RPT PARAMETERS

Radioactive particle tracking (RPT) is a non-intrusive velocimetry method which is used to study the hydrodynamics of single and multiphase systems. Launching a RPT simulation for photon count calculation in Lethe requires a solver which is rpt_3d, a parameter file, two files including detector and particle positions inside the vessel. The particle positions file includes either the experimental calibration positions or a set of generated points inside the vessel by the user. Detector positions file contains the position of detector face canter and the position of a point inside the detector on its axis. In Lethe-RPT code the middle bottom of the cylinder is considered as the origin. An example of these two files can be find here This section aims at describing the various parameters available within Lethe-RPT.

Parameters file (.prm)

In the parameter file format, the parameters are established one by one using the following syntax for instance: 'set reactor radius= 0.1' would fix the reactor radius to 0.1 m. The arguments can be either doubles, integers, or a choice between a predefined condition. In the parameter files, comments are preceded by the sharp symbol (e.g., '#comment').

The parameter file is composed of different subsections. In the following, the principal subsections of a RPT parameter template file are explained.

1. RPT parameters
2. Parameter tuning
3. Detector parameters

RPT parameters

This subsection contains the general information for photon count calculation which is described in detail in this part. “Particle position file” defines the file that Lethe uses to read the particle positions inside the vessel. Setting the option “export counts” to true allows exporting the results in a .csv file. The entry “counts file” specifies the mentioned .csv file name. The number of Monte-Carlo iteration (the number of traced gamma-ray from each particle position to the detector within the defined solid angle) is defined by “monte carlo iteration”. Setting “random number seed” to auto allows us to generate random numbers for the Monte-Carlo calculation. Reactor radius defines the radius of the cylindrical tank in which simulations are carried out. “Peak-to-total” ratio is the relation between the area of photopeak and the total area in the spectrum. The amount of time for which the RPT hardware records the photon count at each position is specified by “sampling time”. A sample of this subsection is shown below:

• particle positions file : Filename of the text file with a set of positions inside the reactor
  Options : Any text file with .particle extension with the required header : particle_positions_x particle_positions_y particle_positions_z
• verbosity : Enable to show photon counts numbers results in realtime in terminal
  Options : enable or quite
• export counts : Enable to export photon counts numbers in a file
  Options : true or false
• counts file : Filename of export counts file (.csv or .dat)
  Options : Any text file with .csv or .dat extension
• monte carlo iteration : Defines the number of traced gamma-rays from each particle position to the detector within the defined solid angle
  Options : Any positive integer
• random number seed : Seed number for the random number generator, using a particular number allows to run the same series of numbers
  Options : Any positive integer
• reactor radius : Radius of the reactor vessel (cylinder only)
  Options : Any positive float
• peak-to-total ratio : The proportion of the events appearing in the full energy peak to the total number of events (model parameter)
  Options : Any positive float
• sampling time : Sampling time of detector(s) (model parameter)
  Options : Any positive float
• dead time : Dead time of the detector per accepted pulse (model parameter)
  Options : Any positive float
• activity : Activity of the tracer (model parameter)
  Options : Any positive float
• gamma-rays emitted : Number of gamma-rays emitted by each disintegration (model parameter)
  Options : Any positive float
• attenuation coefficient reactor : Total linear attenuation coefficient of the medium, reactor wall and air (model parameter)
  Options : Any positive float
• attenuation coefficient detector : Total linear attenuation coefficient of the detector (model parameter)
  Options : Any positive float

# --------------------------------------------------
# RPT Monte Carlo technique
#---------------------------------------------------
subsection rpt parameters
    set particle positions file          = positions.particle
    set verbosity                        = verbose
    set export counts                    = true
    set counts file                      = counts.csv
    set monte carlo iteration            = 100000
    set random number seed               = 0
    set reactor radius       		 = 0.1
    set peak-to-total ratio  		 = 0.4
    set sampling time        		 = 1
    set dead time       		 = 1e-5
    set activity  			 = 2e6
    set gamma-rays emitted        	 = 2
    set attenuation coefficient reactor  = 10
    set attenuation coefficient detector = 21.477
end

Parameter tuning

This subsection contains information regarding the photon count calculation which has also been used in the detector’s parameter tunning process. Setting “tuning” to false allows Lethe-RPT to calculates the photon counts at each particle position with respect to the detector. In order to tune the detector’s parameter this option musted be set to true. “Gamma-rays emitted” indicates the number gamma-rays emitted by each disintegration. RPT technique works based on the attenuation of the light which travels from the tracer particle to the detector through the reactor. Therefore, the attenuation coefficient of the detector’s crystal and that of the materials inside the reactor must be specified. These parameters respectively are defined as “attenuation coefficient detector” and “attenuation coefficient reactor”. The “dead time” specifies detector dead time parameter and “activity” declares the radioactive source activity.

• tuning : Enable to tuning parameters with NOMAD through showing the cost function in terminal
  Options : true or false
• cost function type : Type of cost function to evaluate
  Options : Larachi, l1 or l2
• experimental data file : Filename of the text file with experimental/artificial counts
  Options : Any text file with .experimental extension with the required header : experimental_counts

# --------------------------------------------------
# Tuning with NOMAD
#---------------------------------------------------
subsection parameter tuning
	set tuning                           = true
	set cost function type               = larachi
	set experimental data file           = noisy_counts.experimental
end

Detector parameters

This subsection contains the specific information of the detector. “Detector position file” defines the files that Lethe uses to read the detector positions. This file includes the position of detector face center and the position of a point inside the detector on its axis. The options “radius” and “length” respectively defines the detector’s crystal radius and length.

• detector positions file : Filename of the text file with positions of detector(s)
  Options : Any text file with .detector extension with the required header : face_positions_x face_positions_y face_positions_z middle_positions_x middle_positions_y middle_positions_z
• radius : Radius of detector(s) (all detectors must have the same dimensions)
  Options : Any positive float
• length : Length of detector(s) (all detectors must have the same dimensions)
  Options : Any positive float

#---------------------------------------------------
# Detector parameters
#---------------------------------------------------
    set detector positions file          = positions.detector
    set radius       			 = 0.0381 
    set length                           = 0.0762
end