Residual Gas Effect on 

aCORN

WILLIAM (DREW) BYRON 
Acknowledgements

Thanks to: 

◦My advisor Scott Dewey.

◦My professor Fred Wietfeldt. 

◦Tulane University 

◦The SURF Program and Directors 

◦NIST 

◦DOE and NSF 
Outline 

1. aCORN Experiment Overview

2. Residual Gas Effect Overview

3. Residual Gas Simulation Results

4. Measurement of the Residual Gas Composition

5. How do we arrive at a final value for the size of this systematic effect? 
Outline 

1. aCORN Experiment Overview

2. Residual Gas Effect Overview

3. Residual Gas Simulation Results

4. Measurement of the Residual Gas Composition

5. How do we arrive at a final value for the size of this systematic effect? 
Neutron Decay 
What is aCORN? 

Little a CORrelation

in Neutron Decay
Neutron Decay Parameters
aCORN Schematic

Proton Momentum Space Diagram 

Group I (Fast) Group II (Slow)
The Wishbone 

• Group I = Fast, Small TOF

• Group II = Slow, Large TOF

a = 

1

vvβ

K(EEβ) NNFFFFFFFF−NNSSSSSSSS

NNFFFFFFFF+NNSSSSSSSS
The aCORN Apparatus 

Proton Collimator Location 

Solenoid

Decay Volume Location 
Outline 

1. aCORN Experiment Overview

2. Residual Gas Effect Overview

3. Residual Gas Simulation Results

4. Measurement of the Residual Gas Composition

5. How do we arrive at a final value for the size of this systematic effect? 
Residual Gas Effect

•The gas remaining in the vacuum apparatus during data collection will have an effect on the 

asymmetry we are measuring.

•We are attempting to measure the size of this residual gas effect by increasing the pressure in 

the apparatus and recording the effect this has on the asymmetry. Is the effect linear? 

•We are also modelling the effect in order to guarantee that the physics of the effect is well 

enough understood to trust the experimental data. 

High Pressure = 100 times running 

pressure

Effect = -.1 (deviation in asymmetry) 

Running Pressure

Effect = -.1/100 = -.001 
Residual Gas Effect Continued 

185 cm 

Proton Detector

Residual Gas

Neutron Decay!
Residual Gas Experiment 

= High pressure

(2.5x10-5 torr)

= Normal pressure

(4x10-7 torr)

Asymmetry vs. Beta Energy (keV)
Outline 

1. aCORN Experiment Overview

2. Residual Gas Effect Overview

3. Residual Gas Simulation Results

4. Measurement of the Residual Gas Composition

5. How do we arrive at a final value for the size of this systematic effect? 
Geant 4 Simulation 

= Running pressure

(4x10-7 torr)

= High pressure

(2.5x10-5 torr)

Asymmetry vs. Beta Energy (keV)
Residual Gas Simulation: Things to Consider

•Neutralization and Scattering

•Residual gas composition: Initially assumed to be H2

•Pressure and Temperature in proton collimator: 4 x 10-7 torr, 200K
Cross Sections: Proton in H2 Gas 

A . V . P H E L P S , J O U R N A L O F P H Y S I C A L A N D C H E M I C A L R E F E R E N C E D A T A , 6 5 3 - 6 7 5 ( 1 9 9 0 )
Cross Section Derivatives 

Protons born here. 

• Recall: 

Asymmetry = NNFFFFFFFF−NNSSSSSSSS

NNFFFFFFFF+NNSSSSSSSS

• Thus:

• Intuitively, a positive 

derivative would result in 

a negative change in 

asymmetry.

• And a negative derivative 

would result in a positive 

change in asymmetry. 
Neutralization 

Method: 

•Alteration of existing Runge-Kutta Monte Carlo to incorporate the effect of 

neutralization. Proton Energy and distance traveled per step are used to 

calculate proton weight. 

•Weighti = Weighti-1 ∗ ee−σ∗Δxxii

∗nn

1 .99

.98

Neutron Decay! Step 1 Step 2 Step 3 

Step 4 

Proton Detector
Scattering 

• Energy considerations lead us to believe that 

the H2 molecules are in their vibrational 

ground state. 

• The large relative energy of the proton makes 

many vibrational states accessible for 

excitation. 

Method: 

•Alteration of existing Runge-Kutta Monte Carlo 

to incorporate the effect of scattering. When 

scattered the proton recoil velocity direction is 

randomized within a cone. 
Monte Carlo Results 

• The change in asymmetry 

(delta asymmetry) caused 

by the residual gas found in 

the MC is both the right sign 

and comparable in size to 

the preliminary 

experimental results. 
Outline 

1. aCORN Experiment Overview

2. Residual Gas Effect Overview

3. Residual Gas Simulation Results

4. Measurement of the Residual Gas Composition

5. How do we arrive at a final value for the size of this systematic effect? 
Residual Gas Analyzer (RGA) Results

Running Pressure (1.04 x 10-6 torr) High Pressure (5.89 x 10-5 torr) 

Highest Gas Load: H2 and H2O Highest Gas Load: N2 and O2
Residual Gas Analyzer (RGA) Results

Running Pressure: 

Highest Gas Load: H2 and H2O 

High Pressure: 

Highest Gas Load: N2 and O2
Outline 

1. aCORN Experiment Overview

2. Residual Gas Effect Overview

3. Residual Gas Simulation Results

4. Measurement of the Residual Gas Composition

5. How do we arrive at a final value for the size of this systematic effect? 
How to measure this Effect? 

•Our proposed method for measuring the size of the effect will probably not 

suffice. 

•We may need to just scale the pressure by a factor of 4 as opposed to a factor of 

40 in order to maintain the same gas composition. But measuring the effect this 

way will take longer and be more difficult because the pressure effect will be 

smaller. 

•Cross sections for low energy protons in H2O must be found and put into the 

Monte-Carlo simulation. 
Conclusion 

•The residual gas systematic effect is relevant to several major neutron decay 

experiments (aCORN, Beam Lifetime, Nab) and thus a thorough investigation is 

necessary, especially when programs like Geant 4 are failing to aptly model the 

problem. 

Thank you!
What are the energies of these protons? 

• I recorded the energy of the 

proton for each step in the 

Monte Carlo for a given 

proton group (Fast or Slow) 

and Beta energy.
Residual Gas Analyzer (RGA) Results

Running Pressure (1.04 x 10-6 torr) High Pressure (5.89 x 10-5 torr) 

Highest Gas Load: H2 and H2O Highest Gas Load: N2 and O2
RGA Results Continued 

•Our proposed method for measuring the size of the effect will not work as we had thought. 

•The regular running pressure, temp, RG composition. And the high pressure temp and RG 

composition? 

•The purpose of the MC and the purpose of the experiment. And how the experimental value is 

used to actually get a value for the size of the systematic uncertainty. 

a = 

1

vvβ

K(EEβ) NNFFFFFFFF−NNSSSSSSSS

NNFFFFFFFF+NNSSSSSSSS

Type equation here.