1 BVR, July 18th 2005, CB + T. Iwamoto CALIBRATION AND MONITORING METHODS (C&M) FOR THE LIQUID XENON CALORIMETER AND FOR THE WHOLE MEG DETECTOR Xe calorimeter, wire-chamber spectrometer, timing counters an updated discussion on : advantages, disadvantages, open problems, etc. of proposed methods C&M for the entire MEG: at any time during PSI beam-off periods, tuning.... efficient use of beam-on periods

2 an internal note requested by the INFN MEG Referees (MEG-TN027)

3 MEG internal note and then NIM collaboration paper

4 KEEP MEG UNDER CONTROL PARTICULARLY AT HIGH (AND VARIABLE) BEAM INTENSITIES frequent checks of calorimeter energy scale, linearity and stability checks of LXe optical properties energy resolution, spacial resolution, time resolution shower properties at the right  energy (  53 MeV), but also at other energies..... BR   e  ~ Beam Intensity ~  /s

5 no single calibration method has all the required characteristics use complementary (and redundant ) methods, make the best use of their intrinsic properties 1)MAINTAIN THE MEG ENERGY, SPACE AND TIME RESOLUTIONS OPTIMIZED OVER LONG PERIODS OF TIME 2)HAVE RECORDED PROOFS OF MEG PERFORMANCES (WHATEVER THE FINAL MEG RESULT ON BR   e  ) emphasize the reliability of our experiment ! GOOD C&M IS THE KEY TO MEG SUCCESS TWO MAIN TARGETS:

6 attempt to grade the different C&M methods

7 500 KV PROTON ACCELERATOR AND LITIUM TARGET FOR A 17.6 MEV GAMMA LINE Potentialities : strongly exothermic nuclear reaction unique:  -emission much favoured over  -emission obtainable: at resonance (E p = 440 keV  14 keV)  10 6  /s (isotropic) for I p  50  A from LiF target at COBRA center;  ’s on the whole cal. entrance face energy and position calibration; shower properties rather frequent use privilege simple, fast, (semi-automatic) mechanical system for proton beam and LiF target introduction and positioning (give up the use for the calorimeter monitoring from the back) [P.R. 73, 666 (1948), N.P (1960), Zeitschrift f. Physik A (1995)] 3 7 Li (p,  ) 4 8 Be

8 further studies: compatibility with normal beam and target COBRA field, accelerator (and focusing element) position project for easiness of target-tube mounting p-beam divergence and protons on target; p  29 MeV/c post-acceleration to scan the resonance thin-target, thick-target H2+ ions, effects on  -line, (H2+ elimination by a mag.-triplet)

9 E sigma error S-factor error (MeV) (b) (b) (MeV b) (MeV b) E E E E E E E E E E E E E E E E-03 at the T p * 384 keV resonance and compound nucleus formation + non resonant direct reaction elsewhere E E E E-05 astrophysics data

Li (p,  ) 4 8 Be E  0 = 17.6 MeV E  1 = B peak  0 /(  0+  1 )= 0.72  0.07 NaI 12”x12”  spectrum 11 00 resonant at E p = 440 keV  =14 keV  peak = 5 mb

B (p,  ) 6 12 C lower proton energy ! lower rate at 50  A !! other interesting possibilities..... : 1 3 H (p,  ) 2 4 He E  ~ 20 MeV !! used in SNO  in : Hahn et al. PRC (1995) but Tritium....and low rate Cecil et al. NP A (1992) 10x10 cm NaI crystal resonant at E p = 163 keV  = 7 keV E  0 = 16.1 MeV  peak = 5.5  b E  1 =  peak = 152  b  750  0 /s (isotropic)   1 /s for I p  50  A

12 ENERGY, TARGET THICKNESS AND  -LINE QUALITY correspondence between resonance  and range interval  R “thin target”  R   “thick target”  R >>  if T p = 445 keV and  R =   R =  N=7 x LiF/cm 2 at 80  A I p N p = 5x10 14 p/s N  = 1.8x10 6  /s (up to 1.6x10 5 in calorimeter) very clean  -line (more difficult calibration tuning) if T p = 445 keV and  R = Range (445 keV) >>   R = 413  N = 2.5 x LiF/cm 2 at 80  A I p N p = 5x10 14 p/s N  = 6x10 5  /s (+ N  =1.8x10 6  /s)  -line with appreciable left shoulder from 17.6 to 17.1 MeV (simple calibration tuning) of the total 5x10 14 p/s, 2x10 6 p/s produce photons at resonance, some of the residual 2.5x10 8 p/s produce direct photons of lower energy (if T p > resonant energy, right tail also ) H2+ ion effects (30% of CW-beam)

13 N  = 1.8x10 6  /s over 4  (up to 1.6x10 5  /s into the whole calorimeter) (PMT non linearity over I a = 4  A, therefore at about 2x10 5  /s in the calorimeter) Very high  -intensity (other optional reactions have smaller cross-section) (possibility of using low-efficiency selective triggers) MEG aquisition rate is about 100 Hz The accelerator current can be easily limited, but one can also test the calorimeter and the PMT behaviour as a function of an increasing  -rate in the calorimeter......

14 CHOICE OF THE ACCELERATOR Cockroft-Walton, Van der Graaf, Radio Frequency Quadrupole HV Engineering, NEC, AccSys, Neue Technologien GmbH overall price, guarantees, delivery time, test, assistance, spare parts, etc. energy interval of operation, current, stability, beam phase space, background radiation, etc. simplicity of use, reliability, type of computer control source duration, 1-year without servicing, etc. fast conditioning and tuning beam height possibility of moving the accelerator system availability and possible use at the beginning of the experiment The collection of information on all points is a slow, multistep process......: visits to experiments using similar accelerators visit to accelerator factories discussion with national lab. experts

15 STRONG PREFERENCE FOR A COCKROFT-WALTON reliable system, in use for several precision experiments, visits to GS good assistance in mounting and test; “nearby” factory large energy interval of machine operation visit to HV in Amersfoort and visit of HV to Pisa (Legnaro lab. expert present) adequate current, good beam properties, stability fast tuning and operation if 1 MV machine in the same tank of the 0.5 MV machine. (15% increase in price) very low-background machine well interfaced, good safety system, interlocks, good software (and program source available) compact machine in pressurized (and shielding) container one year operation without service If one wants to use the machine for the MEG start-up an order must be issued as soon as possible (September !)

16 model: “coaxial SINGLETRON”

17 θ E (MeV) PRECISE CALIBRATION Potentialities : energy and position calibration shower properties and reconstruction at E   55 MeV, the proper energy ! fully tested in “large prototype” runs Open problems: definition of  -lines by collimators or by  -hit reconstruction (for   ~ 180 º ). NaI set-up. Several positions. NaI behind coils. H 2 cryogenics, negative beam, different target, target introduction in COBRA. how often it can be performed ? BVR February 2005 FULLY TESTED......

18 TWO POSSIBLE WAYS TO PERFORM THE  º CALIBRATION IN MEG 1)EXTRAPOLATION FROM PREVIOUS TESTS FOR MEG Movable NaI system Safe solution at the beginning of the experiment. 2)CONVERSION METHOD No movable parts. More comprehensive applications (wire-chambers,timing counters). It depends on a trigger systems which is presently untested. Both methods allow Xe calorimeter calibration in 1-2 days

19 NaI Detector Stage design NaI detector (~100kg) needs to be moved 2 dimensionally at the opposite side of the xenon detector. The movable stage and motor need to be magnetic tolerable with reasonable positioning accuracy. Test under COBRA field  OK Anti Counter up down target 0000   Linear slider Motor No bearing ball Prism guide Screw drive Example Linear slider: Motor:

20 an interesting possibility for a  calibration in MEG abandon NaI detector in coincidence illuminate the whole calorimeter at the same time with  -2 convert the  -1 in a 0.1 X 0 converter close to the H 2 target detect conversion and measure conversion point with a “special counter” measure e + branch of the pair in the chambers use part of the information for selecting  - 1 by trigger angle between  ’ s defined by impact points on LXe-Cal and “ special counter” (angles  useful for calibrating at different energies) loss at conversion but huge increase in solid angle MC METHOD SIMULATION RESULTS (F.Cei)

21 A FULL TEST OF THE WIRE-CHAMBERS SPECTROMETER CAN ALSO BE PERFORMED ! TRIGGER UNDER STUDY Ingredients: LXe Cal. and QSUM threshold “special counter” good time resolution, pixelization for conversion point reconstruction, separation of e + e -- pairs from single particles positron (from n  ) or pair trajectory (from n  ) by the wire-chamber trigger timing-counters depending on the particular calibration

22 but also the Cockroft-Walton allows a calibration of the LXe Cal and, wire-chamber spectrometer, timing counters CW use is much simpler than  calibration ! LXe Cal illuminated by 17.6 MeV  ’s at high rate Use of  -converter for testing the wire-chambers spectrometer maximum COBRA field for LXe Cal test half COBRA field for wire-chamber spectrometer test WIRE CHAMBER SPECTROMETER AND TIMING COUNTERS TEST (at full COBRA field) by  - p   0 n and  - 1 conversion into an e + e – pair and also by  - p  n  and  conversion into an e + e – pair (a pair spectrometer and a  -line !!)

23  energy release: increased statistics 0.1 X 0, NDC > 4, relative angle >  Intrinsic width for photons emitted with relative angle emitted with relative angle > : 0.3 %. > : 0.3 %.  Leakage effects: ~ 1 %.  Remaining contributions from natural angular width from natural angular width of e + e - pair production and of e + e - pair production and multiple scattering in the multiple scattering in the target. target.FWHM 2.6  0.3 %

24   p  n  (129 MeV)  e + e -   p  n  (129 MeV)  e + e -  Main purpose: calibration of wire-chamber spectrometer and timing counters. spectrometer and timing counters.  Use e + e - pair production from 129 MeV gamma conversion in Tungsten. gamma conversion in Tungsten.  Both e + and e - must be detected and their tracks reconstructed. Pair spectrometer ! tracks reconstructed. Pair spectrometer !  Interesting thing: it provides a fixed (total) energy calibration point for the wire-chamber energy calibration point for the wire-chamber spectrometer spectrometer (normally not easily obtainable......). (normally not easily obtainable......).

25 Efficiency vs converter thickness Thickness (X 0 )  ( o, FWHM) Single particle efficiency (e + or e -, %) Double particle efficiency (e + && e -, %) Generated events in the whole solid angle (  ).  4 chambers required for detection Large errors due to small statistics, but promising results; 0.1 X 0 looks the best choice events (> 4 chambers) ~ 400 Hz

26 Total momentum distribution No reconstruction included This FWHM must be compared with the value quoted in the Proposal: Thickness 0.1 X 0 FWHM ~ 0.7  0.9 % e + + e - momentum (MeV)

27 Wire presently mounted in “Large Prototype” Am SOURCES ON WIRE AND WALLS Potentialities : PMT quantum efficiencies Xenon optical properties low-energy position and energy calibration use in Xe gas and liquid stability checks ? a unique method for cryogenic liquid detectors !! Sources in production. Soon available for all LXe devices. Open problems: will the method be usable under full intensity beam conditions ? To be verified by test ! BVR February 2005

28 reconstruction of the 8  -source positions in gaseous Xe. Recent measurement with the large-prototype. (Po-source produced in Genoa)

29 RINGS IN LIQUID XENON the ring radius depends on the Rayleigh scattering length in LXe

30 Determination of the relative QE for 4 different PMTs by the use of 4 dot-wire-sources in Xe gas of the large-prototype the relative QEs are given by the slope of the linear fits.

31 large-prototype in the large-prototype the line is worse..... (thermal neutrons in LXe !) the measurement must be repeated, protecting LXe from thermal neutrons by a borated-foil NaI  /E=2.5% C&M by NEUTRONS AND NICKEL-LINE, AT THE BACK OF THE CALORIMETER

32 CONCLUSIONS Several C&M methods tested with satisfactory results: wire-sources  0 and  from  – charge exchange thermal neutrons and nickel  -line Other C&M methods in preparation or being modified for MEG: CW accelerator and 3 7 Li (p,  ) 4 8 Be reaction new methods for  0 and  from  – charge exchange

33 EXTRA SLIDES

34 Some distributions – a) P e+ + P e- = E  129 MeV Energy loss and MS Thickness 0.15 X 0

35 Some distributions – b) e+/e- momenta At least 4 chambers (7 hits) required Region to be selected (both e + /e - seen) Thickness 0.15 X 0  energy Relativeangle

36 RADIO FREQUENCY QUADRUPOLE ACCELERATOR practically monoenergetic pulsed operation; frequency 100 Hz 100  s pulses average current 50  A, pulsed current 5 mA beam energy bin approx. 10 keV small vessel, pre-accelerator beam optical properties ? 1mm ; 20 mR RF radiation ? No proton source ? Plasma cost ? acceptable (AccSys), (Neue Tech.) !!!!! special design....time to produce ? One year not an out-of-the-shelf machine Companies: AccSys, Neue Technologien GMBH

37

38 MC ingredients  Liquid hydrogen (LH 2 ) target close to the muon stopping target (10 cm length x 5 cm diameter); stopping target (10 cm length x 5 cm diameter);  Thin tungsten converter adjacent to the LH 2 target; thickness between 0.05 X 0 and 0.3 X 0 ; target; thickness between 0.05 X 0 and 0.3 X 0 ;   0 decay & n  pair generated in the LH 2 target with the correct energy and angular distributions; the correct energy and angular distributions;  Tracking of photons from   decay;  Tracking of electron & positron from photon conversion;  Multiple scattering in tungsten included;  Minimum number of chambers (4) in DC system required to define a track; required to define a track;  Energy/momentum reconstructions: work in progress  Increase of MC statistics: under way

39   p  n  0 Some distributions 1 st  –e + relative angle and multiple scattering effect 2 nd  –e + relative angle vs energy loss in LXe Region to be selected for energy calibration  E (MeV)  E in LXe(MeV) Higher density of points for  E  60 MeV Beforeconverter FWHM < 2 0 After converter FWHM ~ 6 0   0) Converter thickness 0.15 X 0

40 Impact point and  energy release in LXe Converter thickness 0.15 X 0 Relative angle  2 -e +  > FWHM(energy)  4 - 5% FWHM  cos (  ) Uniform coverage of the whole calorimeter  1 -e +

41 Efficiency vs converter thickness Efficiency vs converter thickness Thickness (X 0 )  ( o, FWHM) Events with 50<  E<60 MeV Generated events in the solid angle covered by the LXe calorimeter (10 %)  4 chambers; relative angle  events (> 4 chambers) ~ 23 Hz

42 Rough estimate of the time needed for the LXe calibration    (20  30)/10 5 /10 = (20  30) x  R = R   x  = (R   /10 6 ) x 10 6 x (20  30) x = (20  30) x (R   /10 6 ) Hz (max.MEG acquisition rate  100 Hz) (20  30) x (R   /10 6 ) Hz (max.MEG acquisition rate  100 Hz)  Events/day  8.64 x 10 4 R  2 x 10 6 x (R   /10 6 )  Assuming  50 locations to be calibrated (216 PMTs in groups of 4): (216 PMTs in groups of 4): (< 1000 events/location would be sufficient) (< 1000 events/location would be sufficient) 1000 events/50 s total for 50 locations 2500 s < 1 h Solid angle factor Solid angle factor Reconstruction and trigger efficiencies under evaluation

43 Assuming N 0 = MeV photons/s: N(e + e - pairs detected)/s = N 0 x  pair ~ 400/s. N 0 x  pair ~ 400/s. Requiring 10 6 pairs in the wire-chamber spectrometer (at a rate of 100 Hz: spectrometer (at a rate of 100 Hz: Time = 10 6 /(100/s) = 10 4 s Time = 10 6 /(100/s) = 10 4 s (less than three hours). (less than three hours).