Radiation Characteristics of the FN-II Plasma Focus Device Julio Herrera 1, Fermín Castillo 1, Guillermo Espinosa 2, Isabel Gamboa 1, José Ignacio Golzarri 2, José Rangel Gutiérrez 1 Instituto de Ciencias Nucleares Instituto de Física Universidad Nacional Autónoma de México

Outline Description of thel FN-II device and its diagnostics Soft X rays observed with pinhole camera and PIN diode. Measurement of the hard X-rays angular distribution by means of TLD-100 dosimeters. Use of needle point electrode for contrast radiography. Dosimetry using TLD-200 dosimeters. Hard X-rays and neutron detection using scintillator- photomultiplier arrays. Conclusions

Sketch of the FN-II Dense Plasma Focus Device

Main parameters (F. Castillo et al., Brazilian J. Phys. 32 (2002) 3) Inner electrode:length 40 mm (oxygen-free copper)diam.50 mm Outer electrode:squirrel cage type (12 bars) (oxygen-free copper) diam.100 mm Insulator (Pyrex ® )length19 mm diam.50 mm Usual operating voltage:36 kV Stored energy:4.8 kJ Current350 kA

Main diagnostics Current (Rogowski coil) Neutrons 2 Silver activation counters CR 39 nuclear track detectors 5 BC-400 Scintillator-photomultiplier systems X-rays Pinhole cameras PIN diodes TLD-100 and TLD-200 dosimeters 5 BC-400 Scintillator-photomultiplier systems

Cámara de agujero

Soft X-rays – Hollow electrode

Rogowski and PIN in Hydrogen

Bored electrode after several hundreds of shots

Electrode with a 2 mm INOX-304 inserted needle

Pin-hole image of a discharge with a needle

Worn out 2 mm needle point after a few shots

Worn out 1 mm needle point after a few shots

Erosion of the needle

TLD-100 dosimetry (1) 60 shots for the hollow electrode. Number of counts at 20 o /90º for the activation detectors is 68774/65312 → 4.12  10 8 / 6.13  10 8 neutrons/steradian 54 TLD-100 dosimeters were placed 1 m away from the focus, measurenments were made of four groups of 9 dosimeters: 1. 9 Unfiltered Filtered by 0.5 mm Al Filtered by 1.0 mm Al A dose estimate is made using the Unfiltered group, using their TL response and the calibration factor previously obtained by means of 60 Co irradiation.

TLD-100 dosimetry (2) 79 shots for the needle electrode. Number of counts at 20 o /90º for the activation detectors is 63448/58478 → 3.81  10 8 /5.49  10 8 neutrons per steradian. 63 TLD-100 dosimeters were placed 1 m away from the focus, measurenments were made of four groups of dosimeters: 1. 9 Unfiltered Filtered by 0.5 mm Al Filtered by 1.0 mm Al Filtered by 2.0 mm Al A dose estimate is made using the Unfiltered group, using their TL response and the calibration factor previously obtained by means of 60 Co irradiation.

Attenuation of the thermoluminiscent (TL) response of filtered TLD-100 dosimeters placed at 1m from the focus.

TLD-100 dosimetry (3) Hollow electrode: 60 shots  10 8 / 6.13  10 8 neutrons/steradian at silver activation detectors 4.6 ± 0.5 mGy → ± mGy Needle electrode: 79 shots  10 8 /5.49  10 8 neutrons per steradian. at silver activation detectors 8.5 ± 0.5 mGy → 0.11 ± 0.01 mGy (Environmental daily dose ~ 0.01 mGy)

Sketch of the FN-II Dense Plasma Focus Device

Angular distribution of X-rays measured with TLD-200 dosimeters Sensibilty peaked at ~ 30keV

Tracks at upper detectors (400X)

Tracks at lower detectors (400X)

Angular distribution of protrons

Angular distribution of neutrons at upper detectors

Angular distribution for neutrons at lower detectors

Isotropic and Anisotropic Contributions

Comparisson Between Both Samples I BHσKA1A1 A2A2 AIso.Aniso. Neutrons up Neutrons down

Comparisson Between Both Samples II  10 6 Y Iso  10 6 Y Aniso  10 6 Y Total  10 6 Iso % Aniso % Neutrons up    Neutrons down   

Scintillator-Photomultiplier systems

Rogowski and PIN in Deuterium

Shielding on scintillators

No. de detectorShielding (Pb cm)Channel 1(PMT only)5Osc. 1-Ch (water)Osc. 2-Ch.2 35Osc. 2-Ch.3 410Osc. 1-Ch.4 510Osc. 1-Ch.3

Shielding on scintillators Shot 7735-Low performance

Shielding on scintillators Shot 7726-High performance

Response of the neutron detectors

Conclusions (1) The signals from the pin-hole camera show that the plasma column is around 3 cm long and 3 mm diam. The PIN signals are peaked at the moment of the dip in the B-dot signal, and then show a platform which lasts about 1μm. By placing a needle at the tip of the elecrode, the hard X- rays can be well localised, and the dosimetry shows that the yield is doubled. Further work needs to be done in order to estimate the spectrum. The better concentration of the x-ray bright-spot allows their use for high contrast radiography. Some of the hard X rays are highly energetic (gamma rays?) In “high performance” shots the rate and production of gamma rays is highly increased, masking the neutron signal (scattering of gamma rays?).

Conclusions (2) The angular distribution of neutrons shows an isotropic and an anisotropic component. The latter usually contributes less than 30%. The angular distribution of protons is aligned to the axis and falls rapidly for angles greater than  40 º, implying the existence of strong magnetic field. The X-rays show a bimodal angular distribution. While the needle slightly increases the X-ray emission, the hollow electrode is best for neutron generation.

References [1] F. Castillo, J.J.E. Herrera, J. Rangel, A. Alfaro, M.A. Maza y V. Sakaguchi "Neutron Anisotropy and X-Ray Production of the FN-II Dense Plasma Focus" Brazilian Journal of Physics 32, 3-12(2002). [2] F. Castillo, J.J.E. Herrera, J. Rangel, J.I. Golzarri y G. Espinosa "Neutron Angular Distribution in a Plasma Focus Obtained Through Nuclear Track Detectors", Radiation Protection Dosimetry 101, (2002). [3] F. Castillo, J.J.E. Herrera, J. Rangel, M. Milanese, R.Moroso, J. Pouzo, J.I. Golzarri y G. Espinosa "Isotropic and Anisotropic Components of the Neutron Emissions at the FN-II and PACO Dense Plasma Focus Devices” Plasma Physics and Controlled Fusion 45, (2003). [4] F. Castillo, J.J.E. Herrera, J. Rangel, I. Gamboa, G. Espinosa y J.I. Golzarri “Angular Distribution of fusion products and X-rays emitted by a small dense plasma focus machine” Journal of Applied Physics (2007).