Speaker
Dr
Joachim Wolf
(Karlsruhe Institute of Technology (KIT))
Description
The objective of the Karlsruhe Tritium Neutrino experiment (KATRIN) at the Karlsruhe Institute of
Technology (KIT) is the measurement of the effective electron neutrino mass with a sensitivity of
200 meV/c$^2$. A central component is the Main Spectrometer (MS), a MAC-E filter type
electrostatic high pass filter for electrons. It measures the energy of $\beta$-electrons from
tritium decay close to the endpoint at 18.6 keV with high precision at a low count rate of
$10^{-2}$ cps in the last eV of the spectrum. The
target value for the background rate in the KATRIN design is $10^{-2}$ cps. The large
ultra-high-vacuum chamber of the MS has a volume of 1240 m$^3$ and is operated at an
ultra-low pressure in the range of $10^{-11}$ mbar, which is required to reduce the
background rate. The pumping system of the MS consists of turbo-molecular pumps
and large-scale getter pumps (SAES St707 non-evaporable getter (NEG) strips).
The NEG strips ($^{219}$Rn, $t_{1/2} = 3.9$ s), as well as the stainless steel
walls ($^{220}$Rn, $t_{1/2} = 56$ s) are known to emanate small amounts of radon atoms,
increasing the intrinsic background rate by 0.5 cps, if no further countermeasures are taken.
Therefore, three LN2-cooled cryogenic baffles (1.7-m diameter), made of L-shaped copper panels,
have been installed in front of the NEG-pumps, reducing the transmission of radon into the main
volume. Radon from the walls and welds of the vacuum chamber, which is directly emanated into the
main volume, has to be removed quickly enough before it decays. However, radon does not stick
to a cold surface indefinitely. It either desorbs after a limited sojourn time, or it decays into
polonium while still on the cold baffle. In the first case, it can contribute again to the
background rate.
This talk describes the simulation of the effectiveness of the radon suppression with
the Test-Particle Monte Carlo (TPMC) code MolFlow+ and presents data from an extensive
measurement program. The simulation takes the effect of the half-lifes of the different
radon isotopes into account, as well as the temperature dependent sojourn time of radon on
a cold surface. By comparing measured rates with TPMC simulations for different sojourn times
(temperatures), we learned more about possible surface conditions of the baffles (Cu, Cu2O, H2O)
and the corresponding desorption enthalpies. The measurements with the MS showed that the radon
suppression with cold baffles works sufficiently well, so that the remaining background is no
longer radon dominated. This work has been supported by the German BMBF (05A14VK2).
Summary
Short-lived radon isotopes are a serious source of background for the measurement of the
neutrino mass with the KATRIN experiment. This talk describes a method to suppress the radon
rate with cold baffles in the ultra-high vacuum chamber of the KATRIN Main Spectrometer and
compares simulations with measured data. The effectiveness of the method depends not only on
the half-life of the radon isotopes, but also on the temperature of the cryogenic baffles.
Primary author
Dr
Joachim Wolf
(Karlsruhe Institute of Technology (KIT))
