Add optional protonabsorber.zStartInMu2e: absolute IPA position by oksuzian · Pull Request #1913 · Mu2e/Offline

oksuzian · 2026-08-01T03:36:43Z

Motivation

The MECO-style inner proton absorber is placed relative to the stopping
target: MECOStyleProtonAbsorberMaker computes targetEnd from the live
StoppingTarget and derives everything from it — body placement, DS2/DS3
segmentation, the cone-radius interpolation anchor, and the IPA
support-wire ring positions. But the IPA is fixed hardware;
protonAbsorber_cylindrical_v04.txt itself documents
distFromTargetEnd = 625.; //mu2e z positions are 6901-7901 mm.

Anyone varying stopping-target geometry gets a silently displaced
absorber. In our stopping-target optimization study a longer foil stack
dragged the IPA 133.33 mm downstream as a rigid body (measured with
protonabsorber.verbosityLevel = 1: 7034.35–8034.35 vs the design
6901.02–7901.02) through 414 grid evaluations before it was noticed. For
a cylindrical IPA this is exactly compensable by adjusting
distFromTargetEnd; for the conical variant it is not (the taper anchor
moves with the target) — hence a first-class option.

Change

New optional key protonabsorber.zStartInMu2e. When present, the maker
derives its reference plane from the given absolute z
(targetEnd = zStartInMu2e - distFromTargetEnd), so the absorber's
upstream end lands exactly at the given z and its full geometry (length,
DS2/DS3 split, cone radii, support wires) is independent of the stopping
target. When absent — the default everywhere — the new branch never
executes and output is bit-identical to today. One commented example
added to protonAbsorber_cylindrical_v04.txt.

Validation (Offline v13_32_10 + SimJob/Run1Bap backing)

Six single-event G4 runs with full surface check
(protonabsorber.verbosityLevel = 1), stopping-target stack extents
400 / 800 / 1100 mm, each extent probed twice — config-compensated
distFromTargetEnd (no new key) vs stock 625 + zStartInMu2e = 6901.02:

probe	mechanism	absorber Z extent printed
extent 400, expression	distFromTargetEnd = 825.0	constructProtonAbsorber protonabs1 Z extent in Mu2e : 6901.02, 7901.02
extent 800, expression	distFromTargetEnd = 625.0	constructProtonAbsorber protonabs1 Z extent in Mu2e : 6901.02, 7901.02
extent 1100, expression	distFromTargetEnd = 475.0	constructProtonAbsorber protonabs1 Z extent in Mu2e : 6901.02, 7901.02
extent 400, option only	zStartInMu2e = 6901.02, dist = stock 625	constructProtonAbsorber protonabs1 Z extent in Mu2e : 6901.02, 7901.02
extent 800, option only	zStartInMu2e = 6901.02, dist = stock 625	constructProtonAbsorber protonabs1 Z extent in Mu2e : 6901.02, 7901.02
extent 1100, option only	zStartInMu2e = 6901.02, dist = stock 625	constructProtonAbsorber protonabs1 Z extent in Mu2e : 6901.02, 7901.02

All six print the identical absorber extent 6901.02–7901.02 with zero
overlaps. With the key unset, a golden-parity harness (real G4 preflight
replay) reproduces baseline output unchanged.

🤖 Generated with Claude Code

The MECO-style inner proton absorber is placed relative to the stopping target (targetEnd), but it is fixed hardware -- the geometry file itself documents 'mu2e z positions are 6901-7901 mm'. Studies that vary the stopping-target geometry silently drag the absorber (and its support-wire rings) with it. When the new key is set, the maker derives its reference plane from the given absolute z instead of the live target; unset, the code path is untouched and output is bit-identical. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01R5HnfdYMwXXrJGAkYVE48c

FNALbuild · 2026-08-01T03:37:11Z

Hi @oksuzian,
You have proposed changes to files in these packages:

GeometryService
Mu2eG4

which require these tests: build.

@Mu2e/write, @Mu2e/fnalbuild-users have access to CI actions on main.

⌛ The following tests have been triggered for a894d5d: build (Build queue - API unavailable)

About FNALbuild. Code review on Mu2e/Offline.

FNALbuild · 2026-08-01T03:47:07Z

☔ The build is failing at a894d5d.

scons: *** [build/al9-prof-e29-p103/Offline/tmp/CosmicReco/src/LineFinder_module.os] Error 1

Test	Result	Details
test with	✅	Command did not list any other PRs to include
merge	✅	Merged `a894d5d` at `e9b2fbd`
build (prof)	❌	Log file.
ceSimReco	〰️	Log file.
g4test_03MT	〰️	Log file.
transportOnly	〰️	Log file.
POT	〰️	Log file.
g4study	〰️	Log file.
cosmicSimReco	〰️	Log file.
cosmicOffSpill	〰️	Log file.
ceSteps	〰️	Log file.
ceDigi	〰️	Log file.
muDauSteps	〰️	Log file.
ceMix	〰️	Log file.
rootOverlaps	〰️	Log file.
g4surfaceCheck	〰️	Log file.
trigger	❗	Log file.
check_cmake	〰️	Log file.
FIXME, TODO	✅	TODO (0) FIXME (0) in 1 files
clang-tidy	➡️	0 errors 59 warnings
whitespace check	✅	no whitespace errors found

N.B. These results were obtained from a build of this Pull Request at a894d5d after being merged into the base branch at e9b2fbd.

For more information, please check the job page here.
Build artifacts are deleted after 5 days. If this is not desired, select Keep this build forever on the job page.

FNALbuild · 2026-08-01T03:47:11Z

📝 The HEAD of main has changed to ed58b12. Tests are now out of date.

oksuzian · 2026-08-01T03:48:48Z

@FNALbuild run build test

FNALbuild · 2026-08-01T03:49:11Z

⌛ The following tests have been triggered for a894d5d: build (Build queue - API unavailable)

FNALbuild · 2026-08-01T04:11:05Z

☀️ The build tests passed at a894d5d.

Test	Result	Details
test with	✅	Command did not list any other PRs to include
merge	✅	Merged `a894d5d` at `ed58b12`
build (prof)	✅	Log file. Build time: 10 min 46 sec
ceSimReco	✅	Log file.
g4test_03MT	✅	Log file.
transportOnly	✅	Log file.
POT	✅	Log file.
g4study	✅	Log file.
cosmicSimReco	✅	Log file.
cosmicOffSpill	✅	Log file.
ceSteps	✅	Log file.
ceDigi	✅	Log file.
muDauSteps	✅	Log file.
ceMix	✅	Log file.
rootOverlaps	✅	Log file.
g4surfaceCheck	✅	Log file.
trigger	✅	Log file.
check_cmake	✅	Log file.
FIXME, TODO	✅	TODO (0) FIXME (0) in 1 files
clang-tidy	➡️	0 errors 59 warnings
whitespace check	✅	no whitespace errors found

N.B. These results were obtained from a build of this Pull Request at a894d5d after being merged into the base branch at ed58b12.

For more information, please check the job page here.
Build artifacts are deleted after 5 days. If this is not desired, select Keep this build forever on the job page.

AndrewEdmonds11

This is OK with me as a patch. I think ideally we would number a new version and then have new code that would not refer to the target position at all but I appreciate that that would be a lot of effort

AndrewEdmonds11 · 2026-08-03T13:40:13Z

+    // stopping target; distFromTargetEnd then only anchors the cone-radius
+    // interpolation. Unset (default): stock target-relative behavior.
+    if ( _config.hasName("protonabsorber.zStartInMu2e") ) {
+      targetEnd = _config.getDouble("protonabsorber.zStartInMu2e") - distFromTargetEnd;


Does the distFromTargetEnd parameter still need to be set correctly? By default it is zero and so here the targetEnd would be the starting position of the IPA...

FNALbuild added build pending build running and removed build pending labels Aug 1, 2026

oksuzian requested a review from AndrewEdmonds11 August 1, 2026 03:46

FNALbuild added build pending and removed build running labels Aug 1, 2026

FNALbuild added build running and removed build pending labels Aug 1, 2026

FNALbuild added build finished and removed build running labels Aug 1, 2026

AndrewEdmonds11 approved these changes Aug 3, 2026

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oksuzian mentioned this pull request Aug 3, 2026

Fix IPA EndRing parameters and add debugging for wire support #1915

Merged

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Add optional protonabsorber.zStartInMu2e: absolute IPA position - #1913

Add optional protonabsorber.zStartInMu2e: absolute IPA position#1913
oksuzian wants to merge 1 commit into
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oksuzian:ipa-absolute-position

oksuzian commented Aug 1, 2026 •

edited

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oksuzian commented Aug 1, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Motivation

Change

Validation (Offline v13_32_10 + SimJob/Run1Bap backing)

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oksuzian commented Aug 1, 2026 •

edited

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