Stopped Muon/Pion Measurements

Jim Miller, BUMay 2012

UW Test Beam Meeting

Possible Measurements with Stopped Muons and Pions• For beam normalization: Collect muonic, pionic xray data in singles mode

– Ideally muons and pions are done separately in pure beams– How serious is background from electrons/positrons in beam?– Run rate up artificially with a radioactive source to test performance of Ge at high

rates• Look at activation gammas following mu and pi capture as alternative to xrays for

normalization; run beam for a while with known total number of stopped muons to calibrate– Pi activation could confuse us but there are far fewer stopped pi’s than stopped mu’s – Other sources of excited Al? e.g. e?

27 27

27 27

27 27

( , ) (13%)

* ; 9.458 m

*

e

Al Mg

Mg Al e

Al Al

Examples of muonic x ray spectra

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F.J.Hartman, et al. Z. Phys. A 305, 189 (1982)

J. Miller - Director's CD-1 Review

Magnesium Lyman Series

Aluminum Balmer Series

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Physics Requirements- Xray Monitor

J. Miller - Director's CD-1 Review

On the choice of detector type• Excellent energy resolution (~ a few keV)• Sufficiently large photopeak efficiency (>50%)• Ability to tolerate background particles including neutrons and low energy

photons• Handle potentially high count rates, especially at injection time n-type intrinsic germanium detector with a fast reset preamp (size > 45cm3,

depth > 3cm)

On the location of the detector • The detector should only view the target.

– good collimation ahead of the detector.

• Far from the stopping target due to the large xray production rate ~1010 Hz, the detector must be far from the source,

• Path from target: low attenuation of xrays• A sweeper magnet should be used to eliminate charged particles• The detector must be lie beyond the DS magnetic field and enclosure where it

can be serviced periodically and annealed to repair neutron damage.

R. Ray - Director's CD-1 Review 5

Location of stopping target monitor

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Conceptual Design

J. Miller - Director's CD-1 Review

The Muon Stopping Target Monitor is placed along the axis of the Detector Solenoid, at the downstream end of the end cap steel.

End cap shielding

Permanent magnet dipole

Ge detector1 mm thick SS window

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Conceptual Design

J. Miller - Director's CD-1 Review

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Conceptual Design

J. Miller - Director's CD-1 Review

Selected specs:•Photopeak efficiency at 1.33 MeV = 50%•Resolution = 2.2 keV (FWHM) at 1.33 MeV•Count rate = up to 106/s at 1 MeV

An example of commercially available Ge detector•Ortec GMX HPGe detector GMX50P4 with X-cooler II mechanical cooler and a ultra-high count rate preamplifier option

R. Ray - Director's CD-1 Review 9

Estimated Rates• Muon stopping rate ~ 1x1010 Hz• 2p-1s xray production rate ~0.8x1010

• 2p-1s xray detection rate in photopeak, assuming 50% photopeak efficiency, 80% through 2p-1s: 1x1010x 0.5(photopeak efficiency)x2.8x10-6(geometric acceptance)x0.8(BR of 2p-1s)=11000 Hz

• Estimate of neutron damage: anneal every 1-3 months

Other Measurements• Other alternatives to xray stopping monitor for flux monitoring

– Look at high energy photons from radiative DIO (RDIO) of stopped muons (flux drops fast above 53 MeV), also similar photon spectrum from bremsstrahlung in target following regular DIO [BR 1.4%]

– Look at flux of photons from RMC (BR quite low but may have higher flux than gammas from RDIO at high energies).

– Neutrons from stopped muons• Could high-E neutrons be used to monitor muon capture rate?

• Neutrons, Protons from muon capture• 70 MeV positrons from• Calibration photons(need thin converter to make e+-e-) from

– Use e.g. Lithium borate, see how clean the 129 MeV peak is.

ee

0

(129 MeV)

(51-88 MeV)

p n

p n

Contaminated Targets

• Oxygen has a higher endpoint energy than either Al or Ti, therefore the DIO’s can create background

• With Oxidized targets of Al and Ti, measure relative number of xrays and hence relative number of muons stopped in O compared to Al or Ti (e.g. Al2O3, …)

Comment

• We should select one or two measurements for the TRIUMF run