LXe Alignment with X-rays W. Molzon for UCI group

Goal of Alignment In-situ measurement of MPPC positions in fully operational configuration Technique that does not require beam time, modest time for measurement, repeatable Technique with cross-checks of optical survey allowing direct correlation between LXe and DC alignment

Overview of Technique Produce collimated X-ray beam of known position and orientation Detect electrons from Compton scattering of photons immediately in front of MPPC – direct measurement of MPPC transverse ( Rφ and Z ) position X-ray requirements Small beam size in at least one direction – spot small compared with MPPC dimension Sufficient rate to do measurement in reasonable time Beam aligned with spatial precision of order 200 μm, angle of order 0.2 mrad Energy large enough to have large transmission probability into LXe Energy low enough so that absorption length in LXe is short so photon scatters near in front of MPPC. Technique with monitor of beam alignment as it is repositioned Cross-check of X-ray beam position using small X-ray detectors on the cryostat exterior Collect data with standard DAQ Implement trigger with lower limit on energy in restricted set of MPPCs, veto on energy above some threshold in full calorimeter to reduce CR rate

Choice of Source Could use X-ray tube or radioactive source Advantages/disadvantages of X-ray tube Can get very high intensity – for some applications, can produce a very small beam spot Can get arbitrary energy For energy > 100 keV, devices become large For energy > ~40 keV, only get bremsstrahlung X-rays, most flux is well below the electron beam endpoint Advantages/disadvantages of radioactive source Can get a variety of mostly mono-chromatic beams Sources can be small, no power required Rates for any particular source are limited – in most cases cannot make a very highly collimated spot source at reasonable rate. For now, focus on radioactive source

Energy Considerations 1 Electron range by dE/dx is ~ 6MeV/cm in LXe ⇒ X-ray energy below 600 keV will produce electron with range below 1 mm. X-ray range in various materials can be found in a web-based calculator Preferably keep energy < 200 keV to keep scattering in LXe close to MPPC Potential problem if significant LXe between cryostat and MPPC Absorption in COBRA and LXe cryostat is manageable for energies down to ~100 keV – table below for 124 keV and estimate of COBRA/Lxe Some modulation of transmission fraction due to construction (e.g. SC cable in Al cable) – might be able to measure coil positions by transmission rate

Energy Considerations 2 Table on the right shows potentially relevant X-ray sources Table below shows energy, activity, branching fraction, and dimension (radius of disk) for readily available sources Note 1 mCi is 3.6x107 decays/sec – will show that ~100 mCi is needed

Expected Rate 1 Imagine a beam with spot size < 1 mm by few cm at LXe Use a collimator ~100 mm long, slit 0.1 x 5-10 mm wide Measure 1 coordinate at a time, using the narrow beam Illuminate a few MPPCs in the non measurement direction Only a fraction of the source is used due to small slit – note beam is triangular 0.1 mm 20 mm 3.0 mm 5 mm 100 mm

Expected data collection time Scan in steps ~ 1/10 of MPPC size in the measurement direction Cover 3 MPPCs in the non-measurement direction, total of 4000 MPPCs Individual measurement time of 6s, including repositioning, assuming ~200 events per MPPC per position – see MC results below Total time per orientation (Rφ or Z) of 22 hours Potential impacts on measurement time Attenuation might be larger (MC gives transmission of 0.3 vs. 0.37 estimated here LXe between cryostat and MPPC could substantially attenuate X-rays – estimate from Toshihuki is 1-2 mm, which would increase statistical uncertainty by 21-46% Could increase the beam width (non-measurement direction) to increase rate Assume an efficient trigger can be made and that DAQ can handle rates up to few hundred Hz Assume that deadtime due to CR interactions is small

MC simulation of alignment scheme Use default MEG2 MC Generate isotropic distribution of 124 keV X-rays -50<φ< 50 and -25<θ<25 mrad. Equivalent to 100 measurement points with a beam 1 mrad in φ, 50 mrad in θ 150000 events generated, transmission to LXe ≃ 30% (compare simple estimate 30%) Corresponds to ~2.5 s of data per measurement point (~3 MPPCs per measurement) Default MPPC fill factor, QE, avalanche efficiency included in the number of generated scintillation photons (Ryu), reflection included in GEANT Only MPPC signal used, directly from MC (no bartender)

Sample analysis of alignment data 1 Assume that the data are equivalent to a scan to measure the φ position Analyze the position of the MPPC at Z=0, θ=π/2 Associate X-rays in a bin of 1 mrad in generated φ distribution with X-rays from a single measurement (recall that the beam is actual triangular, not square) Apply simple selection criteria sum of NPE in central MPPC and those above and below it > 50 sum of NPE in this central column greater than that in column on either side Plot number of photons in central MPPC vs generated φ of X-ray Make a profile plot of the mean number of photons in central MPPC vs. φ of X-ray Mean from a Gaussian fit (not best) determined with precision 0.06 mrad

Sample analysis of alignment data 2 Generate new MC event samples with the calorimeter rotated in φ by varying amounts as a test of some systematic errors Plot measured position of central MPPC vs rotation angle – fit to straight line and plot deviations from straight line Deviation from linear fit is larger than statistical precision on fit Including scatter of these deviations as a measure of some systematic errors still gives expected precision below 0.2 mrad (below 0.2 mm).

Some comments on technical implementation Translation stage supported on a beam cantilevered in from downstream of COBRA Translation stage with motion 600 mm will fully cover the Z extent of the calorimeter Mount a rotation stage on the table of the translation stage, with the rotation axis in Z direction, and with the collimator and source mounted on the rotation stage Clearance for the X-ray beam to be rotated through full φ extent of the calorimeter Can probably be made small enough to allow insertion with DC in place Appropriate commercially available stages have been found 0.01 mm in translation, ~0.5 mrad in rotation Variation in pitch (rot. about x), roll (rot. about z), yaw (rot. about y) of < 1 mrad Various checks of x-ray beam alignment Electronic level on the translation stage – measure pitch and roll at each position Use a laser on the translation stage aligned with Z axis: monitor pitch and yaw by monitoring spot at a fixed plane 1-2 m away from the translation stage Install a few small X-ray detectors (small scintillator chip with MPPC) on COBRA and LXe cryostat, compare position from optical survey with position from X-ray survey

Some comments on possible additional measurements Radial coordinate of MPPC by stereo – requires two measurements with source displaced by dy = ± 7 cm. Precision of ~ 1mm possible Possible check of wire positions Use a low-energy X-ray tube with very small spot See a wire signal when X-ray hits the wire – this technique has been used with good results Complicated because only Z and Rφ are measured and wires are stereo Possible check of coordinates of timing counters – look for interactions in the scintillator Signal is small compared to minimum ionizing Requires repositioning the translation stage in Z

Next steps Confirm noise and background levels and their contribution to the alignment precision Confirm material between source and MPPC for absorption calculation Design improved technique for analysis to determine MPPC position Prepare a technical design of the support of the cantilevered beam that supports the translation stage, preferably allowing use of the device with the DC installed. Prepare a technical design for the collimator and the support for the rotary and translational stages. Prepare a technical design for the ancillary measuring devices that would be used to check the optical alignment of the X-ray beam Electronic gravity level Laser and 4 quadrant photodiode for pitch and yaw control Request – support from collaboration for this measurement to allow proposal to DOE for resources Check of technique with small prototype – requires MPPCs near cryostat edge