Photomultiplier Tubes, or PMTs,
are electron tube devices which convert light into a measurable electric
current. They
consist of a photocathade and a series of dynodes
in an evacuated glass enclosure (see figure). When photons strike
the photocathode, electrons are emitted via the photoelectric effect.
The electrons are focused onto the first dynode by an electric field, and
when they strike the dynodes, more electrons are emitted. The current
of electrons multiplies as it moves down the successive dynodes until it
reaches the anode.
Basic Photomultiplier
Summary of BooNE's PMT Specs.
Made By Hamamatsu.
| Hamamatsu
Product No. |
Min. Effective Area (mm) | Spectral Response Range (mm) | Peak (mm) | Photocathode Material | Window Material |
| R5912 | 190 dia. | 300 to 650 | 420 | Bialkali
SbRb - Cs |
Borosilicate |
Photocathode:
The electrons
are emitted from the photoemissive cathode by the photoelectric effect.
When energy, in this case light, strikes the cathode, the electrons in
the atoms absorb the energy and leave the energy levels of the atoms.
A potential difference between the photocathode and the first dynode
causes the electrons to accelerate toward the dynode and an electric field
focuses them onto the dynode. For the photocathode's used in BooNE,
the maximum effeciency is apporximately 420 nm, or blue light. The
photocathode material is a bialkali composed of antimony an alkali
metals. This forms a semiconductor material, much more favorable
than metals or other photoelectric substances because semiconductors have
much greater quantum effeciencies for converting a photon to a usable electron.
Electron-Optical Input System:
The electron-optical
input system is responsible for collecting and focusing the electrons onto
the first stage of the multiplier section. It is therefore important
for the system to be capable of collecting, effeciently, as many electrons
as possible and making sure that the time it takes for an emitted electron
to travel from the photocathode to the first dynode is as independent as
possible of the point of the electron's emission.
Focusing Electrode:
The focusing
electrode is responsible for focusing the emitted electrons onto the first
stage of the multiplier section, the first dynode. Between the photocathode
and the first dynode is what's called an accelerating-electrode.
The accelerating-electrode looks much like a wall with a small gate in
it (noticable in the top figure). Positioned behind the gate is the
first dynode. The accelerating-electrode sets up a high potential
between
itself and the photocathode which accelerates the emitted electrons toward
the "wall." It is the job of the focusing electrode, using an electric
field, to focus the accelerating electrons through the "gate" and onto
the first dynode.
Electron-Multiplier System:
The electron-multiplier
system amplifies the primary photocurrent using a series of dynodes to
produce a measurable current at the anode of the photomultiplier.
When electrons strike the dynodes, energy is transmitted into the dynode
material allowing secondary electrons to escape. This is very similar
to the photoelectric effect except that the photon is replaced with an
electron. This process snowballs, thus creating a current.
The dynode materials are also made of semiconductors and insulators.
However, unlike the photocathode, the dynodes must supply a constant electric
field to keep the electrons accelerating through the multiplier.
Thus the secondary emission material must be deposited on a conducting
material. This can be done by forming an alloy of an alkali or alkaline
earth metal with a more noble metal. There are three important characteristics
of a good dynode material:
1) high secondary emission factor,
i.e., a high average number of secondary electrons emitted per primary
electron
2) stability of secondary emission
effect under high currents
3) low thermonic emission, i.e.,
low noise.
The PMT used by BooNE
is sturctured in what is called Box and Line form with 10 stages of dynodes.
Anode:
The anode is connected in series to a load resistance, the anode voltage
will fall as the anode current increases. Thus a change in the potential
difference between the anode and the last dynode will occur. This
allows for the entire chain of dynodes to be sustained at a constant voltage.
The output signal at the anode, then, is a current or charge pulse whose
total charge is proportional to the initial nimber of electrons emitted
by the photocathode.
Detailed
Specifications and Diagrams of BooNE's Photomultiplier Tubes R5912