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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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 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

9. 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
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)

10. 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

11. 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).

12. 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

13. 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

14. 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