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Optimization Studies on PGNAA Coal Analysis Improvement for Patent Development

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### Optimization Studies on PGNAA Coal Analysis Improvement for Patent Development

AgendaAgendaAgenda

JiaxinWang and Robin P. Gardner

Oct 6th 2011, CEAR at NC State University, Raleigh, NC

Agenda

1. Overview

2. Detector Response Function

3. Code CEARCPG

4. Prompt Gamma-ray Modeling

5. Simulation Results

6. Mc(do)lls Quantitative Analysis

7. Conclusions and Future work

Overview-PGNAA

- Disadvantages
- Inherently large background
- Interference from the neutron excitation source.
- Natural background
- Structure materials
- Detector activation (NaI)
- Summing and pulse pile-up effect
- Hydrogen Peak

- Advantages:
- Nondestructive
- Simultaneous
- In Situ
- Quantitative
- sensitive to the entire periodic table.
- shape of the sample are relatively unimportant.

Overview-CPGNAA

- Solution: introduce gamma – gamma coincidence technique
- Advantages
- Increase the signal – to – noise ratio
- Reduce the interference of background
- Eliminate the hydrogen prompt gamma-ray peak

- Disdvantages
- The coincidence response is about 2 order of magnitude lower than single response
- Long measurement time

Overview-CPGNAA

- Source Bulk Sample Detector
- Reach maximum prompt gamma ray/coincidence prompt gamma ray counting rates under certain neutron source (cf-252) strength

Overview-MC Simulation

MCNP5

The general purpose Monte Carlo simulation

parameter study, distribution maps

CEARCPG

Specified code for prompt gamma and coincidence prompt gamma

Pulse height spectra, elemental library spectra

Computation power

CEAR ‘Spectral’ cluster with 41 running nodes, each with a Quad-core CPU.

Overview-MC Simulation

- Detector response function-> More scintillators, more shapes, more size, etc.

-> New DRF generation code – CEARDRFs

- CEARCPG was written for serial computation only

-> Parallel feature implement of CEARCPG

- MCLLS Quantification

-> Differential Operator implement in CEARCPG

Overview-Quantification

- Peak analysis
- Matrix effect?
- Detector resolution (NaI, BGO, or HPGE)?
- Monte Carlo Library Least Square

Overview-Quantification

MCLLS - procedure

Compositions of a unknown sample are assumed and the PGNAA measurement is simulated

Elemental library spectra are generated with the simulation

Least-squares fit for the experimentally measured sample spectrum to obtain compositions of it.

Compare calculated values with the originally assumed ones, if not close enough, repeat the process from step 1.

Agenda

1. Overview

2. Detector Response Function

3. Code CEARCPG

4. Prompt Gamma-ray Modeling

5. Mc(do)lls Quantitative Analysis

6. Conclusions and Future work

DRF-MC simulation

Because the same detector has been repeatedly used under different situations, the particle-transport inside the detector (DRF) could be pre-calculated through MC simulation to improve future simulation speed and accuracy.

MCNP5

General purpose for neutron, photon and electron transport

G03

Specific for Cylindrical NaI detector

*CEARDRFs

For more shapes, more scintillation detector: BGO, plastic, etc.

DRF-Advantages

- (1) on the order of one-half of the calculations per history can be omitted by the use of a DRF
- (2) use of the DRF has a natural smoothing effect which reduces the number of histories necessary for the desired accuracy by a factor of about 100
- (3) use of the DRF yields better accuracy in spectral simulations because they can be more accurate than calculations of particle transport with existing physics inside the detector.

DRF-Accuracy and Speed

The DRFs generated by CEARDRFs have much better agreement with experiments than commonly used MCNP5

The speed of CEARDRFs is very fast. It costs 69 seconds for 2.754 MeV energy and 29 seconds for 0.662 MeV, which almost hundreds of times faster than original MCNP5. Thus, a complete set of DRF could be simulated in a reasonable time.

MC simulation outside detector (CEARCPG)DRF-Usage

Convolute the incident gamma flux with DRF

Simulated pulse height spectra

Elemental analysis

DRF generation (CEARDRF)

Experimental spectra

1. A complete set of DRF needs to be generated by MC simulation, i.e. CEARDRFs. For example, an energy range from 0 to 11 MeV in 1024 channels.

2. Build up the model of surrounding geometry of detector, run the MC simulation to record the photon energy flux reaching the detector surface and its path length.

3. Adjust the photon weight according to the path length and convolute the recorded energy flux with DRF to get the final simulated spectra.

Agenda

1. Overview

2. Detector Response Function

3. Code CEARCPG

4. Prompt Gamma-ray Modeling

5. Simulation Results

6. Mc(do)lls Quantitative Analysis

7. Conclusions and Future work

CEARCPG-Overview

CEARCPG (Han, 2005) was developed as the first specific code that can be used to simulate both the single and coincidence spectrum of coincidence PGNAA, including relatively complicated neutron and photon transportation.

The most important contribution of CEARCPG is a new algorithm is developed to sample the neutron-produced coincidence gamma-rays following nuclear structure.

CEARCPG-Parallel implement

Random seeds generated and distributed to slave nodes

File I/O path preparation

Master node collects recorded data from each slave nodes

CEARCPG-DO

The Differential Operator method is very powerful tool for measurement sensitivity study and system optimization. The basic idea of the differential operator technique is, if the magnitude of perturbation is very small, the ratio of changed response can be found by using Taylor series expansion.

Agenda

1. Overview

2. Detector Response Function

3. Code CEARCPG

4. Prompt Gamma-ray Modeling

5. Simulation Results

6. Mc(do)lls Quantitative Analysis

7. Conclusions and Future work

Prompt Gamma-ray Modeling

Source

Moderator

Bulk sample

Detector 1

Detector 2

- General optimization
- Moderator
- Neutron distribution
- Prompt gamma-ray distribution
- Detector
- Cross-section
- Detector Efficiency
- Neutron response
- Plastic detector setup
- Geometry arrangement
- Lab sample
- Large sample

Modeling-Neutron Maps

Neutron capture rate

Thermal neutron

Fast neutron(1-10MeV)

Radioactive capture reaction happens all through the coal sample with highest production in the center area, if the source is placed under the sample

Thus, it is better to place the source under the large size bulk sample. No moderator is needed as the self moderation of sample is enough for 252Cf neutron source

Modeling-Photon Maps

- Photon flux spatial distribution maps around the large rectangular shape coal sample and the conveyor belt shape coal sample.

In coincidence detection, it is better to place the detectors facing the top and bottom surfaces separately if possible. Otherwise, placing the two detectors together on the opposite side of neutron source is also a good arrangement.

Modeling-Geometry

Lab size sample (55cm x 9.7cm x 6.7 cm)

6”x6” NaI Cylindrical detector

2”x4”x16” Slab NaI detector

Modeling-Geometry

Thinner paraffin (7.3cm)will increase the overall detector response about a factor of 4.3 and3.4for single andcoincidence, respectively.

Changing the 6”x6” detectors position from bottom to left-right sides can further increase the overall detector response another factor around 1.6 and 3.8.

Two slab detectors replacing the 6”x6” cylindrical NaI detectors can gain another increase of a factor around 9.5 and 17.2.

In sum, the slab detector left-right arrangement can detect around 65 and 223 times more gamma-ray events than the reference setup.

The ratio of increase (ROI) for different setup as a function of energy: Higher efficiency for higher energy

Modeling-Geometry

* All ROI values are calculated based on reference setup

Modeling-Geometry

Large size sample (25cm x 100cm x 100 cm)

6”x6” NaI Cylindrical detector

2”x4”x16” Slab NaI detector

70cm x 50cm x 10cm plastic detector

Modeling-Geometry

Replacing the two 6”x6” cylindrical detectors with two 2”x4”x16” slab NaI detectors could gain the ROI of 1.6 and 6.2 in single response and coincidence response, respectively

The plastic/NaI special setup could gain the ROI of 2.5 and 1.7.

NaI detector in the special setup has a better efficiency to high energy gamma-rays in single response while the slab detectors setup has better efficiency to high energy gamma-rays in coincidence response

1. Overview

2. Detector Response Function

3. Code CEARCPG

4. Prompt Gamma-ray Modeling

5. Simulation Results

6. Mc(do)lls Quantitative Analysis

7. Conclusions and Future work

Results

Through CEARCPG, the 2D coincidence spectrum of these setups has been simulated with a coal sample (H-2.892%, C-5.28%, N-%1.4, O-5.487%, Na-1.121%, Al-2.38%, Si-1.943%, S-5.6%, Cl-1.729, Hg-2.168%).

Three setups: slab detectors for lab and large sample, the special setup with plastic detector.

Results-Interference

Fission gamma and prompt gamma-rays from structure materials still contribute to true coincidence.

Results-Interference

Everything source of gamma-rays could be included in the coincidence response through chance coincidence.

When , The chance coincidence counting rate is only 2% of the true coincidence rate.

However, when the single detector counting rate increases to 105/s the chance coincidence counting rate is 20% of the true coincidence rate

Results-Dose Rate

MCNP5 F4 mesh tally and FM card (flux-to-dose conversion factor for human)

For neutron and photon separately.

If a 10 microgram (μg) source is used, it is allowed to stay close the device behind the shielding material for 2000 hours annually, even under the public limits

1. Overview

2. Detector Response Function

3. Code CEARCPG

4. Prompt Gamma-ray Modeling

5. Simulation Results

6. Mc(do)lls Quantitative Analysis

7. Conclusions and Future work

MC(DO)LLS-Fitting Results

- Sample 2, the results of both sulfur and mercury are improved through Q-value projection.
- Sample 1, the result of sulfur is improved while the result of mercury has degradation.
- This result is reasonable since there is little interference in the high-energy window. The reason of mercury result in sample 1 is that the 8-9 MeV windows is too close to Mercury Q-value to include the whole peaks.
- When the concentration of Mercury is low as in sample 1, the benefited of less interference might be canceled out by the drop of signal due to energy window projection.

1. Overview

2. Detector Response Function

3. Code CEARCPG

4. Prompt Gamma-ray Modeling

5. Simulation Results

6. Mc(do)lls Quantitative Analysis

7. Conclusions and Future work

Conclusions

1. A new code named CEARDRFs has been developed to generate pretty accurate detector response function at a very fast speed to improve accuracy and efficiency of CEARCPG.

2. Parallel computation feature has been implemented in CEARCPG by a simple script approach, which dramatically simplified the job while keeping all the original features and could nearly reach the ideally linear speed-up feature.

3. With derivatives to second order Taylor expansion, the DO has also been implemented into CEARCPG and validated, including the consideration of collision kernel, transportation kernel and variance reduction kernel.

Conclusions

4. For lab size sample, replacing the detectors with two 2”x4”x16” slab NaI detectors could gain the ROI of 66.5 and 223.7 for single and coincidence response, with higher efficiency for higher energy gamma-rays.

5. For large size sample, two 2”x4”x16” slab NaI detectors setup could gain the ROI of 1.6 and 6.2 in single response and coincidence response, respectively and the special setup of plastic VS NaI could gain the ROI of 2.5 and 1.7. The NaI detector in the special setup has a better efficiency to high energy gamma-rays in single response while the slab detectors setup has better efficiency to high energy gamma-rays in coincidence response

Conclusions

6. The simulated 2D coincidence spectra show the feasibility of using the plastic detector as a trigger to another detector that has better energy resolution.

7. Among all the interference, in the total coincidence spectra, the fission gamma remains the major factor while the interference from structure material still contributes.

8. Q-value projection on the 2D spectra could further suppress the interference. The MCLLS analysis on the Q-value projected spectra shows better accuracy than using the total coincidence spectra.

9. With proper shielding, the dose rate around the analyzer is pretty low.

Future Works

1. Validate the results with benchmark experiments,

2. New elemental analysis method is also need to be developed with elemental libraries, eg. Restraind LLS, true 2D LLS.

3. Other neutron sources like D-T generator are worth a look.

4. Looking for more complete nuclear structure data, especially angular correlations between prompt gamma-rays

5. The light transport in large size detector is also an interesting area to look.

Thank you!

The authors are also grateful for the financial support of CEAR through the Associates Program for Nuclear Techniques in Oil Well Logging presently supported by Baker Hughes, Weatherford, EXXON Mobil, Halliburton, Pathfinder, and Los Alamos National Laboratory

Questions and comments?

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