EXPERIMENTAL INVESTIGATION OF GAMMA AND NEUTRON RADIATION FROM

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5th Workshop on Fast Ignition of Fusion Targets, Funchal, 18-22 June 2001                              45-48



 EXPERIMENTAL INVESTIGATION OF GAMMA AND NEUTRON RADIATION
                               FROM LASER-PRODUCED PLASMA
                                          A.Matafonov, V.Belyaev
       Russian Aviation Space Agency, Central Research Institute of Machine Building,
                   4, Pionerskaya Str., Korolev, Moscow Region, 141070, Russia
          Phone: 7 095 5135444 Fax: 7 095 1871588 E-mail: matafonov@mtu-net.ru


      In the work [1] a variety of theoretical investigations were performed, as well as
numerical predictions as for the plasma parameters and fast particles (electrons)
characteristics were defined more precisely within the range of laser fluxes q =
1017-1018 W/cm2. It was demonstrated that for q = 1017 W/cm2 one can expect hot electron
temperature Th = 60 keV, and for q = 1018 W/cm2 - Th = 155 keV. The important feature of the
model with which the hot electrons temperature values were obtained is predicted existence of
the hot electrons maximum energy (electron spectrum "cut-off energy"). It was shown
theoretically that the maximum energy of fast electrons can reach Wmax = 300-400 keV for q =
1017 W/cm2 and Wmax = 900-1000 keV for q = 1018 W/cm2. Our experimental investigations
were aimed at determination of the maximum energy and the quantity of fast electrons in the
laser-produced plasma on different targets at q = 1017 W/cm2.
      We have performed a systematic measurements of the γ-quanta cut-off energy from
laser plasma produced on targets made of Be, Al, Cu, Ta, Bi. γ-radiation has been detected
with detector based on the nonorganic crystal NaI(Tl) with the sizes ∅40×40 mm3. The
scheme of experiments is shown in Fig. 1.
                                                         Laser radiation




  U = +1800 V NaI(Tl)              Filter of Pb                                   Reference
              crystal              up to 1 cm                                     detector


                                                                  Target

        Protection of Pb 1 cm
  To crate CAMAC                                                                          To crate CAMAC
                                                                              Vacuum
                                              70 cm                         chamber ∅30


                                       Fig. 1. The scheme of experiments.




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          Laser radiation was directed to the target surface through the aspherical lens. The laser
radiation intensity on the target surface was 1017 W/cm2. The contrast of laser radiation was
3⋅103. In the experiments we used Be, Al, Cu, Ta, Bi targets measures 20×20 mm2. The
thickness of the targets was 1 mm or 2 mm. The focused pulse is incident on the target at 10°
and 25° from the normal with p-polarization. The targets were installed within the vacuum
chamber. The NaI(Tl) detector was external to the vacuum chamber and was mounted at 90°
to the laser beam. The distance between the NaI(Tl) detector and the target was 70 cm. The
detector with stilbene crystal ∅50×50 mm3 was the reference one. The NaI(Tl) detector is
connected to the amplifier in Crate-CAMAC. Signals from amplifier are passed to the input of
the digital oscilloscope TEKTRONIX TDS3032. To evaluate the maximum energy and the
quantity of γ-quanta Pb-filters of various thickness were installed in front of the NaI(Tl)
detector. By the measurements of γ-flow attenuation depending on Pb-filter thickness the
maximum energy of γ-quanta was evaluated. Given the thickness of the Pb-filter in front of
the detector and the solid angle of γ-quanta detection the total quantity of maximum-energy γ-
quanta generated in the laser-produced plasma can be evaluated.
          Fig. 2 show the attenuation of γ-quanta flux from Cu target in depend on thickness of Pb
filter.
          With all the targets the maximum γ-quanta energies were found to be 240±40 keV. The
number of γ-quanta possessing his maximum energy made up (5,0±2,0)⋅105. These data will
be used to correct our physical models.
          It was shown in [2] that the neutron yield of DD reactions depends essentially on a
prepulse intensity (CD2 target was irradiated by a 1017 W/cm2 pulse). As the prepulse had the
intensity of 3.1011 W/cm2 (a weak prepulse), the calculated neutron yield was N = 1.4.103. In
the case of a 3.1013 W/cm2 (strong prepulse) the neutron yield is small (N~10). By the present
time, the neutron yield for the case of the strong prepulse has been experimentally measured.
          Neutrons were detected with all-wave neutron detector based on four He3 counters. The
detection of neutrons follow the order given below. Laser radiation 1 µm wave length, 1 J
pulse energy, 1 psec pulse duration, was directed to the target surface through the aspherical
lens 140 mm focal length. The diameter of the laser beam cross-section was 40 mm. The
aspherical lens allowed focussing of the laser radiation on the target surface into a spot 30 µm
diameter. The diameter of the focal spot was measured with obscure camera equipped by set
of diaphragm. Thus, the laser radiation intensity on the target surface was 1017 W/cm2. The
contrast of laser radiation was 3⋅103. The intensity of prepulse was 3⋅1013 W/cm2 (strong



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       J,
      A.U.

       100
        80
        70
        60
        50
         40
         30


         20
                                                                       Cu, ϕ = 25°
                                                                                 °
         10
          8
          7
          6
          5
          4
          3


          2



          1
              0         1         2          3          4          5         6               , mm Pb
                                                                                          




              Fig. 2. The attenuation of γ-quanta in depend on thickness of Pb filter.


prepulse). The prepulse occurs for about 13 nsec before the main pulse. In the experiments we
used deuterated polyethylene targets (CD2)n measures 10×10 mm2 and thickness of 250 µm.
The focused pulse is incident on a target at 10° and 25° from the normal. The target was
installed within the vacuum chamber. The measuring detector – all-wave neutron detector
based on four He3 counters was external to the vacuum chamber. The detector was mounted at
90° to the laser beam. The distance between the measuring detector and the target was 6 cm.
      The detector with stilbene crystal ∅50×50 mm3 was the reference one. The reference
detector was placed at 15 cm from the target. The employment of the reference detector
allowed evaluating the repeatability of the value of the laser radiation intensity on the target in
different experiments. The counters He3 are connected to the amplification assembly – four
amplification units in Crate-CAMAC. Signals from amplification assembly are passed to
input of the digital oscilloscope TEKTRONIX TDS 3032. In front of the all-wave neutron
detector there were not moderator – thermalizator disks, that is it was set to detector of




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neutrons within the range between thermal energies and 2.5 MeV. We performed experiments
to investigate the neutron radiation from (CD2)n – target with laser radiation parameters
mentioned previously. We registered the yield of neutrons which corresponded to the
predicted value of N ~ 10 in 4π steradian. The calculations show that in order to increase the
neutron yield, it is necessary to improve the energy contrast. Work is currently under way on
improving of the energy contrast with use Pockkels cell and two Glan prisms.
     The present work was supported by International Science and Technology Center
(ISTC) under project number 856.


     References.
1. V.S.Belyaev,     G.A.Vergunova,       S.Yu.Gus'kov,   N.N.Demchenko,        A.P.Matafonov,
   V.B.Rozanov, γ-ray generation in the picosecond-laser plasma with taking into account
   prepulse influence, Abstract of ECLIM 2000, 12 - 16 June, Prague, Czech. Rep., p. 210.
2. N.N.Demchenko, V.B.Rozanov, Hydrodynamic model of picosecond laser pulses with
   condensed targets interaction, Preprint of Lebedev Physical Institute of the Russian
   Academy of Sciences, Moscow, 2001.




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