LASER-DRIVEN GAMMA IMAGING STUDIES AND EXPERIMENTS FOR APPLICATIONS DEVELOPMENT

Objectives of proposed experiment:

The scientific and experimental work proposed in this program will finally
allow us to predict and optimize various laser-driven gamma beam imaging
configurations, as a starting tool for further potential development of
stand-alone microfocus gamma ray sources prototype for being used in the
development of medical, scientific and industrial high-energy gamma
microfocus imaging applications.

EXPERIMENTS FLOW DESCRIPTION

• The GAMMA IMAGING EXPERIMENTS at ELI
  NP was running for three weeks, between 27 January and 10 February, and
  with two additional extension days granted, for 13 th and 14 th February.
• Most of the first two weeks time have been spent for new main optical setup, probe beam and laser beam microscope
  mounting and aligning , for mechanical design of valve and target movement stages, magnet and microscope positioning
  stages, for parabola beam blocker stage, target alignment and for doing first preliminary experiments with Argon gas.
  Since no accelerated electrons
  were detected with Argon gas, in the last week we have decided to change the gas with
  He98%+N2% and the first results with the tantalum target came out, obtaining low quality gamma images due to the
  unstable operation of the gas valve.
  • Because the valve continued to create problems, main issue being shots without gas which created a partial damage of a
    laser mirror by returned scatter laser beam, we continue the experiments only with electrons, but a poor image with
    electrons was obtained in this case.
  • In the last day of the given three weeks time, have been analysed the valve problem and an algorithm for controlling it
    by watching the interaction chamber vacuum values was developed and the first experiments have revealed first very
    promising results.
  • In the last two additional days, for avoiding to make more damages, we decided to move the bremsstrahlung target to
    the Aluminium exit port Flange thinned at 2mm , and to put outside the chamber the electrons deflecting magnet ,
    setup being presented in Figure 1;
  • In the last
    day hours have been achieved the experimental gamma imaging data presented in following.

  
  

  Experiment Parameters

  Laser energy on target: 1,7
  2.04 J , Argon pressure: 2 bar, Gas valve predelay: 5 ms ; Valve open time: 10 ms , Laser beam valve height: 1.5 mm;
  Laser beam focal size (FWHM): 24 microns, Laser pulse duration: around 30 fs;
  
  

  GAMMA IMAGING EXPERIMENTS PARTICIPANTS

  IFIN HH/ ELI NP Team

  Ovidiu TESILEANU / CoPI
  Liviu NEAGU CoPI
  Yoshihide NAKAMIYA
  Georgiana GIUBEGA
  Florin NEGOITA
  Gabriel COJOCARU
  Andrei BERCEANU
  Jianfuh
  ONG
  Petru
  GHENUCHE
  Madalin
  ROSU
  Catalin CHIOCHIU
  Bogdan
  TATULEA
  Iani
  MITU
  Laser Group members

  AccentPro2000 S.R.L. (AP2K)

  Team
  Mihai
  IOVEA / PI
  Marian NEAGU
  Edward Hermann
  Mihaela POTERASU
  Monica MIREA
  External collaborators
  Gabriel SULIMAN ( Politehnica Univ & ELI NP)
  Laura NITA (PhD student
  Politehnica Univ )
  GAMMA IMAGING EXPERIMENTS
  
  

  CONCLUSIONS

  • Definitely the three
    weeks granted beam time was totally insufficient for completing such complex experiments.
  • We have learned how to look for a stable point of
    gamma imaging experiment, where Laser shots are almost
    continuously good shots, based on the probe beam optimum acceleration channel image, interaction chamber
    vacuum variation on valve shots, EMP induced signal level and watching the Laser parameters.
  • The Argon gas is more suitable for the “gamma imaging applications” and low gas pressures data looks more stable.
  • The gas valve malfunctions created most of the experiments issues and should be further upgraded or replaced.
  • Despite the problems encountered, the
    final day hours gamma imaging experiments show promising results, an
    experiment continuation being necessary;
  • Main gamma imaging issues remained the
    gamma beam divergence and pointing variations that reached roughly up
    to 35 % of the field of view size, and a special further investigations for compensating these drawbacks are mandatory
    for the future of this set up imaging capability.
  • The Gamma emission during our experiments still was under optimum, being room for more experimental
    improvements.

  
  

  FUTURE DEVELOPMENT

  • We intend to continue our experiment by requesting a new beamtime for focusing on key mechanism of accelerating electrons by
    las er gas
    interactions for our specific needs: relatively low energy range useful electrons (up to 10 20 MeV) for producing useful gamma ray spectrum up to
    5 6 MeV. This requirement is due to the photon energies bigger than 5 6 MeV that are depositing negligible energy in thin standa rd scintillator
    (typically 0.3 1 mm) that is currently used for gamma high resolution imaging.
  • We will focus on finding the experiment parameters where are obtained stable shots imaging results, where Laser successive sh
    ots are almost
    similar in terms on intensities, beam size and centroid positions, even if in this case we are not reaching the maximum inten sit ies of electrons or
    photons .
  • We will further improve the
    use of the instruments for characterizing the interactions, by optimizing the probe beam visualization of the
    acceleration channel, computing the plasma electrons density with Phasics device, introducing a probe set up for an electro optical crystal for
    calculating the shot electrons bunch duration, improving the valve functionality and monitoring its behavior, monitoring the EMP induced signal
    level, watching the Laser parameters (power, near field intensity shape, spectrum) stability and corelating their values with th e quality of gamma
    imaging
  • An improved mechanical design of gas valve and target moving stages will reduce many of the issues encountered;
  • Developing a gamma imaging set
    up and a dedicated software capable to analyze in real time each shot imaging by computing the image number
    of photons, field of view dimensions, average energy of incident gamma beam, pointing positions and showing continuously thei r v ariations from
    successive shots.
  • We
    intend to extend our applications investigation, from actual gamma imaging goals in in Since and industrial Non destructive Testing,
  • to
    acquiring gamma movies of objects in continuous movement/rotation by synchronizing the laser shots with the scanned object position
    and
  • by analyzing the
    ultrashort gamma irradiation effect on bio samples, for studies regarding the effects of ultra short Gamma Ray shots in
    imaging techniques, where two specialized bio scientific groups will be partnering