A UV LED-based fast-pulsed photoelectron source for time-of-flight studies

Abstract

We report on spectroscopy and time-of-flight measurements using an 18 keV fast-pulsed photoelectron source of adjustable intensity, ranging from single photoelectrons per pulse to 5 photoelectrons per microsecond at pulse repetition rates of up to 10 kHz. Short pulses between 40 ns and 40 microseconds in length were produced by switching light emitting diodes with central output wavelengths of 265 nm and 257 nm, in the deep ultraviolet (or UV-C) regime, at kHz frequencies. Such photoelectron sources can be useful calibration devices for testing the properties of high-resolution electrostatic spectrometers, like the ones used in current neutrino mass searches. Comment: 16 pages, 11 figures

Full text

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arXiv:0902.2305v2 [physics.ins-det] 17 Jun 2009
A UV LED-based fast-pulsed photoelectron source
for time-of-flight studies
K. Valerius1,‡, M. Beck1, H. Arlinghaus1, J. Bonn2,
V. M. Hannen1, H. Hein1, B. Ostrick1,2, S. Streubel1,
Ch. Weinheimer1, and M. Zboˇ ril1,3
1Institut f¨ ur Kernphysik, Westf¨ alische Wilhelms-Universit¨ at M¨ unster, Germany
2Institut f¨ ur Physik, Johannes Gutenberg-Universit¨ at Mainz, Germany
3Nuclear Physics Institute ASCR,ˇReˇ z near Prague, Czech Republic
E-mail: valerius@uni-muenster.de
Abstract.
18keV fast-pulsed photoelectron source of adjustable intensity, ranging from single
photoelectrons per pulse to 5 photoelectrons per µs at pulse repetition rates of up to
10 kHz. Short pulses between 40ns and 40µs in length were produced by switching
light emitting diodes with central output wavelengths of 265nm and 257nm, in the
deep ultraviolet (or UV-C) regime, at kHz frequencies. Such photoelectron sources can
be useful calibration devices for testing the properties of high-resolution electrostatic
spectrometers, like the ones used in current neutrino mass searches.
We report on spectroscopy and time-of-flight measurements using an
PACS numbers: 29.25.Bx, 29.30.Aj, 29.30.Dn
‡ corresponding author

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A UV LED-based fast-pulsed photoelectron source for time-of-flight studies
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1. Introduction
We describe the application of ultraviolet light emitting diodes to produce
photoelectrons for a calibration source at electron energies in the keV range.
Electrostatic filters with magnetic adiabatic collimation (‘MAC-E filters’) [1, 2, 3, 4,
5] have proven to be ideal instruments for neutrino mass searches based on high-precision
measurements of the tritium beta decay spectrum near the endpoint [6, 7, 8, 9]. The
upcoming Karlsruhe Tritium Neutrino experiment (KATRIN) [8] aims at increasing
the sensitivity on m(νe) by an order of magnitude with respect to its predecessors.
This requires an electron spectrometer with a resolving power of E/∆E = 2 · 104,
corresponding to an energy resolution of ∆E ? 1eV at the endpoint of the tritium beta
spectrum (E = 18.6keV).
Calibration sources at these keV energies offering line widths of the order of or even
below this resolution are necessary for different purposes:
For continuous monitoring of fluctuations of the retardation potential, an electron
source with high stability both of the energy and the intensity is given by a
nuclear/atomic standard using 17.8keV conversion electrons from83mKr [10]. The K-
shell conversion line exhibits a natural width of Γ = 2.7eV [11], which is small enough to
monitor the retardation potential, but too broad for detailed studies of the transmission
properties of the MAC-E filter. Such a conversion electron source is presently being
developed for KATRIN [8, 12].
However, in order to investigate the details of the transmission properties of the
MAC-E filter, an electron source in the keV range is needed, which can deliver fast-pulsed
electrons with a sub-eV and well-defined energy spread and tunable beam intensities
between single electrons and up to 105electrons per second.
A pulsed electron source can serve as a means to test a particular “time-of-flight”
operational mode of the MAC-E filter that turns it from a high-pass into a band-pass
filter and at the same time allows a more detailed and faster characterization of its
transmission properties. This idea has been introduced in [13], where a first experimental
study validating the concept is described. Such a source requires a fast timing with pulse
lengths τ both smaller than the time-of-flight of the electrons through the MAC-E filter
and smaller than the expected variation of the time-of-flight§.
A potentially suitable method to set up an electron source fulfilling the
abovementioned requirements is to irradiate a cathode on high voltage with narrow-band
UV light. This can be realized in a simple manner using modern deep-UV light emitting
diodes (LEDs). Typical work functions Φ of metals lie in the range of about 4.2eV (Ag)
to 5.1eV (Au), with Φ = 4.4(2)eV [4] for stainless steel, corresponding to a wavelength
of the light of λ =hc
output wavelengths in the deep-UV range. However, such (high-power) laser systems are
rather expensive, immobile and require particular safety installations in the laboratory.
E≈ 282nm. Standard laser technology can also be used to obtain
§ In addition, a fast data acquisition and analysis is required, with a timing resolution again smaller
than both the time-of-flight and its expected variation.

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A UV LED-based fast-pulsed photoelectron source for time-of-flight studies
3
−1
−0.5
0
0.5
1
−3 −2−1 0 1 2 3
y position [m]
x position [m]
photoelectron
source
movable
detector
(i)
(i)
(ii)
(iii)
(iv)
(v) (vi)
spectrometer
Figure 1. Schematic of the experimental setup to test the photoelectron source.
From left to right: vacuum chamber with photoelectron source, electrostatic filter
with magnetic adiabatic guiding and detector.
include: (i) two superconducting solenoids to produce the magnetic guiding field of
the MAC-E filter (ii) the electrode configuration comprising a vacuum tank on ground
potential and an inner high-voltage electrode system, (iii) field-shaping air coils, (iv)
the vacuum chamber of the photoelectron source (see figure 2), (v) the stainless steel
cathode and (vi) an additional water-cooled coil for local enhancement of the magnetic
field strength. Magnetic field lines connecting the photocathode and the detector are
indicated as dashed curves. The analyzing plane of the spectrometer at x = 0 is defined
by the maximum of the retardation potential |Uspec| coinciding with the minimum
magnetic field strength Bmin.
The details shown in the sketch
By contrast, light emitting diodes operating in the deep-UV domain are advantageous
due to their versatility and ease of use, albeit offering only a fraction of the optical
output power? as well as inferior spectral and beam profile characteristics compared
to laser systems. Certain fields of application, however, benefit from this feature of
moderate and highly controllable output power combined with fast pulsing (pulse length
τ ? 25ns, switching slopes of the order of a few ns, see for example references [15, 16]) at
repetition rates of (1−10)kHz. In the particular application of photoelectron creation,
the drawback of line half-widths of the order of δλ ≈ 15nm typically obtained for such
LEDs can partially be compensated by an appropriate matching of the work function
Φ of the photocathode material and the photon energy Ephoton. If Φ and Ephoton are
chosen such that only photons from the high-energy part of the distribution can cause
photoemission, the energy spread of the photoelectrons will be reduced accordingly.
In this article we will show that photoelectrons created with UV light from a
pulsed LED can be used to investigate the transmission function of electrostatic electron
spectrometers and to perform time-of-flight measurements. The article is structured
as follows: Section 2 contains an overview of the setup and equipment used for our
? At emission wavelengths as short as 247nm, an optical power output of 300µW for continuous
operation and ? 10mW in pulsed mode has been reported [14].

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A UV LED-based fast-pulsed photoelectron source for time-of-flight studies
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towards spectrometer
windows
DN 40 CF
DN 100 CF
bellow
UV light source
photoelectrons
cathode
disc−shaped
DN 150 CF
Figure 2. Schematic of the photoelectron source: top view of the vacuum chamber
housing the disc-shaped cathode on high voltage (Usource) seen edge-on. The diameter
of the stainless steel disc is D = 180mm and its thickness about d = 4mm (with
a ∅12mm rounded bead at the rim to prevent field emission when high voltage is
applied). The chamber is equipped with four windows, two of which are made of UV-
light transmitting quartz glass that allows to illuminate the cathode with an external
UV light source. Photoelectrons are accelerated in the strong electric field and later
guided magnetically into the spectrometer. (The magnetic field lines are not shown.)
measurements, focusing on the photoelectron source and the retardation spectrometer.
In section 3 we demonstrate the use of short UV light pulses to obtain tunable
photoelectron rates. To determine the angular emission and the energy width of the
photoelectron source, we measured with the MAC-E-Filter at Mainz an integrated
energy spectrum and a time-of-flight spectrum, which are both presented in section
4. The paper concludes with a discussion of the results and an outlook in section 5.
2. Experimental setup
Figure 1 shows the setup used for the measurements reported on in this paper, the three
main components being the photoelectron source, the electrostatic spectrometer and an
electron detector.
Photoelectron source:
The photocathode consists of a mechanically polished plane disc made of stainless steel
with a diameter of D = 180mm and a thickness of d = 4mm, encased in a cylindrical
vacuum chamber with an inner diameter of 200mm. Stainless steel was selected as
the cathode material because of its easy handling and ready machinability, as well as
its work function of Φ = 4.4(2)eV, which is just within the photon energy reach of
commercially available UV LEDs. As light sources we employed UV LEDs of the type
A sketch of the photoelectron source is presented in figure 2.

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A UV LED-based fast-pulsed photoelectron source for time-of-flight studies
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T9B26C and T9B25C from Seoul Semiconductor Co., Ltd, with central wavelengths of
λcentral= 265nm and 257nm at a spectral width of ∆λFWHM= 15.3nm and 13.8nm,
respectively (compare measured light spectra in figure 3). The LEDs provide a maximum
cw optical power output of Popt≤ 400µW (T9B26C) and Popt≤ 150µW (T9B25C)
[17, 18], and each is equipped with a focusing ball lens. The energy spread derived from
the width of the UV light peak is FWHM = 0.27eV for the 265 nm LED at a central
energy of 4.68eV and FWHM = 0.26eV for the 257 nm LED at a central energy of
4.82eV. By comparing this with the work function of stainless steel of 4.4(2)eV it is
apparent that the low-energy part of the UV light may be cut off by the work function.
However, the photocathode surface was not especially prepared and the vacuum at the
photocathode of p ? 5 · 10−9mbar was not good enough to guarantee the homogeneity
of the work function of the photocathode. In section 4 we determine the energy width
of the photoelectrons to be compatible with a Gaussian standard deviation of about
0.2 eV, which matches the numbers of the expected work function inhomogeneity of
0.2 eV and energy width of the incident photons.
0
200
400
600
800
1000
1200
1400
1600
1800
230 240 250 260
wavelength [nm]
270 280 290 300
5.25.0 4.8 4.64.44.2
intensity [a.u.]
photon energy [eV]
257 nm LED
265 nm LED
Figure 3. Spectra of deep-UV LED type T9B25C (⋄) and T9B26C (•), recorded
with a grating spectrograph and a silicon PIN diode. In addition to the ultraviolet
peak, a less intense emission component of visible light is present (not shown). This
measurement was made at ILED= 12mA (continuous operation of the LED).
AFG
U
LED
U
R
Figure 4. Circuit for driving the UV LED via the function generator (AFG). The
voltage UAFGis supplied to the UV LED in series with a resistor of R = 90Ω. The
voltage at the UV LED ULEDwas typically 5V to 7.6V. The corresponding current was
measured via the voltage drop across the resistor R and varied between ILED< 1mA
and ILED≈ 20mA in pulsed operation of the LED.