KAMLAND pulser

Kamland Note: Calibration−011107
A Compact LED Light Source for KamLAND
Gerald Przybylski, Herbert Steiner, Fred Bieser, John Wolf
Lawrence Berkeley National Laboratory, Berkeley, CA 94720
November 7, 2001
We have constructed two compact free−running light pulsers using Nichia NSHU550E (uv) and
NSPB520S (blue) LEDs to generate pulses having a width, ß, of about 2 ns at a rate of 100 Hz.
Initially they will be suspended from a string or cable, and then used for the initial timing
calibration of the 2000 photomultiplier tubes of the KamLAND detector. A typical pulse shape
is shown in figure 1.
Figure 1: LED light pulse as viewed by two photomultiplier tubes
(a) Hamamatsu R1924 (black) and (b) RCA 8575 (green).
Time scale is 10 ns per division.
Even faster pules (¯100 ps) could be generated with the newly−developed laser LEDs (e.g.,
Nichia NLHB 500X−01), but they are still quite expensive, so we have not yet used them.
The schematic of the driving circuit is shown in figure 2.

Figure 2: Schematic drawing of the LED light pulser circuit
The circuit is very compact, with all electronic components mounted on a printed circuit card
about 2 cm x 3 cm in size. At present the pulse rate of our units is about 100 Hz, but it can be
adjusted by varying the resistor (R9) and/or the capacitor (C12). Two small disk−shaped 3−volt
batteries (Panasonic #CR1220−3V) in series, and good for about ten hours of continuous
operation, are used to power the circuit. The batteries can be exchanged easily. Alternatively it
is possible to replace the batteries by a metallic slug, and supply power with an external power
supply. In our battery−driven version both of the diodes D1 and D2 are shorted with pieces of
wire in order to maximize the light intensity. Lower light levels are possible when the diodes
are not bypassed. Of course, with a continuously variable external power supply the LED light
output can be set to any desired level. It is possible to insert the LED into a 3/4" diameter Delrin
diffusing ball to produce a more isotropic light distribution.
The circuit is spring−mounted on a stainless steel plug, which is used to insert and then
hermetically seal the whole assembly inside a hollowed−out acrylic cylinder. A hermetic BNC
connector together with a tethered shorting plug can be used to switch the batteries on and off
from the outside. Externally supplied power can be injected into the same BNC connector via a
coaxial cable. The weight of the unit when submerged in the liquid scintillator is about 50 gm.
Additional weights can easily be attached if desired. Some photos of the pulsers are shown in
figures 3 and 4.

Figure 3: Photo of the assembled LED light pulsers.

Figure 4: Photo of LED circuit, stainless steel mounting plug and acrylic tube.