Module 04: The HC Threshold
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What Hydrocarbons Do to a Beam
When photons or electrons strike a surface contaminated with hydrocarbons, they crack the molecules and deposit amorphous carbon. On an X-ray mirror or undulator chamber, just a few nanometers of carbon changes reflectivity, absorbs photons, and degrades beam quality.
This is not a slow degradation — it is an irreversible surface damage that requires removal and re-polishing. That is why hydrocarbon contamination is the go/no-go criterion for putting beam into a beamline. The HC Threshold quantifies this risk.
This is not a slow degradation — it is an irreversible surface damage that requires removal and re-polishing. That is why hydrocarbon contamination is the go/no-go criterion for putting beam into a beamline. The HC Threshold quantifies this risk.
Why Masses 47–100?
The RGA spectrum below mass 47 is crowded with innocent species: H₂ (2), water (18,17,16), CO/N₂ (28), CO₂ (44), and all their fragments. These are always present, even in the cleanest system. But above mass 46, any signal means hydrocarbon contamination.
M1–46: Common zone (always present)
M47–100: HC zone (contamination indicator)
Why not M44? CO₂ is ubiquitous. Why not M45–46? Isotopologues of CO₂ (13CO₂ at 45, C18OO at 46). M47 is the first mass where signal = real contamination.
The HC Threshold Sum
HC Threshold Sum = the sum of all partial pressures from M47 to M100 that exceed the noise floor. It is a single number that captures the total HC contamination level. The pass/fail threshold is 1 × 10−12 Torr.
HC Threshold Sum = Σm=47100 max(Pm − Pnoise, 0)
1.0 × 10−13 Torr
PASS
Power-Law Fit & ETA
During a bake, the HC Threshold decreases over time. We fit a power-law: HC Threshold(t) = A · t−b. Extrapolating this fit tells us when the system will pass — the ETA.
The dashed line below shows the projection. The red horizontal line is the pass threshold. Where they cross is the ETA.
The dashed line below shows the projection. The red horizontal line is the pass threshold. Where they cross is the ETA.
Why Monte Carlo? The fit has uncertainty in both A and b. A simple extrapolation gives an optimistic ETA. Instead, we perturb the parameters 1000 times within their error bars, compute 1000 ETAs, and report the 80th percentile. This is more conservative and accounts for measurement noise.
Common HC Sources
Recognizing the fingerprint of each HC source helps you identify and fix the root cause faster.
Pump Oil Backstreaming
Turbo/roughing pump oil migrates upstream. Produces saturated hydrocarbon fragments with a regular pattern of odd masses.
Key masses: M55, 57, 69, 71 (CnH2n+1 series)
Fingerprints / Skin Oils
A single fingerprint deposits ~1 µg of long-chain fatty acids. Produces a broad, diffuse forest across the entire HC range.
Pattern: broad M39–100, no dominant single peak
Solvents (IPA, Acetone)
Residual cleaning solvents trapped in crevices. Each has a characteristic parent ion.
Acetone: M43 (parent), M58 | IPA: M45, M31
O-ring Outgassing (Silicone)
Silicone elastomers release cyclic siloxane fragments. Regular pattern with 14 AMU spacing.
Key masses: M41, 55, 69 (Si–O–CH3 fragments)
Machining Oils / Cutting Fluids
Residual paraffin-based fluids from manufacturing. Long-chain alkane fragments with ~14 AMU spacing.
Key masses: M57, 71, 85 (CnH2n+1 paraffin series)
Origin & Theory
The HC Threshold is an RGA-based hydrocarbon cleanliness criterion, originally formulated at Fermilab (FNAL TD-09-005, also known as Fermilab Procedure 14) for the ILC/SRF superconducting cavity qualification program. The criterion defines a single number — the sum of partial pressures from M47 to M100 above the noise floor — that captures total HC contamination. It is used today as a general go/no-go check wherever surface cleanliness drives beam reliability.
HC outgassing (power-law):
qHC(t) = q0 · (t / t0)−α
α = 0.5–1.5 depending on HC species and surface treatment. Heavier HC have larger α (faster decay).
qHC(t) = q0 · (t / t0)−α
α = 0.5–1.5 depending on HC species and surface treatment. Heavier HC have larger α (faster decay).
Arrhenius for HC desorption:
r(T) = ν · exp(−Ea / kBT)
Ea = 0.7–1.0 eV for hydrocarbons (cf. 0.45–0.9 eV for water). Higher Ea means HC are harder to remove, hence longer bakes.
r(T) = ν · exp(−Ea / kBT)
Ea = 0.7–1.0 eV for hydrocarbons (cf. 0.45–0.9 eV for water). Higher Ea means HC are harder to remove, hence longer bakes.
| Species type | Ea (eV) | α exponent | Bake response |
|---|---|---|---|
| Light HC (C1–C3) | 0.5–0.7 | 1.0–1.5 | Fast removal at 100°C+ |
| Medium HC (C4–C8) | 0.7–0.9 | 0.7–1.0 | Moderate, needs 120°C+ |
| Heavy HC (C9+, oils) | 0.9–1.2 | 0.5–0.7 | Slow, needs 150°C+ extended |
| Silicone (O-rings) | 0.8–1.0 | 0.6–0.8 | Very slow, difficult to bake out |
Monte Carlo ETA Algorithm:
1. Fit power-law HC Threshold(t) = A · t−b to last N data points → get (A, b, σA, σb)
2. For i = 1 to 1000:
Ai = A + randn() · σA ; bi = b + randn() · σb
ETAi = tnow · (Ai / threshold)1/bi
3. Sort ETA values, take P80 = 80th percentile
4. Report ETA = P80 (conservative: 80% of simulations pass by this time)
1. Fit power-law HC Threshold(t) = A · t−b to last N data points → get (A, b, σA, σb)
2. For i = 1 to 1000:
Ai = A + randn() · σA ; bi = b + randn() · σb
ETAi = tnow · (Ai / threshold)1/bi
3. Sort ETA values, take P80 = 80th percentile
4. Report ETA = P80 (conservative: 80% of simulations pass by this time)
Carbon deposition rate on photon optics:
dC/dt ∝ Φphoton · PHC · σcrack
Φ = photon flux (ph/s/mm²), PHC = HC partial pressure, σcrack = cracking cross-section. Critical dose for X-ray mirrors: ~1018 ph/mm² at 10−10 Torr HC.
dC/dt ∝ Φphoton · PHC · σcrack
Φ = photon flux (ph/s/mm²), PHC = HC partial pressure, σcrack = cracking cross-section. Critical dose for X-ray mirrors: ~1018 ph/mm² at 10−10 Torr HC.
References
[1] FNAL TD-09-005, "Procedure for RGA Qualification of SRF Cavities," Fermilab Technical Division, 2009.
[2] J.F. O'Hanlon, A User's Guide to Vacuum Technology, 3rd ed., Wiley, 2003 — Ch. 7: Outgassing.
[3] K. Jousten (ed.), Handbook of Vacuum Technology, 2nd ed., Wiley-VCH, 2016 — Ch. 12: Partial Pressure Measurement.
[4] R. Reiser, "Carbon contamination of synchrotron radiation optics," Nucl. Instr. Methods A 467–468 (2001) 1041–1044.
[5] SLAC-TN-23-003, "Vacuum bake-out procedures for LCLS-II cryomodules," 2023.
[2] J.F. O'Hanlon, A User's Guide to Vacuum Technology, 3rd ed., Wiley, 2003 — Ch. 7: Outgassing.
[3] K. Jousten (ed.), Handbook of Vacuum Technology, 2nd ed., Wiley-VCH, 2016 — Ch. 12: Partial Pressure Measurement.
[4] R. Reiser, "Carbon contamination of synchrotron radiation optics," Nucl. Instr. Methods A 467–468 (2001) 1041–1044.
[5] SLAC-TN-23-003, "Vacuum bake-out procedures for LCLS-II cryomodules," 2023.