused" components
measured by the three PMTs, we have done the following:

We first normalized each one of the LED waveforms to the number of
p.e.'s per VEM measured with the corresponding PMT. This accounts for
the differences in the optical coupling and in the quantum efficiencies.
The result is shown in Figure 10. One sees that the "diffused" component
from PMT2 (in black) matches very well those of PMT1 and PMT3. We can
than subtract the diffuse component from PMT2 using the waveform from
PMT1 or PMT3. The result (PMT2 -- (PMT1+PMT3)/2) is show by the blue
histogram in Figure 11. This visualizes the "direct" light component of
PMT2.

![](media/image11.png){width="6.7027777777777775in"
height="2.0694444444444446in"}Figure 9: Waveforms from PMT1, PMT2 and
PMT3 averaged over 1000 LED flasher events. The time abscissa is in ns.
The exponential fits gives 106 ns, 94 ns, and 99 ns, very close to the
muon events fits with the noticeable exception of PMT2 for which the
lifetime is found to be significantly shorter. This is understood to be
due to an underlying contribution from short distance light
trajectories. The effect of after-pulses is even clearer than with
muons.

Finally, to quantify the fraction of direct light and the relative
balance of the diffuse light, we simply integrate the normalized
histograms. The results are given in Table 2 .

![](media/image12.png){width="3.2895833333333333in"
height="3.0909722222222222in"}Figure 10: The waveforms for PMT2 (in
black) PMT1 (in red) and PMT3 (in green) after normalization using the
calibration of p.e./VEM from section 5. The leading edge time difference
is clearly seen as well as the spike of direct light. The scattered
light exponential decay components are almost identical in shape and in
intensity.

![](media/image13.png){width="3.486111111111111in"
height="3.282638888888889in"}Figure 11: The "direct" light component on
PMT2, shown in blue, has been isolated by subtracting the normalized
waveform of PMT2 by the average of the normalized waveforms of PMT1 and
PMT3.

Table 2: Estimating the fraction of direct light and comparing the
scattered light components. Here, w~i~(t) is the waveform normalized by
the calibration factor, Q~i~ is the integrated signal expressed in units
of VEM, D~21~ and D~23~ are the estimate for the "direct" light
component estimated from the difference ot PMT2-PMT1 and from PMT2-PMT3,
\<D~2~\> is the average of the previous estimates. Finally, S~2~ is the
estimate of the "scattered" light component for PMT2. On can see that
the scattered light component is indeed balanced on the three tubes to
better than 10%.

  ----------------------------------------------------------------------------------------------- --------------------------------------------------------------- ----------------------------------------------------------------------------------------------- -------------------------------------------
  $$Q_{1} = \int_{}^{}{w_{1}(t)\text{dt}}$$                                                       $$Q_{2} = \int_{}^{}{w_{2}(t)\text{dt}}$$                                                                                                                       $$Q_{3} = \int_{}^{}{w_{3}(t)\text{dt}}$$

  Q~1~ = 3.09 VEM                                                                                 Q~2~ = 4.60 VEM                                                                                                                                                 Q~2~ = 2.71 VEM

  Q~2~/Q~1~ = 1.48                                                                                                                                                Q~2~/Q~3~ = 1.69                                                                                

  $$D_{\text{21}} = \int_{}^{}{\left( w_{2}(t) - w_{1}(t) \right)\ \text{dt}} = Q_{2} - Q_{1}$$                                                                   $$D_{\text{23}} = \int_{}^{}{\left( w_{2}(t) - w_{3}(t) \right)\ \text{dt}} = Q_{2} - Q_{3}$$   

  D~21~ = 1.51 VEM                                                                                                                                                D~23~ = 1.89 VEM                                                                                

  D~21~ / Q~2~ = 32 %                                                                                                                                             D~23~ / Q~2~ = 41 %                                                                             

  \<D~2~\> = (D~21~ + D~23~) = 1.70 VEM                                                                                                                                                                                                                           

  \<D~2~\> / Q~2~ = 37%                                                                                                                                                                                                                                           

  $$\begin{array}{r}                                                                              $$\begin{array}{r}                                                                                                                                              $$\begin{array}{r}
  S_{1} \equiv Q_{1} \\                                                                           S_{2} \equiv Q_{2} - D_{2} > = 2\text{.}\text{90}\ \text{VEM}                                                                                                   S_{3} \equiv Q_{3} \\
  3\text{.}\text{09}\ \text{VEM}                                                                  \end{array}$$                                                                                                                                                   2\text{.}\text{71}\ \text{VEM}
  \end{array}$$                                                                                                                                                                                                                                                   \end{array}$$
  ----------------------------------------------------------------------------------------------- --------------------------------------------------------------- ----------------------------------------------------------------------------------------------- -------------------------------------------

## Conclusions

Although the LED was installed in FATII in a very off-centred position,
we found that the balance between the PMTs is surprisingly good. The
nearby (28 cm) PMT is affected by an excess of "direct" light which
amounts to \~ 37 % of the total light. After subtracting this component,
the remaining scattered light is balanced between the three PMTs to
about 10%. Further more, assuming that the direct light component can be
eliminated in the future by improving the LED support and window, and
that a the dedicated LED port can be installed somewhat half way between
the centre and the PMT's, we are confident that the light of the LED
pulser will be naturally balanced between the PMTs to a very good
precision.

## Bibliography

\[1\] J.S.Kapustinsky et al \[1\]. *Nucl. Instr. Meth,* A241 612 1985.

\[2\] J.E.McMillan, L.F.Thompson, R.A. Brook, private communication and\
ANTARES internal note CALI/2000-008 and technical note 3-INS-03-01-A\
ANTARES Technical Design Report http://antares.in2p3.fr/