Vacuum and Surface Contamination Problems in Experiments with Ultracold Neutrons

Transcription

1 Vacuum and Surface Contamination Problems in Experiments with Ultracold Neutrons Reinhold Henneck Paul-Scherrer-Institut, CH-5232 Villigen, Switzerland Ultracold neutrons (UCN) are used in several experiments to measure fundamental properties of the neutron, e.g. the neutron electric dipole moment, the neutron lifetime and neutron decay parameters. Most of these experiments exploit the fact that due to their very low kinetic energy (< 300 nev) UCN can be stored in material bottles for hundreds of seconds. The probability for total reflection from material walls however depends critically on the surface cleanliness. Due to the 1/v dependence of the absorption cross sections UCN experience huge losses, specifically for wall contaminants containing hydrogen and nitrogen. Therefore clean surfaces are essential to obtain long storage times in these experiments. Moreover, the problem of anomalous losses for UCN is intimately linked to the knowledge of surface contamination. The hydrogen cleansing strategies reported in the literature so far in this context will be reviewed. It should be pointed out that on the other hand the extreme selectivity of UCN to hydrogen could also be exploited for surface characterization for penetration depth up to several tens of nm.

2 PAUL SCHERRER INSTITUT Vacuum and surface contamination problems in experiments with ultracold neutrons Background / motivation Review of some literature studies some results on Be coated surfaces Further info: 05/05/2003 R. Henneck

3 E ~ 10-7 ev ~ 1 mk V 7 m/s l 500 Å sensitive to very weak forces Ultracold Neutrons (UCN) heavy particle with light character Surface potential: e.g. V(Be) = 252 nev total reflection under any angle Magnetic potential : ~ 4 T magnetic bottle Gravitational potential: mg ~ nev/ cm Storage times in material bottles 860 s achieved, corresponding to wall storage times 10 4 s! fundamental studies via properties of n (EDM, lifetime, decay asymmetries, neutron antineutron oscillations, ) solid state physics, material sciences (elastic/ inelastic scattering NESSIE, UCN microscopy, ) Surface and near surface studies (l 500 Å) via characteristic (n,g) and total reflection studies

4 UCN surface reflection / interaction total UCN reflection surface penetration ~ 10 nm (1/e) µ 2η f(e,v) : loss probability per wall collision η= W/V = (s abs + s inel ) / 2λ Re(b) s abs,s inel ~ 1/v ; s inel ~ f(t) m Be» number of collisions n τ stor = n t 10 6 s!!! would give a 10-3 correction for a direct neutron lifetime measurement! But experimentally m Be» 10-5! anomalous losses H D Be C N O Ti 10 B s abs [b] s inel [b]

5 Remove H to a level n H/cm 2 within 10 nm at /below surface!! Keep it so!! 05/05/2003 R. Henneck

6 05/05/2003 R. Henneck

7 15 N + 1 H 12 C + 4 He + γ (4.43 MeV) Γ res 0.3 kev Surface hydrogen detection Similarly: 11 B + 1 H 3 4 He!! Depth resolution 20 nm 05/05/2003 R. Henneck

8 Surface hydrogen detection Elastic Recoil Detection Analysis (ERDA) Depth resolution 200 nm, but one can measure H and D simultaneously! 05/05/2003 R. Henneck

9 05/05/2003 R. Henneck continuous evaporation of Al, Pb

10 Heating to high temperature Re-contamination depends on foil thickness takes longer to fill bulk!

11 discharge cleaning with Ar / O 2 / H 2 / He B sputtering: vacuum dependence

12 Ion sputtering: 11 B B sputtering: recontamination

13 Holes in Be coating Fig.4a: Optical microscope, picture height about 40 µm. Note the diagonal scratch extending over the whole area. Fig.4b: AFM picture of same hole. The structure is high as compared to the surroundings.

14 Holes in heated Be samples (200 C, 52 d)

15 XPS (X-ray Photoelectron Spectroscopy) surface penetration depth: UCN ~ 100 Å, XPS ~ 50 Å Element sensitivity (for Be on Cu): ~ 10-5 over area < 1 mm 2 ~ n x 10-5 over area ~ 0.4 cm 2 Make depth profiling by Ar sputtering Scanning Auger Spectroscopy (SAM) Table 2: typical, absolute, raw surface composition for a raw, untreated sample after insertion into vacuum element Energy [ev] contribution Be O/ BeO C N Cu ± /05/2003 R. Henneck

16 XPS: depth profiling with Ar sputtering N ~ 5Å

17 conclusions Ultracold neutrons are extremely sensitive to surface H contamination problem advantage (D/H sensitive!!) H should be reduced to a level n H/cm 2 and be kept there Can in principle be done by high temperature baking / ion sputtering or discharge cleaning. Problem is recombination in vacuum

18 Manufacturing of Large UHV Vessel Xavier Sauge and Sylvain Blanchard SDMS, Saint-Romans, France 1 GEOMETRICAL SHAPE AND MAIN CHARACTERISTCS 2 ASSUMPTION AND CALCULATION RESULTS - Calculation method by means of finite element analysis such as ANSYS 3 MATERIAL AND SURFACE PREPARATION 4 CONSTRUCTION METHOD - Shop manufacture and site works 5 PERFORMANCES TO BE EXPECTED - Expected outgassing rate 6 MAGNETIC PERMEABILITY 7 SIZE OPTIMISATION AND PRECAUTION 8 WELDING CONSIDERATIONS IN RESPECT WITH UHV CONDITIONS

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20 SDMS Xavier SAUGE Sylvain BLANCHARD

21 Manufacturing of large UHV vessels

22 Plan SDMS presentation General informations Achievements (ALCATEL, VIRGO) Tritium-β spectrometer

23 A few figures Founded in 1962 Located near Grenoble (France) Issued capital Euros Turnover 2002 : Euros (50% export) Workforce : 100

24 SDMS (19000 m 2 )

25 SDMS (19000 m 2 )

26 Workshop J (20 tons & controlled atm.)

27 Workshop C (50 tons)

28 Workshop H (clean room)

29 Quality Assurance ISO 9001 since 2001 by BVQI ISO 9002 from 1992 to 2001 by AFAQ Certified AD - HP0 by the TÜV Saarland (German standards)

30 Plan SDMS presentation General informations Achievements (ALCATEL, VIRGO) Tritium-β spectrometer

31 Process Cleanliness Not for an attractive appearance : to fulfil the requirements Cleanliness during procurement and manufacturing Qualified machining process (oils) Materials cleaning (tests and experience) degreasing / rinsing / alkaline solution / rinsing / Drying / Bake- out Cleanliness during the assembly - clean rooms - handling with gloves - tools - straps... Shipment At each stage (contamination, scratch )

32 Materials and surfaces Use of stainless steel X2CrNi ( L) or X2CrNiMo ( L) or (X2CrNiMoN ) 3) or (X2CrNiMoN ) Mechanical polishing of plates in order to obtain a surface roughness value about µ (beginning) Electropolishing of plates in order to obtain a final surface roughness value about 0.2µ à 0.4µ Cleaning (cleaning workshop) Baking at 400 C during 7 to 10 days

33 Performances Leak rate (< mbar l / s with RGA) Outgassing see SDMS booklet «Outgassing - Results and Economic Consequences - SDMS» Materials / Roughness Magnetic permeability Dimensional requirements

34 Plan SDMS presentation General informations Achievements (ALCATEL, VIRGO) Tritium-β spectrometer

35 SDMS achievements VIRGO UHV chambers (UHV / Outgassing rate) ALCATEL SPACE INDUSTRIES simulation chamber (dimensions) Manufacturing process (quality plan) Dimensional requirements (flanges) Gaskets (HELICOFLEX) Outgassing (oven, procedure) Helium tests, RGA Pumping system

36 LAPP VIRGO (18 tons) Date : 05/98 10 chambers 20m 3 Gravitational waves measurements UNS S30400 (304) UNS S30403 (304L)

37 LAPP (idem) mbar Mecanical and inner electropolishing

38 LAPP (idem) HT 400 C (200 h) Outgassing rate mbar l / s. Cm 2 On site : Bake-out 150 C

39 Alcatel Space Industries 130m 3 Simulation chamber 5m in diameter HELIOS 1 Ra 0.8µm

40 Alcatel Space Industries Chamber 450 m 3 Date : 12/97 UNS S30403 / 304L / Internal diameter : 6m Total Length : 16m Thickness 15mm HELIOS2 tests (launched( by ARIANE5) Clean room class 100

41 Alcatel Space (1/10th scale model)

42 Flange (lower)

43 Wall plates on jig protected by plastic layer (class 100)

44 Lower part (class 100 inside)

45 Upper and middle parts

46 Handling

47 Cranes

48 Shipment

49 Plan SDMS presentation General informations Achievements (ALCATEL, VIRGO) Tritium-b spectrometer

50 Tritium-b spectrometer GEOMETRICAL SHAPE AND MAIN CHARACTERISTCS ASSUMPTION AND CALCULATION RESULTS Calculation method by means of finite element analysis such as ANSYS MATERIAL AND SURFACE PREPARATION CONSTRUCTION METHOD Shop manufacture and site works PERFORMANCES TO BE EXPECTED Expected outgassing rate MAGNETIC PERMEABILITY SIZE OPTIMISATION AND PRECAUTION WELDING CONSIDERATIONS IN RESPECT WITH ULTRA-VACUUM CONDITIONS

51 General drawing

52 Updated General Drawing

53 Details

54 Updated Details

55 Lower part

56 Upper part

57 Support

58 Assembly

59 Results CODAP / Working temperature at 400 C SHELL Ø7000 / 20 mm STIFFENING RINGS / 200 x 40 CONICAL SHELL / 20 mm STIFFENING RINGS / 200 x 40 HEMI-HEAD HEAD / 15 mm SHELL Ø1000 / 10 mm

60 Manufacturing process Hot rolled plates (thickness >6mm) Mechanical polishing (+ plastic layer) Cutting / Forming Assembly using a specific jig ( the protective plastic layer is kept during manufacturing operations) Nozzles DN1500, DN1000 and others Manufacture of a specific tool for site assembly works Transportation and assembly on site, tests and adjustments Welding Electropolishing Final cleaning

61 Conclusions Outgassing rate : mbar.l.s -1.cm -2 after several baking treatments at the temperature of 400 C Leak rate < mbar l / s Magnetic permeability : X2CrNiMoN or X2CrNiMoN or Aknowledgements : SDMS staff VIRGO staff (Mr MUGNIER)

62 SDMS Thank you for your attention