- Email: [email protected]
Measurements with the KATRIN pre-spectrometer
Progress in Particle and Nuclear Physics 57 (2006) 49–57 www.elsevier.com/locate/ppnp
Review
Measurements with the KATRIN pre-spectrometer L. Bornschein ∗ , for the KATRIN collaboration Institute for Experimental Nuclear Physics, University Karlsruhe (TH), 76021 Karlsruhe, Germany
Abstract The objective of the KArlsruhe TRItium Neutrino experiment (KATRIN) is a direct measurement of the absolute mass of the electron (anti)neutrino by means of precise spectroscopy of the tritium β-spectrum near its endpoint. The pre-spectrometer is part of the KATRIN reference set-up where it will work as a pre-filter for low energy β-decay electrons that are inessential for the determination of the ν-mass. Since its delivery in autumn 2003 the pre-spectrometer has been the first major hardware component of KATRIN in operation at Forschungszentrum Karlsruhe (FZK). The vacuum measurements were successfully completed in early 2005. The main results are an outgassing rate for the stainless steel surface of the pre-spectrometer of 10−12 mbar l/s cm2 at room temperature and a final pressure below 10−11 mbar. This corresponds to the specification of the main spectrometer. The amount of Non-Evaporable-Getter (NEG) strips needed can be restricted to about 3000 m and the additional cooling of the main spectrometer is optional, if a combined pumping system of NEG and turbo-molecular pumps (TMPs) is installed, that will provide a sufficient pumping speed. The modification of the pre-spectrometer set-up for the electromagnetic measurements is nearly completed, and the measurements will start in spring 2006. c 2006 Elsevier B.V. All rights reserved. Keywords: Neutrinos; Neutrino mass; Vacuum; Tritium; Hydrogen; Astroparticle physics; Particle physics
1. Introduction With the evidence for massive neutrinos from recent ν-oscillation experiments (e.g. [1–5]), one of the most fundamental tasks of particle physics over the next few years will be the ∗ Corresponding address: Universit¨at Karlsruhe (TH), Institut f¨ur experimentelle Kernphysik, Postfach 6980, D-76128 Karlsruhe, Germany. E-mail address: [email protected].
c 2006 Elsevier B.V. All rights reserved. 0146-6410/$ - see front matter doi:10.1016/j.ppnp.2005.12.011
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Fig. 1. Schematic view of the KATRIN reference set-up. The main components of the system comprise a windowless gaseous tritium source (WGTS, (a)), an electron transport and differential pumping system with space for quench condensed sources (b), two MAC-E-Filters (pre-spectrometer and main spectrometer, (c)) and an electron detector (d). The overall length of this linear set-up amounts to about 70 m. The magnetic flux transported from source to detector will be 191 T cm2 .
determination of the absolute mass scale of neutrinos, which has crucial implications for cosmology and particle physics. In cosmology, neutrino hot dark matter could play an important role in the evolution of large scale structures (LSS). In particle physics, a measurement of m ν would discriminate among different ν-mass models, commonly grouped as either of hierarchical type (m 1 m 2 m 3 ) or of quasi-degenerate type (m 1 m 2 m 3 ). So far, the study of LSS evolution with galaxy surveys (2dFGRS, SDSS) and cosmic microwave background radiation experiments (WMAP) has led to inconclusive statements on neutrino masses [6,7]. Therefore, it is essential to probe sub-eV neutrino masses with laboratory experiments. The spectroscopy of β-decay just below the kinematic endpoint is the only direct and model independent way to investigate neutrino masses with a sensitivity in the (sub-)eV range [8]. These experiments are relying just on the relativistic energy–momentum relation E 2 = p 2 c2 + m 2 c4 . The current tritium β-decay experiments at Troitsk [9,10] (using a windowless gaseous source) and Mainz [11] (using a quench condensed source) have approached their sensitivity limit of about 2 eV/c2 . The KArlsruhe TRItium Neutrino (KATRIN) experiment is a next-generation direct neutrino mass experiment with a sensitivity to cosmologically important sub-eV ν-masses [8,12]. 2. The KATRIN experiment The KATRIN experiment combines an ultraluminous windowless gaseous molecular tritium source (WGTS) with a high resolution electrostatic retarding spectrometer (MAC-E-Filter) to measure the spectral shape of β-decay electrons close to the endpoint at 18.6 keV with unprecedented precision. If no neutrino mass signal is found, the KATRIN sensitivity after three years of measurements is m ν < 0.2 eV/c2 (90% C.L.); a ν-mass signal of m ν = 0.35 eV/c2 can be measured with 5σ evidence. A schematic view of the KATRIN reference set-up is shown in Fig. 1. The WGTS (a) will consist of a 16 m long cylindrical tube of 90 mm diameter filled with molecular tritium gas of high isotopic purity (>95%) and kept at 27 K. The tritium gas will be injected through a capillary at the middle of the tube. It diffuses to both ends of the tube, resulting in a decrease of the tritium density by at least a factor of 20 from the injection point to the ends of the tube. This tritium inventory will deliver 1011β-decay electrons per second. The tritium tube will be placed inside a chain of superconducting solenoids. The solenoids will generate a homogeneous magnetic field of 3.6 T, which adiabatically guides the decay electrons to the tube ends. At the end of the WGTS a first differential pumping section will recover more then 99.9% of the tritium molecules. They
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Fig. 2. Two electrostatic spectrometers of KATRIN.
will be sent via a purification system to a tritium buffer vessel. From there the tritium is sent back to the “pressure controlled WGTS Supply buffer Vessel” and reinjected into the WGTS tube. This so called “inner loop” guarantees a high tritium throughput with a reasonable tritium inventory (for a more detailed description see [13]). The WGTS is followed by an electron transport system (Fig. 1(b)), which will guide the β-decay electrons adiabatically to the spectrometer, while at the same time reducing the tritium flow rate towards the spectrometer. Since the maximal allowed rate of tritium flow into the prespectrometer is of the order of 10−11 mbar l/s, a tritium suppression factor of more than 1010 is needed between the outlet of the WGTS tube and the entrance of the pre-spectrometer. This will be achieved by a combination of differential (DPS-F) and cryogenic (CPS-F) pumping sections. The cryo-pumps consist of the liquid helium cold surface of the transport tube, covered by a thin layer of argon frost for better trapping. The split coil magnet will enable the insertion of quench condensed sources into the beamline. The energy of the β-decay electrons is analyzed in a system of two MAC-E-Filter (Magnetic Adiabatic Collimation combined with an Electrostatic Filter) spectrometers (Fig. 1(c)): a prespectrometer and a large main spectrometer (see Fig. 2). On their way to the detector the β-decay electrons first have to pass the smaller pre-spectrometer, a stainless steel UHV vessel with a length of 3.4 m, a diameter of 1.7 m and a volume of 8.5 m3 . In the normal tritium mode of the experiment it works at a fixed retarding potential, acting as a pre-filter for electrons with energies below 18.3 keV. The rejection of these low energy β-decay electrons that do not carry relevant information on the ν-mass reduces the total flux of electrons from the source into the main spectrometer by six orders of magnitude. The energy spectrum of the remaining electrons close to the β-decay endpoint is then scanned in the much larger main spectrometer with a resolution of 0.93 eV. The enormous measurements of this UHV vessel are 23.4 m length, 10 m diameter and 1400 m3 volume. Finally the detector (Fig. 1(d)) has to detect all β-electrons which passed the energy filter, and besides this it is used for systematic investigations of the whole KATRIN experiment. Therefore, the detector has to have a high efficiency for electron detection (>90%), a good position resolution to enable the radial monitoring of the source density, a high background suppression (<1 mHz), which is correlated with a good passive and active shielding and a good energy resolution (<600 eV). The present detector concept is based on a large monolithic array of PIN diodes. The typical pixel size of 5 × 5 mm2 leads to about 500 read-out channels. This will allow a detailed radial source mapping.
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Fig. 3. Principle of a MAC-E-Filter.
3. The electrostatic spectrometers The main features of the MAC-E-Filter are illustrated in Fig. 3. Two superconducting solenoids are providing a guiding magnetic field. The β-electrons, which are starting from the tritium source in the left solenoid and going into the forward hemisphere, are guided magnetically in a cyclotron motion along the magnetic field lines into the spectrometer, thus resulting in an accepted solid angle of up to 2π. On their way into the center of the spectrometer the magnetic field B drops by many orders of magnitude. Therefore, the magnetic gradient force transforms most of the cyclotron energy E ⊥ into longitudinal motion. This is illustrated in Fig. 3 by a momentum vector. Due to the slowly varying magnetic field the momentum transforms adiabatically and therefore the magnetic moment µ keeps constant (the equation is given in the non-relativistic approximation) µ=
E⊥ = const. B
(1)
This transformation can be summarized as follows: The β-electrons, isotopically emitted at the source, are transformed into a broad beam of electrons flying almost parallel to the magnetic field lines. This parallel beam of electrons is energetically analyzed by applying an electrostatic potential generated by a system of cylindrical electrodes. All electrons which have enough energy to pass the electrostatic barrier are reaccelerated and collimated onto a detector, all others are reflected. The spectrometer, therefore, acts as an integrating high energy pass filter. The relative sharpness E/E of this filter is given by the ratio of the minimum magnetic field Bmin in the analyzing plane to the maximum magnetic field Bmax between β-electron source and spectrometer: Bmin E = . E Bmax
(2)
The β-spectrum is scanned by varying the electrostatic retarding potential. The main challenges for the operation of the spectrometers are providing a retarding potential of sufficient stability (U/U < 10−6 ) and minimizing the background event rate coming from the spectrometer volume. The novel electromagnetic design concept for the KATRIN spectrometers foresees connecting the retarding high voltage directly to the spectrometer vessel.
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Fig. 4. The KATRIN pre-spectrometer. The diagram on the left side shows the components: (a) vessel with a DN1700 flange at the right-hand end, (b) 45◦ pumping port with DN500 flange, (c) horizontal pumping port with DN500 flange, (d) DN160CF flanges for high voltage feedthroughs, (e) ground electrode flanges (DN500, beamline), (f) spectrometer magnets. The picture on the right shows the pre-spectrometer at FZK with installed turbo-molecular pumps and permanent thermal insulation (spring 2004).
A nearly massless inner wire electrode [14] at slightly larger negative potential than the vessel itself suppresses low energy electrons emanating from the inner surfaces of the spectrometer walls, representing a potential source of background. In addition, this inner electrode fine-tunes the electrostatic field to avoid the occurrence of traps for charged particles and to optimize the adiabatic transmission properties. Both spectrometers are equipped with large metal sealed vacuum flanges (diameter: 1.7 m, developed at Forschungszentrum Karlsruhe (FZK)) for the installation of the inner wire electrodes. As a fundamental method for reducing the background rate a stringent requirement is set on the vacuum conditions of the KATRIN experiment: the pressure inside the pre-spectrometer and the main spectrometer must be better than 10−11 mbar. This minimizes the probability for ionizations of residual gas molecules inside the main spectrometer. The foreseen vacuum concept is based on the combination of non-evaporable getter pumps (NEG pumps, SAES St707) mainly for hydrogen and nitrogen and cascaded turbo-molecular pumps (TMP) for non-getterable gases like noble gases and methane. All mechanical pumps are dry pumps. Hydrogen outgassing from the electro-polished stainless steel walls of the spectrometers is expected to be the limiting factor for the final pressure. Therefore the vessels can be heated up to 350 ◦ C to clean the surfaces and to activate the NEG pumps. In normal operation mode the temperature of the spectrometers will be stabilized at room temperature (≈20 ◦ C) with the option of cooling down to −20 ◦ C, which would reduce the outgassing rate of the surfaces by almost an order of magnitude. 4. The pre-spectrometer measurements The KATRIN pre-spectrometer plays a major role during the tritium measurements as well as during the R&D phase. During normal tritium measurements the pre-spectrometer will act as a pre-filter for low energy β-decay electrons (see Section 2). In a second application, the pre-spectrometer can act as a fast switch for running the main spectrometer in a non-integrating time-of-flight mode (see [12]). An important task for the pre-spectrometer is to serve as a prototype for the main spectrometer. This holds for the vacuum concept as well as for the electromagnetic design concept. After manufacturing by the company SDMS (F) and delivery in autumn 2003, the prespectrometer became the first major hardware component of KATRIN in operation at FZK.
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Fig. 5. Pictures of the pre-spectrometer set-up: the left-hand picture shows a detailed view of the newly developed DN500 and DN1700 flanges, the picture in the middle gives an impression of the dimensions of the dry-air compartment around the pre-spectrometer, the right-hand picture shows the fully equipped NEG cartridge ready for installation.
A diagram and a picture are shown in Fig. 4. Two pumping ports of 1 m length and 0.5 m diameter provide space for the NEG pump holders. The first pumping stage consists of two TMP that have been installed at the end cap of the horizontal pumping port, where they will be in a sufficiently low magnetic field. The pumps can be independently separated from the volume by two DN200 gate valves. The outlet of the first stage pumps goes via another gate valve into the inlet of the second-stage TMP, that is backed by a dry forepump. The pre-spectrometer is connected to a heating–cooling system based on temperature controlled thermal oil circulating in a closed loop. A thermal insulation of 20 cm mineral wool, which can be removed in parts, was installed around the vessel. In addition, the pre-spectrometer is surrounded by a nearly airtight compartment with lowered dew point (see Fig. 5), which prevents ice formation when operating at temperatures below 0 ◦ C. The pre-spectrometer was successfully operated at temperatures between −20 and +230 ◦ C. In addition electrical heating power for pump ports was installed to enable the activating temperature of the NEG pumps to be reached (> +350 ◦ C). During the measurements the pressure readings of two inverted magnetron gauges (IM) were recorded, where possible residual gas spectra were recorded (quadrupole mass spectrometer) and an extractor gauge was used to verify the measured absolute pressure values from the IM gauges. 4.1. Vacuum measurements with the KATRIN pre-spectrometer The initial result of the vacuum tests was that the newly developed sealings for the DN500 and DN1700 flanges (Fig. 5) were absolutely leaktight. The inner, silver coated spring-energized metal seal ring worked exceedingly well, so that we did not have to make use of the possibility of evacuating the volume between the two seal rings. For a reliable scaling of the required pumping speed for the main spectrometer one needs to know two input parameters: the final pressure and a realistic estimate of the hydrogen outgassing rate of the inner surface. Therefore the pre-spectrometer was baked in four steps to temperatures between 150 and 230 ◦ C. Before the first and after the following baking steps, the outgassing rate of the pre-spectrometer was measured at room temperature (20 ◦ C). For the highest baking temperature it was also measured at vessel temperatures of 0 and −20 ◦ C. The outgassing rates were determined with the pressure rise method, that means after pumping down the prespectrometer to the final pressure the gate valves in front of the first stage TMPs were closed simultaneously and the absolute pressure as well as the RGA spectra were recorded for several
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Fig. 6. Diagram of the pre-spectrometer set-up for electromagnetic measurements. The set-up shown in Fig. 4 is completed by a photoelectron gun and the prototype detector. Representative for the precision HV supply, a picture of the newly built precision HV divider is shown, which was built by the University of M¨unster.
hours. The results of the measurements are very promising: after baking the spectrometer to 230 ◦ C, an outgassing rate of 10−12 mbar l/s cm2 was reached for room temperature. This corresponds to the specification of the main spectrometer. At a temperature of −20 ◦ C the value determined for the outgassing rate was 1.6 × 10−13 mbar l/s cm2 (for details see [15]). In these measurements with the TMP branches only (effective pumping speed 900 l/s) a final pressure of 2.9 × 10−10 mbar was reached with a residual gas spectrum largely dominated by hydrogen (RGA measurements). To meet the requirements for the final pressure in the spectrometer section of KATRIN (<10−11 mbar, see Section 3), a getter cartridge of 0.5 m diameter and 1 m length was equipped with 90 m of St707 NEG strips (see Fig. 5). The cartridge was installed into the 45◦ pumping port and the getter was activated by heating up to 350 ◦ C during the pre-spectrometer baking. After the vessel was cooled down and the temperature stabilized at 20 ◦ C, the IM gauges with a sensitivity of 10−11 mbar showed no measurable pressure. The conservative assumption of a final pressure of 1 × 10−11 mbar combined with the known outgassing rate leads to a total pumping speed of 25 000 l/s for this getter pump. Based on this value, a comparison with simulations of the getter geometry for various sticking probabilities [16] indicates a sticking probability of 2% for St707 strips. This hitherto unknown value is a necessary input for simulations of the main spectrometer vacuum conditions under realistic assumptions. 4.2. Electromagnetic measurements with the KATRIN pre-spectrometer The primary task of the electromagnetic measurements with the pre-spectrometer is to investigate the performance and properties of the new KATRIN high voltage (HV) concept and the electromagnetic design. The HV system should be tested on the achievable stability of the retarding potential with the aim of meeting the requirements of 10−6 at 18.6 keV for the main spectrometer voltage. It is also necessary to look for the influence of High Frequency (HF) noise on the retarding voltage stability and to foresee the installation of appropriate HF filter systems, if necessary. A large part of the pre-spectrometer measurements will deal with the question of the background level under various conditions. Beside simple background measurements, tests with enhanced pressure and various retarding potentials and magnetic fields are planned. In addition, the enhancement of the background rate due to charged particles from the inner wall of the spectrometer created by the irradiation of external X-ray or γ -ray sources will be tested. This also includes tests of the wire electrode system in the monopole mode (repelling low energy
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Fig. 7. Left: Diagram of the wire electrode system inside the KATRIN pre-spectrometer set-up. Right: View into the pre-spectrometer through the open DN1700 flange after the successful installation of the wire electrode.
electrons coming from the wall before entering the magnetic flux tube that will be focused on the detector) as well as in the dipole mode (to expel stored charged particles from the spectrometer volume). Testing the electromagnetic design of the pre-spectrometer means measuring a set of typical parameters of the MAC-E-Filter (e.g. the transmission function, the homogeneity of the retarding potential and magnetic field, the validity of the adiabatic momentum transformation) and comparing them with the result of simulations performed in the design phase of the prespectrometer. To perform these tests additional components such as a photoelectron gun (a source of nearly monoenergetic electrons) and a prototype detector (see Fig. 6) will be connected to the prespectrometer. The electrode system (see Fig. 7 was designed and built by the UW in Seattle (USA) and successfully installed into the pre-spectrometer in May 2005. The responsibilities for the major components of the pre-spectrometer measurement set-up show the interconnection between the KATRIN collaborators: HV equipment (University of M¨unster (D)), the photoelectron gun (INR Troitsk (RUS) and Institute for Nuclear Physics (IK) at FZK (D)), the prototype detector (Institute for Process-data processing and Electronics (IPE) at FZK (D), IK and UW), magnets (Institute for Technical Physics (ITP) at FZK (D), vacuum system and system integration (University of Karlsruhe (D) and IK). In the meantime the ground electrode and the pre-spectrometer magnets (see Fig. 4) work properly. So we expect the start of the electromagnetic test measurements with the prespectrometer in spring 2006. 5. Conclusion The pre-spectrometer is the first major component of the KATRIN experiment in operation at FZK. The vacuum measurements have been completed successfully. The results of these measurements went into the optimization of the vacuum system of the KATRIN main spectrometer. Examples are the effective pumping speed of the getter pumps which lead to a required amount of 3000 m of NEG strips, that is about half what was formerly estimated, and the low outgassing rate at room temperature achieved even with moderate baking temperatures, which allows us to handle the cost-intensive additional cooling of the main spectrometer only as an option if a combined pumping system of NEG and TMPs is installed, which will provide a sufficient pumping speed.
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The pre-spectrometer set-up for the electromagnetic measurements is nearly completed, and the measurements will start in spring 2006. Acknowledgment This work was supported in part by the German BMBF (05CK1VK1/7, 05CK1UM1/5 and 05CK2PD1/5). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
T. Kirsten, Rev. Modern Phys. 71 (1999) 1213. Q.R. Ahmad et al., Phys. Rev. Lett. 81 (2001) 071301. Y. Fukuda et al., Phys. Rev. Lett. 85 (2000) 3999. K. Eguchi et al., Phys. Rev. Lett. 90 (2003) 021802. M.H. Ahn et al., Phys. Rev. Lett. 90 (2003) 041801. S. Hannestad, arXiv:hep-ph/0409108v2, 12 September 2004. S.W. Allen, R.W. Schmidt, S.L. Bridle, astro-ph/0306386. Ch. Weinheimer, these proceedings. V.M. Lobashev et al., Phys. Lett. B 460 (1999) 227. V.M. Lobashev, Nuclear Phys. A 719 (2003) 153. Ch. Kraus et al., Eur. Phys. J. C 40 (2005) 447–468. KATRIN Design Report 2004, FZKA Scientific report 7090, 2005. B. Bornschein, these proceedings. K. Valerius, these proceedings. L. Bornschein et al., OPM-2 Uppsala (2005) (submitted for publication). X. Luo et al., Vacuum (2006) (submitted for publication) (EVC-9 Paris).
Review
Measurements with the KATRIN pre-spectrometer L. Bornschein ∗ , for the KATRIN collaboration Institute for Experimental Nuclear Physics, University Karlsruhe (TH), 76021 Karlsruhe, Germany
Abstract The objective of the KArlsruhe TRItium Neutrino experiment (KATRIN) is a direct measurement of the absolute mass of the electron (anti)neutrino by means of precise spectroscopy of the tritium β-spectrum near its endpoint. The pre-spectrometer is part of the KATRIN reference set-up where it will work as a pre-filter for low energy β-decay electrons that are inessential for the determination of the ν-mass. Since its delivery in autumn 2003 the pre-spectrometer has been the first major hardware component of KATRIN in operation at Forschungszentrum Karlsruhe (FZK). The vacuum measurements were successfully completed in early 2005. The main results are an outgassing rate for the stainless steel surface of the pre-spectrometer of 10−12 mbar l/s cm2 at room temperature and a final pressure below 10−11 mbar. This corresponds to the specification of the main spectrometer. The amount of Non-Evaporable-Getter (NEG) strips needed can be restricted to about 3000 m and the additional cooling of the main spectrometer is optional, if a combined pumping system of NEG and turbo-molecular pumps (TMPs) is installed, that will provide a sufficient pumping speed. The modification of the pre-spectrometer set-up for the electromagnetic measurements is nearly completed, and the measurements will start in spring 2006. c 2006 Elsevier B.V. All rights reserved. Keywords: Neutrinos; Neutrino mass; Vacuum; Tritium; Hydrogen; Astroparticle physics; Particle physics
1. Introduction With the evidence for massive neutrinos from recent ν-oscillation experiments (e.g. [1–5]), one of the most fundamental tasks of particle physics over the next few years will be the ∗ Corresponding address: Universit¨at Karlsruhe (TH), Institut f¨ur experimentelle Kernphysik, Postfach 6980, D-76128 Karlsruhe, Germany. E-mail address: [email protected].
c 2006 Elsevier B.V. All rights reserved. 0146-6410/$ - see front matter doi:10.1016/j.ppnp.2005.12.011
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Fig. 1. Schematic view of the KATRIN reference set-up. The main components of the system comprise a windowless gaseous tritium source (WGTS, (a)), an electron transport and differential pumping system with space for quench condensed sources (b), two MAC-E-Filters (pre-spectrometer and main spectrometer, (c)) and an electron detector (d). The overall length of this linear set-up amounts to about 70 m. The magnetic flux transported from source to detector will be 191 T cm2 .
determination of the absolute mass scale of neutrinos, which has crucial implications for cosmology and particle physics. In cosmology, neutrino hot dark matter could play an important role in the evolution of large scale structures (LSS). In particle physics, a measurement of m ν would discriminate among different ν-mass models, commonly grouped as either of hierarchical type (m 1 m 2 m 3 ) or of quasi-degenerate type (m 1 m 2 m 3 ). So far, the study of LSS evolution with galaxy surveys (2dFGRS, SDSS) and cosmic microwave background radiation experiments (WMAP) has led to inconclusive statements on neutrino masses [6,7]. Therefore, it is essential to probe sub-eV neutrino masses with laboratory experiments. The spectroscopy of β-decay just below the kinematic endpoint is the only direct and model independent way to investigate neutrino masses with a sensitivity in the (sub-)eV range [8]. These experiments are relying just on the relativistic energy–momentum relation E 2 = p 2 c2 + m 2 c4 . The current tritium β-decay experiments at Troitsk [9,10] (using a windowless gaseous source) and Mainz [11] (using a quench condensed source) have approached their sensitivity limit of about 2 eV/c2 . The KArlsruhe TRItium Neutrino (KATRIN) experiment is a next-generation direct neutrino mass experiment with a sensitivity to cosmologically important sub-eV ν-masses [8,12]. 2. The KATRIN experiment The KATRIN experiment combines an ultraluminous windowless gaseous molecular tritium source (WGTS) with a high resolution electrostatic retarding spectrometer (MAC-E-Filter) to measure the spectral shape of β-decay electrons close to the endpoint at 18.6 keV with unprecedented precision. If no neutrino mass signal is found, the KATRIN sensitivity after three years of measurements is m ν < 0.2 eV/c2 (90% C.L.); a ν-mass signal of m ν = 0.35 eV/c2 can be measured with 5σ evidence. A schematic view of the KATRIN reference set-up is shown in Fig. 1. The WGTS (a) will consist of a 16 m long cylindrical tube of 90 mm diameter filled with molecular tritium gas of high isotopic purity (>95%) and kept at 27 K. The tritium gas will be injected through a capillary at the middle of the tube. It diffuses to both ends of the tube, resulting in a decrease of the tritium density by at least a factor of 20 from the injection point to the ends of the tube. This tritium inventory will deliver 1011β-decay electrons per second. The tritium tube will be placed inside a chain of superconducting solenoids. The solenoids will generate a homogeneous magnetic field of 3.6 T, which adiabatically guides the decay electrons to the tube ends. At the end of the WGTS a first differential pumping section will recover more then 99.9% of the tritium molecules. They
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Fig. 2. Two electrostatic spectrometers of KATRIN.
will be sent via a purification system to a tritium buffer vessel. From there the tritium is sent back to the “pressure controlled WGTS Supply buffer Vessel” and reinjected into the WGTS tube. This so called “inner loop” guarantees a high tritium throughput with a reasonable tritium inventory (for a more detailed description see [13]). The WGTS is followed by an electron transport system (Fig. 1(b)), which will guide the β-decay electrons adiabatically to the spectrometer, while at the same time reducing the tritium flow rate towards the spectrometer. Since the maximal allowed rate of tritium flow into the prespectrometer is of the order of 10−11 mbar l/s, a tritium suppression factor of more than 1010 is needed between the outlet of the WGTS tube and the entrance of the pre-spectrometer. This will be achieved by a combination of differential (DPS-F) and cryogenic (CPS-F) pumping sections. The cryo-pumps consist of the liquid helium cold surface of the transport tube, covered by a thin layer of argon frost for better trapping. The split coil magnet will enable the insertion of quench condensed sources into the beamline. The energy of the β-decay electrons is analyzed in a system of two MAC-E-Filter (Magnetic Adiabatic Collimation combined with an Electrostatic Filter) spectrometers (Fig. 1(c)): a prespectrometer and a large main spectrometer (see Fig. 2). On their way to the detector the β-decay electrons first have to pass the smaller pre-spectrometer, a stainless steel UHV vessel with a length of 3.4 m, a diameter of 1.7 m and a volume of 8.5 m3 . In the normal tritium mode of the experiment it works at a fixed retarding potential, acting as a pre-filter for electrons with energies below 18.3 keV. The rejection of these low energy β-decay electrons that do not carry relevant information on the ν-mass reduces the total flux of electrons from the source into the main spectrometer by six orders of magnitude. The energy spectrum of the remaining electrons close to the β-decay endpoint is then scanned in the much larger main spectrometer with a resolution of 0.93 eV. The enormous measurements of this UHV vessel are 23.4 m length, 10 m diameter and 1400 m3 volume. Finally the detector (Fig. 1(d)) has to detect all β-electrons which passed the energy filter, and besides this it is used for systematic investigations of the whole KATRIN experiment. Therefore, the detector has to have a high efficiency for electron detection (>90%), a good position resolution to enable the radial monitoring of the source density, a high background suppression (<1 mHz), which is correlated with a good passive and active shielding and a good energy resolution (<600 eV). The present detector concept is based on a large monolithic array of PIN diodes. The typical pixel size of 5 × 5 mm2 leads to about 500 read-out channels. This will allow a detailed radial source mapping.
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Fig. 3. Principle of a MAC-E-Filter.
3. The electrostatic spectrometers The main features of the MAC-E-Filter are illustrated in Fig. 3. Two superconducting solenoids are providing a guiding magnetic field. The β-electrons, which are starting from the tritium source in the left solenoid and going into the forward hemisphere, are guided magnetically in a cyclotron motion along the magnetic field lines into the spectrometer, thus resulting in an accepted solid angle of up to 2π. On their way into the center of the spectrometer the magnetic field B drops by many orders of magnitude. Therefore, the magnetic gradient force transforms most of the cyclotron energy E ⊥ into longitudinal motion. This is illustrated in Fig. 3 by a momentum vector. Due to the slowly varying magnetic field the momentum transforms adiabatically and therefore the magnetic moment µ keeps constant (the equation is given in the non-relativistic approximation) µ=
E⊥ = const. B
(1)
This transformation can be summarized as follows: The β-electrons, isotopically emitted at the source, are transformed into a broad beam of electrons flying almost parallel to the magnetic field lines. This parallel beam of electrons is energetically analyzed by applying an electrostatic potential generated by a system of cylindrical electrodes. All electrons which have enough energy to pass the electrostatic barrier are reaccelerated and collimated onto a detector, all others are reflected. The spectrometer, therefore, acts as an integrating high energy pass filter. The relative sharpness E/E of this filter is given by the ratio of the minimum magnetic field Bmin in the analyzing plane to the maximum magnetic field Bmax between β-electron source and spectrometer: Bmin E = . E Bmax
(2)
The β-spectrum is scanned by varying the electrostatic retarding potential. The main challenges for the operation of the spectrometers are providing a retarding potential of sufficient stability (U/U < 10−6 ) and minimizing the background event rate coming from the spectrometer volume. The novel electromagnetic design concept for the KATRIN spectrometers foresees connecting the retarding high voltage directly to the spectrometer vessel.
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Fig. 4. The KATRIN pre-spectrometer. The diagram on the left side shows the components: (a) vessel with a DN1700 flange at the right-hand end, (b) 45◦ pumping port with DN500 flange, (c) horizontal pumping port with DN500 flange, (d) DN160CF flanges for high voltage feedthroughs, (e) ground electrode flanges (DN500, beamline), (f) spectrometer magnets. The picture on the right shows the pre-spectrometer at FZK with installed turbo-molecular pumps and permanent thermal insulation (spring 2004).
A nearly massless inner wire electrode [14] at slightly larger negative potential than the vessel itself suppresses low energy electrons emanating from the inner surfaces of the spectrometer walls, representing a potential source of background. In addition, this inner electrode fine-tunes the electrostatic field to avoid the occurrence of traps for charged particles and to optimize the adiabatic transmission properties. Both spectrometers are equipped with large metal sealed vacuum flanges (diameter: 1.7 m, developed at Forschungszentrum Karlsruhe (FZK)) for the installation of the inner wire electrodes. As a fundamental method for reducing the background rate a stringent requirement is set on the vacuum conditions of the KATRIN experiment: the pressure inside the pre-spectrometer and the main spectrometer must be better than 10−11 mbar. This minimizes the probability for ionizations of residual gas molecules inside the main spectrometer. The foreseen vacuum concept is based on the combination of non-evaporable getter pumps (NEG pumps, SAES St707) mainly for hydrogen and nitrogen and cascaded turbo-molecular pumps (TMP) for non-getterable gases like noble gases and methane. All mechanical pumps are dry pumps. Hydrogen outgassing from the electro-polished stainless steel walls of the spectrometers is expected to be the limiting factor for the final pressure. Therefore the vessels can be heated up to 350 ◦ C to clean the surfaces and to activate the NEG pumps. In normal operation mode the temperature of the spectrometers will be stabilized at room temperature (≈20 ◦ C) with the option of cooling down to −20 ◦ C, which would reduce the outgassing rate of the surfaces by almost an order of magnitude. 4. The pre-spectrometer measurements The KATRIN pre-spectrometer plays a major role during the tritium measurements as well as during the R&D phase. During normal tritium measurements the pre-spectrometer will act as a pre-filter for low energy β-decay electrons (see Section 2). In a second application, the pre-spectrometer can act as a fast switch for running the main spectrometer in a non-integrating time-of-flight mode (see [12]). An important task for the pre-spectrometer is to serve as a prototype for the main spectrometer. This holds for the vacuum concept as well as for the electromagnetic design concept. After manufacturing by the company SDMS (F) and delivery in autumn 2003, the prespectrometer became the first major hardware component of KATRIN in operation at FZK.
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Fig. 5. Pictures of the pre-spectrometer set-up: the left-hand picture shows a detailed view of the newly developed DN500 and DN1700 flanges, the picture in the middle gives an impression of the dimensions of the dry-air compartment around the pre-spectrometer, the right-hand picture shows the fully equipped NEG cartridge ready for installation.
A diagram and a picture are shown in Fig. 4. Two pumping ports of 1 m length and 0.5 m diameter provide space for the NEG pump holders. The first pumping stage consists of two TMP that have been installed at the end cap of the horizontal pumping port, where they will be in a sufficiently low magnetic field. The pumps can be independently separated from the volume by two DN200 gate valves. The outlet of the first stage pumps goes via another gate valve into the inlet of the second-stage TMP, that is backed by a dry forepump. The pre-spectrometer is connected to a heating–cooling system based on temperature controlled thermal oil circulating in a closed loop. A thermal insulation of 20 cm mineral wool, which can be removed in parts, was installed around the vessel. In addition, the pre-spectrometer is surrounded by a nearly airtight compartment with lowered dew point (see Fig. 5), which prevents ice formation when operating at temperatures below 0 ◦ C. The pre-spectrometer was successfully operated at temperatures between −20 and +230 ◦ C. In addition electrical heating power for pump ports was installed to enable the activating temperature of the NEG pumps to be reached (> +350 ◦ C). During the measurements the pressure readings of two inverted magnetron gauges (IM) were recorded, where possible residual gas spectra were recorded (quadrupole mass spectrometer) and an extractor gauge was used to verify the measured absolute pressure values from the IM gauges. 4.1. Vacuum measurements with the KATRIN pre-spectrometer The initial result of the vacuum tests was that the newly developed sealings for the DN500 and DN1700 flanges (Fig. 5) were absolutely leaktight. The inner, silver coated spring-energized metal seal ring worked exceedingly well, so that we did not have to make use of the possibility of evacuating the volume between the two seal rings. For a reliable scaling of the required pumping speed for the main spectrometer one needs to know two input parameters: the final pressure and a realistic estimate of the hydrogen outgassing rate of the inner surface. Therefore the pre-spectrometer was baked in four steps to temperatures between 150 and 230 ◦ C. Before the first and after the following baking steps, the outgassing rate of the pre-spectrometer was measured at room temperature (20 ◦ C). For the highest baking temperature it was also measured at vessel temperatures of 0 and −20 ◦ C. The outgassing rates were determined with the pressure rise method, that means after pumping down the prespectrometer to the final pressure the gate valves in front of the first stage TMPs were closed simultaneously and the absolute pressure as well as the RGA spectra were recorded for several
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Fig. 6. Diagram of the pre-spectrometer set-up for electromagnetic measurements. The set-up shown in Fig. 4 is completed by a photoelectron gun and the prototype detector. Representative for the precision HV supply, a picture of the newly built precision HV divider is shown, which was built by the University of M¨unster.
hours. The results of the measurements are very promising: after baking the spectrometer to 230 ◦ C, an outgassing rate of 10−12 mbar l/s cm2 was reached for room temperature. This corresponds to the specification of the main spectrometer. At a temperature of −20 ◦ C the value determined for the outgassing rate was 1.6 × 10−13 mbar l/s cm2 (for details see [15]). In these measurements with the TMP branches only (effective pumping speed 900 l/s) a final pressure of 2.9 × 10−10 mbar was reached with a residual gas spectrum largely dominated by hydrogen (RGA measurements). To meet the requirements for the final pressure in the spectrometer section of KATRIN (<10−11 mbar, see Section 3), a getter cartridge of 0.5 m diameter and 1 m length was equipped with 90 m of St707 NEG strips (see Fig. 5). The cartridge was installed into the 45◦ pumping port and the getter was activated by heating up to 350 ◦ C during the pre-spectrometer baking. After the vessel was cooled down and the temperature stabilized at 20 ◦ C, the IM gauges with a sensitivity of 10−11 mbar showed no measurable pressure. The conservative assumption of a final pressure of 1 × 10−11 mbar combined with the known outgassing rate leads to a total pumping speed of 25 000 l/s for this getter pump. Based on this value, a comparison with simulations of the getter geometry for various sticking probabilities [16] indicates a sticking probability of 2% for St707 strips. This hitherto unknown value is a necessary input for simulations of the main spectrometer vacuum conditions under realistic assumptions. 4.2. Electromagnetic measurements with the KATRIN pre-spectrometer The primary task of the electromagnetic measurements with the pre-spectrometer is to investigate the performance and properties of the new KATRIN high voltage (HV) concept and the electromagnetic design. The HV system should be tested on the achievable stability of the retarding potential with the aim of meeting the requirements of 10−6 at 18.6 keV for the main spectrometer voltage. It is also necessary to look for the influence of High Frequency (HF) noise on the retarding voltage stability and to foresee the installation of appropriate HF filter systems, if necessary. A large part of the pre-spectrometer measurements will deal with the question of the background level under various conditions. Beside simple background measurements, tests with enhanced pressure and various retarding potentials and magnetic fields are planned. In addition, the enhancement of the background rate due to charged particles from the inner wall of the spectrometer created by the irradiation of external X-ray or γ -ray sources will be tested. This also includes tests of the wire electrode system in the monopole mode (repelling low energy
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Fig. 7. Left: Diagram of the wire electrode system inside the KATRIN pre-spectrometer set-up. Right: View into the pre-spectrometer through the open DN1700 flange after the successful installation of the wire electrode.
electrons coming from the wall before entering the magnetic flux tube that will be focused on the detector) as well as in the dipole mode (to expel stored charged particles from the spectrometer volume). Testing the electromagnetic design of the pre-spectrometer means measuring a set of typical parameters of the MAC-E-Filter (e.g. the transmission function, the homogeneity of the retarding potential and magnetic field, the validity of the adiabatic momentum transformation) and comparing them with the result of simulations performed in the design phase of the prespectrometer. To perform these tests additional components such as a photoelectron gun (a source of nearly monoenergetic electrons) and a prototype detector (see Fig. 6) will be connected to the prespectrometer. The electrode system (see Fig. 7 was designed and built by the UW in Seattle (USA) and successfully installed into the pre-spectrometer in May 2005. The responsibilities for the major components of the pre-spectrometer measurement set-up show the interconnection between the KATRIN collaborators: HV equipment (University of M¨unster (D)), the photoelectron gun (INR Troitsk (RUS) and Institute for Nuclear Physics (IK) at FZK (D)), the prototype detector (Institute for Process-data processing and Electronics (IPE) at FZK (D), IK and UW), magnets (Institute for Technical Physics (ITP) at FZK (D), vacuum system and system integration (University of Karlsruhe (D) and IK). In the meantime the ground electrode and the pre-spectrometer magnets (see Fig. 4) work properly. So we expect the start of the electromagnetic test measurements with the prespectrometer in spring 2006. 5. Conclusion The pre-spectrometer is the first major component of the KATRIN experiment in operation at FZK. The vacuum measurements have been completed successfully. The results of these measurements went into the optimization of the vacuum system of the KATRIN main spectrometer. Examples are the effective pumping speed of the getter pumps which lead to a required amount of 3000 m of NEG strips, that is about half what was formerly estimated, and the low outgassing rate at room temperature achieved even with moderate baking temperatures, which allows us to handle the cost-intensive additional cooling of the main spectrometer only as an option if a combined pumping system of NEG and TMPs is installed, which will provide a sufficient pumping speed.
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The pre-spectrometer set-up for the electromagnetic measurements is nearly completed, and the measurements will start in spring 2006. Acknowledgment This work was supported in part by the German BMBF (05CK1VK1/7, 05CK1UM1/5 and 05CK2PD1/5). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
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