FTREE

FTREE is my creation. It lets us look inside radiation tracks.

It generates catalogues of complete radiation cascades. It is a creative use of Monte Carlo simulations. Simulations are intercepted to access instantaneous values of the desired attributes before they get over-written as the simulation progress. Interceptions are passive; none modifies the physics or transport in any way. Rigorous post-processing spins out the awesome family trees. FTREE is not a Monte Carlo radiation transport code. Thus named, with F for FLUKA, and TREE in honour of the most enduring DOS command.

Beginning from a single particle impinging matter, each progeny is tracked under real-life irradiation conditions until it is fully absorbed or goes below preset energy thresholds. The ensuing radiation field is characterised interaction by interaction, accounting for nth secondaries producing (n+1)th secondaries, recursively, in a full inventory from upstream all the way downstream. Each progeny is uniquely differentiated into a family tree, indented by the nth generation of the particle, rooting from the single source particle as the first foreparent. The kinship, no less complicated than real life, between each particle is uniquely identified without ambiguity. This mode of scientific observation, analysis and presentation goes beyond:

• present-day detector technologies;

• conventional Monte Carlo simulations (where statistical convergence is always sought after); and

• standard pedagogy constraints (where interaction types are discussed in isolation, not in the context of radiation cascades).

FTREE also offers a unique opportunity to observe rare events far out in the Gaussian tail which would have been washed out by averaging; such are the events less probable, but no less correct in physics compared to highly probable events populating the Gaussian peak.

The idea for FTREE was first conceived at a workshop, where the maiden work, Carbon stories in calcium, was presented. This was followed by some early developments:

• Sample histories from carbon therapy of a human brain. Chin et al. (2011)

  slides

• Candidate therapeutic ions: a physics account of interactions in and escapes out of the body. Chin et al. (2012)

  slides

• Later, at the International Nuclear Chemistry Congress in Brazil, FTREE found herself a novel experiment technique as much as a novel teaching resource. Chin (2015)

  slides

Here are 5 sample histories:

Carbon therapy of a human brain

480 MeV proton on tungsten I

480 MeV proton on tungsten II

Vertical n_TOF beamline at CERN

Horizontal n_TOF beamline at CERN

Carbon therapy of a human brain¶

We watch how, a single carbon ion impinging the brain produced progenies (also termed secondaries or even contaminants when they are unwanted but inevitable) such as pions, muons, neutrinos, neutrons, protons and electrons in the brain. This history is atypical; most carbon ions in the beam would not produce pions. It has been cherry-picked from simulation of a 200 MeV/nucleon carbon beam entering the side of the head (a lateral beam in clinical terms). A beam consists of many source particles — carbon ions in this case. The simulation mimics one of the treatment options for brain tumour. The human brain was represented by a VIP-Man anthropomorphic voxel phantom, where tissue segmentation traces back to the Visible Human Project. Due to the stochastic nature of radiation interactions, every carbon ion sent in would track a different trajectory, undergo different types of collision and produce different types of particle. This is an instance out of countless possibilities.

➡ TREE VIEW

480 MeV proton on tungsten I¶

Here we see a bold proton which generously produced 25 neutrons in a single inelastic collision. While this is the underlying principle behind spallation neutron sources, 25 is generous of a proton at 480 MeV. This history has been cherry-picked (for the generous supply of spallation neutrons) from a simulation of 480 MeV protons impinging a 12 cm-deep tungsten target of dimensions 8 cm × 5 cm in beam’s eye view. The application is for the spallation neutrons to enter the warm moderator and cold moderator, eventually slowed down to ultra cold energies, stored in a bottled before being released to experimentalists as a unique probe into matter and creation. Particles attempting to escape are hurdled through shielding blocks of steel, concrete and hematite aggregates, to protect personnel, instruments and electronics downstream.

➡ TREE VIEW

480 MeV proton on tungsten II¶

This is a story of a proton out for some fun, producing exotic particles through exotic collisions. The story was told to beautiful people by the beautiful beach of Maresias (Brazil) on 18th September 2014 at the 4th International Nuclear Chemistry Congress; the slides are here. This history has been cherry-picked (for less-probable events) from a simulation of 480 MeV protons impinging a 12 cm-deep tungsten target of dimensions 8 cm × 5 cm in beam’s eye view. The application is for the spallation neutrons to enter the warm moderator and cold moderator, eventually slowed down to ultra cold energies, stored in a bottled before being released to experimentalists as a unique probe into matter and creation. Particles attempting to escape are hurdled through shielding blocks of steel, concrete and hematite aggregates, to protect personnel, instruments and electronics downstream.

➡ TREE VIEW

Vertical n_TOF beamline at CERN¶

This is a story of a gutsy proton producing neutrons which survive 24 m of tight (16 cm radius) collimation (mainly concrete), reaching the experimental gallery, reporting to the experimentalist’s detector. What is so special about this story? It is very special because this proton makes it where most others can’t reach without variance reduction (also known as biasing). For a primer on variance reduction please see Chapter 5 Section 1 of an ancient writing.

➡ TREE VIEW

Horizontal n_TOF tunnel at CERN¶

This is a story of a gutsy proton producing neutrons which survive 200 m of underground tunnel, reaching the experimental gallery, reporting to the experimentalist’s detector. What is so special about this story? It is very special because this proton makes it where most others can’t reach without variance reduction (also known as biasing). For a primer on variance reduction please see Chapter 5 Section 1 of an ancient writing. This history has been cherry-picked (for the greatest survivor) from a simulation of 20 GeV protons impinging a 40 cm-deep lead spallation target. This is the horizontal, older beamline of n_TOF, as opposed to the new vertical flight path. n_TOF is one of the many experiments around CERN’s accelerator complex. It is the brainchild of Carlo Rubbia, Nobel laureate, who was the Director General of CERN. Many cross-section and astrophysical experiments have been reported from here for some years.

➡ TREE VIEW