Photo-fission Studies of Nuclei by Virtual Photon Tagging at MAX-lab
N. Grigoryan, S. Knyazyan, A. Margaryan (cospokesperson), G. Marikyan, L. Parlakyan,
S. Zhamkochyan and H. Vardanyan
Yerevan Physics Institute, 2 Alikhanian Bros. St., Yerevan-36, 375036, Armenia
J. Brudvik, K. Fissum, K. Hansen (cospokesperson), L. Isaksson, M. Lundin, B. Nilsson and B.
Schroder
MAX-lab, Lund University, Sweden
Z. Yasin
Department of Nuclear Engineering, PIEAS, P.O. Nilore, Islamabad, Pakistan
L. Tang
Department of Physics, Hampton University, Hampton VA, USA
D. Dale
Dept. Physics, Idaho State University,
Campus Box 8106, 785 S. 8th Avenue, Pocattelo, ID 83209, USA
A. Gasparian
North Carolina A&T State University, 1601 E. Market St. Greensboro, NC 27411, USA
L. Gan
University of North Carolina Wilmington, 601 S. College Rd., Wilmington, NC 28403, USA
Abstract
We propose a new experimental program to investigate photofission of
nuclei in the wide mass range 50 ≤ A ≤ 240 at MAX-lab. This project aims to
provide precise data of the fission cross sections, mass, velocity and folding
angle distributions of fission fragments, FF.
These investigations will fully utilize the unique parameters of the tagging
system at MAX-lab, and are enabled by the use of the dedicated FF detector
based on low-pressure MWPC technique, for determining velocities and
trajectories of FF's and high-resolution solid state detectors, SSDs, for
measuring energies of FF’s. In addition we propose to use virtual photon
tagging technique instead of real photon tagging, which will increase the
tagged photon intensity more than two orders of magnitude than modern
tagging facilities can provide, and will allow to take nearly complete and high
statistic data with monochromatic photons in the energy range 40<Eγ<140
MeV.
We propose to start these experimental investigations by using 238U, 232Th
and 209Bi targets.
1. Introduction
The excitation of nuclei by electromagnetic probes such as real photons (γ-quanta) or
virtual photons (inelastic electron scattering) offers attractive features for the study of nuclear
phenomena over a broad range of excitation energies. The use of photons in the study of fission
of highly excited nuclei is advantageous, since photons are very effective, due to their volume
absorption, in heating nucleus, transferring at the same time, relatively low angular momentum
to the struck nucleus. In the intermediate and high energy (Eγ≥40 MeV) region the gammanucleus reaction has been currently explored in the framework of a two-step interaction models
[1-7]. In this approach, firstly a rapid intranuclear cascade, INC, develops through binary
intranuclear collisions. During the second stage of the reaction, the excited residual nucleus
slowly reaches its final state through a competition between the fission and the particle–
evaporation process. The two-step picture clearly assumes that fission is a relatively slow process
which samples the target residues only after they have lost a large fraction of their excitation
energy. Therefore, the fission of a heavy nuclear system provides an excellent tool for studying
the both stages of a complex, high-energy nuclear reaction. Coulomb energy systematics give a
clear indication for the binary fission process while fragment angular correlations and mass and
energy distributions can be used to estimate average quantities such as linear momentum transfer
and mean mass and excitation energy of the fissioning system.
Figure1: Excitation energy distribution at the end of the cascade, taken from [3].
The investigations of photofission of heavy nuclei at intermediate (40-140 MeV) energy
range is very convenient because the primary photoexcitation process is well understood.
However even in the quasideuteron region of photoexcitation, where the photoabsorption
mechanism is a simple one, different INC approaches predicts quite different values for
excitation energies as shown in Fig. 1, taken from [3].
In Fig. 1 the normalized excitation energy distributions for the cascade ɣ+Bi at 140 MeV
from the Monte Carlo multicollisional, MCMC , model calculations (solid histograms) and from
Ref. [4] (dashed histograms) are displayed. The result obtained in Ref. [4] clearly exhibits a
broader shape with an average photo-excitation energy Eex ~ 78 (dashed arrow) higher than the
one achieved from the MCMC model Eex ~ 46 MeV (solid arrow). The difference between these
results could be ascribed to the advanced Pauli-blocking mechanism used in MCMC model.
Figure 2: Fission probabilities relative to Np-237 for photoabsorption in U-233, U-235,
U-238, Th-232, and natural Pb, taken from [6]
However, simulations based on these two different approaches results similar values for
fission probabilities after absorption of photon, as demonstrated in Fig. 2 and Fig. 3, taken from
Ref. [5, 6]. The results displayed on Fig. 2 obtained in the framework of a model similar to one
used in Ref. [4], while results displayed on Fig. 3 was obtained in the framework of MCMC
model.
These features indicates necessity for precise measurement of more complete set of
experimental parameters, to develop proper theoretical approach for both stages of the INC.
Figure 3: Relative fissility for Th-232 with respect to that for U-238, taken from[5]
In this respect, interesting possibilities are offered by the measurement of mass-energymomentum distributions of fission fragments, FF, with monochromatic photons, to investigate
the excitation energy dependence of mass distributions comparing the data taken at different
photon energy Eγ [4]. In fact such experimental data allow to study in a clean way the thermal
effects, in particular the excitation energy dependence of the fission barrier [2]. To date current
experimental studies have concentrated on total photofission cross section measurements of the
heavy actinides. On the Fig. 4, taken from [7], the experimental data and calculated photofission
cross sections are displayed. From the Fig. 2 and Fig. 4 follows that precise measurements of
even photofission cross sections of preactinide nuclei such as Au and Pb, can help checking
existing theoretical approaches.
The FF's mass-energy-momentum distributions, have up to now only been determined in
a few experiments with Eγ≥40 MeV monochromatic photons. The mass of FFs was indirectly
determined [8, 9] for actinide targets. Therefore, more data with precise mass-energy-momentum
distributions of FF, measured directly and by monochromatic photons, highly desirable for are
targets in a wide range of mass.
Figure 4: Comparison of calculated photofission cross section on Au-197, Pb-208, Bi209, Th-232, U-233, U-235, U-238, and Np-237 with experimental data, taken from [7].
The MAX-lab, where tagged-photon systems from 15 to 190 MeV exists and
development and construction of a large acceptance, two angles, two energy and two velocity, FF
detector is planned, is uniquely suited to carry out such a program. In addition zero angle
electron scattering or virtual photon tagging technique is envisaged.
According to this concept, the active fissile target, i.e. the FF detector will be placed
instead of radiator (see Fig. 5) and the tagged photon system will be used as a magnetic
spectrometer for zero degree scattering electrons. Consequently, technical problems related to
proper separation of electron beam pipe from the FF detector, must be solved. The spectrum and
flux of virtual photons produced by relativistic electron scattering at zero degrees can be
expressed approximately as tv/ω, where tv ≈ 0.02, is the equivalent radiation length and ω is the
virtual photon energy. According to this formula the intensity of virtual photons for 200 MeV
and 100 nA electron beam will be about 6×107 phton/MeV/sec. In an experiment requiring a thin
target, t (r.l.) < tv, e. g. experiments requiring detection of heavy recoil products or fission
fragments, this technique has a clear advantage over conventional photon tagging [10]. First such
kind of fission experiment, zero degree electro-fission of 238U, has been performed at Yerevan
[11]. The virtual photon tagging technique is currently exploited extensively in the hypernuclear
spectroscopy experiments at JLab (see e.g. [12]), where thin targets are used to minimize the
dE/dx effect.
Fission
Fragment
Detector
Figure 5: Floor plan of fission study experiments by virtual tagging technique.
We propose to carry out zero degree electro-fission studies by using the MAX-lab
tagged photon system in combination with large acceptance FF detector and to perform high
statistic and precise measurements of nearly complete FF parameters, such as mass, velocities
and folding angles, for nuclei in a wide mass range 50 ≤ A ≤ 240.
The calculations of the intranuclear cascade and the subsequent evaporation and
fission predicts a number of correlations between fission fragments in addition to the correlations
between incident energy of particles and excitation energy of residuals, and between excitation
energy of residuals and fission widths[13-15]:
I. the correlation between the average excitation energy and the mass loss;
II. the correlation between the number of protons and neutrons emitted during the cascade
and the mass loss;
III. the correlation between the total kinetic energy of two fragments, their velocities and
number of protons and neutrons emitted before and after scission.
The parameters of the INC as well as the influence of shell and collective effects on
the level density and the decay widths of nuclei which have different excitation energies and
deformations [2, 16] will be determined by using these correlations and analyzing the high
statistic experimental data.
Such an experiment is like a microscope through which we can have a close look at
the nature of excited nuclear matter. In addition the zero degree electron scattering technique at
MAX-lab can be used also to perform deeply bound pionic atom experiments with the help of
the recoilless (γ,p) reaction [17] on the pre-actinide nuclei. In this case the fission of nuclei can
be used as a filter to decrease the contribution of background reactions [18].
2. The Fission Fragment Detector (FFD)
The FFD is based on low-pressure multi-wire proportional chamber (LPMWPC) techniques
[19]. The detector has a modular structure (see illustration in Fig. 6). It consists of four
windowless LPMWPC units, which form two symmetric arms placed above and below the
central beam. The inner chambers have an active area of 12 x 12 cm2 and a separation distance of
3 cm from the anode plane to the central beam. The active area for the outer chambers is 12 x 12
cm2 and they are located 8 cm away from the central beam. The solid state detectors will be
mounted just behind outer chambers and will provide only pulse hight information for evaluation
of the energy of fission fragments. Each arm provides about 2.5 sr of solid angle coverage and
measures the time-of-flight (TOF) as well as the fragment trajectory and energy.
Figure 6: A schematic sketch of the fission fragment detector.
The target is located at the center of the two arms and is made of e. g. a thin foil of 209Bi with
a thickness of 0.15 mg/cm2 to minimize the target straggling. The foil is placed at a small tilt
angle of about 7 degrees with respect to the beam direction so that the effective target thickness
is about 1 mg/cm2 to maximize the production yield. The entire FFD and target is in the same
gas volume without separation windows.
The assembly of the LPMWPC system and experimental target is placed in a cylindrical
vacuum chamber. There are two windows for the incoming beam and outgoing particles. Fast
pre-amplifiers for anode and cathode planes are mounted directly on each chamber plane
mounting frame and have minimized input cable length of about 10 cm to optimize the signal to
noise ratio and response time.
The fast signal from the anode plane of each unit provides the time coordinate, while the two
cathode planes that sandwich the anode plane in the middle measure the position coordinates; the
wires of the two planes are oriented with 90 degrees with respect to each other, providing two
dimensional position information. The position information from each cathode plane is obtained
by a time delay line technique [19]. Solid state detectors will provide pulse hight information.
The vacuum chamber is connected to a vacuum pumping system and can be evacuated to a
pressure of 10-3 Torr. It is equipped with stainless-steel valves for gas handling and two
barometers for pressure measurements. The chamber volume is connected to a reservoir of liquid
heptane (C7H16), with a reducing valve, and filled with heptane gas vapor at a pressure of 2-3
Torr. The gas density is equal to 5.48 microgram/(cm3 Torr). In addition, for test and calibration
purpose, a 252Cf spontaneous fission source in a source holder with a collimator will be
permanently mounted at the center of the top flange of the chamber.
The cathode planes are at a potential of -100 V. There are additional grounded guard
electrode planes with identical structure of the anode plane, outside of the cathode planes, to
avoid charges build-up outside of the MWPC units. The usual potential applied to the anode is
about +400 V. When +370 V applied to the anode plane, the typical signals are about 100 mV.
Typical time resolution from single anode plane in this case is about or better than 150 ps.
The FFD can also be operated in a double step regime by a voltage arrangement that includes
the guard planes. It can have much larger signals, thus higher sensitivity to low Z fragments. The
disadvantage is lower time resolution and rate capability. Excellent time and position resolutions
were achieved by the source tests as well as with intense bremsstrahlung photon beams. More
detailed discussion on the LPMWPC technique and source test results on this FFD can be found
in Ref. [19].
3. Performance of FFD with intensive electron beam
A similar detector have been tested at Jlab, during E02-017 experiment. The E02-017
experiment was run in Hall C after the HKS experiment in 2005. The goal of the experiment was
to measure heavy hypernuclear lifetime and we spent a few days for the commission of FFD and
learned implicit problems of FFD arising high-current electrons.
In this experiment the primary beam passed through the target foil mounted at the center of
FFD with windowless MWPC units mounted aside. The incident beam electrons produced a
huge amount of low-energy δ-electrons. These electrons entered easily into windowless planes of
the LPMWPCs and decreased the electric potential between cathodes and anode. Under the same
operational voltage scheme, the signals from the inner units were significantly smaller. Figure 7
shows the typical signals from the four LPMWPC anode planes, T1, T2, T3, and T4 with 200 nA
incident electron beam current. Here T2 and T3 were from the two inner units.
With such a short run period we successfully measured pion, proton and kaon associated
fission rates in electron-photon interactions with 209Bi target [20]. Observed at Jlab problem
associated with intense electron beam can be avoided and both inner and outer MWPC unites can
be operated successfully with intense electron beams by using a thin foil (~1 μm metalized
Mylar) [9] to separate target cell to FFD gas volume.
Figure 7: The image of oscilloscope signal from FF detector modules. The incident electron
beam intensity Ie ≈ 200 nA.
4. Expected Resolutions and Rates
The expected time resolution of a low-pressure MWPC detector pair is about 200 ps,
position resolution is expected to be less than 0.35 mm and the angular resolution will be less
than one degree. The expected energy resolution of SSD is less than 1 MeV. In principle by
taking enough distance between MWPC detector pair, about 1-2 mass resolution can be reached
for fission fragments. However with realistic geometry, when the distance between MWPC
detectors is about 5 cm, the expected mass resolution will be about 5.
In this experiment thin about 0.2 mg/cm2 , but tilted targets, with effective thickness of about
1 mg/cm2 (~ 2 ∙ 10-4 radiation length for actinides) will be used in the center of FF detector. For
200 MeV and 100 nA electron current, passing through 2 ∙ 10 -4 radiation length target we will
have about 6 ∙ 105 electron/s/MeV rate in the single tagging channel and 6 ∙ 107 virtual
photon/s/MeV flux on target. The expected fission rate for 20 mb typical actinide photofission
cross section, will be:
YFF = 20 10-27 2.5 1018 6 107 = 3 FF/s/MeV.
Taking into account the angular acceptance of the FF detector (~30%), the FF detection
rate will be about 0.9 FF/s/MeV. So, the true coincidence count rate for single channel is:
YTrue = 0.9 count/s.
For the similar detector arrangement but with real photon tagging [9] the total photon
flux over the entire (50 ≤ Eγ ≤ 800 MeV, E0 = 855 MeV) was about 108 γ/sec, i.e. about 3 105 for
single tagging channel. Taking into account the tagging efficiency (~45%) single channel rate
was about 6 105.
The expected fission rate for 20 mb cross section in this case is:
YFF = 20 10-27 6 1017 3 105 = 0.0036 FF/s/channel.
Hear 1 channel corresponds to ~2 MeV. Taking into account the angular acceptance and
efficiency of the FF detector (~40%), the FF detection rate is about 0.0015 FF/s/channel, i.e.
~600 times less than can be obtained with the virtual photon tagging technique. We would like
note that in this case by using tilted target to increase effective length of target is useless due
large size of real photon beam. It is expected that photon beam size be a circle with about 3 cm
in diameter. The electron beam size is expected to be a circle with about few mm in diameter.
The expected coincidence time resolution, i.e. the time resolution between the fragment
detector and scintillation counters in the focal plane of the tagging spectrometer is about 1 ns
(FWHM). The fission cross section for 200 MeV electron 238U interactions is about 4 mb [21].
The expected electro-fission rate for 238U target is:
YFF (e, 238U) = 4 10-27 2.5 1018 6 1011 = 6 103 FF/s.
The expected FF detector rate is:
YFFD = YFF (e, 238U) ∙ 0.3 = 1.8 103 count/s.
The expected accidental coincidence count rate for single channel, in these conditions will be:
YAcc = 1.8 103 6 105 10-9 ~ 1count/s.
i.e. at 100 nA incident electron current the YTrue /YAcc ~ 1.
5. Summary
The MAX-lab tagging photon system with a dedicated FF detector and small angle
electron scattering technique results a powerful experimental setup for performing precise
measurements of nearly complete photofission data in the energy range 40< Eγ <140 MeV. The
expected rates is about 600 times more than can be obtained by using real photon tagging
technique.
We propose these new photofission experiments start with U-238, Th-232 and Bi-209
targets and asking 30, 100 and 200 h (total 330 h) beam time, to take ~106 nearly complete
photofission data in the 10 MeV energy bin at Eγ = 100 MeV, for each of these three targets.
References
1) V. S. Barashenkov et al., Nucl. Phys. A231, 462 (1974)
2) A. S. Iljinov et al., Yad. Fiz. 32 (1980) 322 (Sov. J. Nucl. Phys. 32, 166 (1980)
3) T. E. Rodrigues et al., Phys. Rev. C 69, 064611 (2004)
4) V. Lucherini et al., Preprint LNF-90/019 (R), Frascati, 1990
5) A. Deppman et al., Phys. Rev. Lett. V. 87, n. 18, 182701-1 (2001)
6) L. A. Pshenichnov, et al., Arxiv preprint nucl-th/0303070, 2003
7) S. G. Mashnik et al., J. Nucl. Radiochem. Sci. Vol. 6, No. 2 pp A1-A19 (2005)
8) D. I. Ivanov et al., Sov. J. Nucl. Phys. 55, 506 (1992)
9) Th. Frommhold et al., Z. Phys. A 350, 249 (1994)
10) C. E. Hyde-Wright, W. Bertozzi, J. M. Finn, Electron Scattering at 0°: A Photon Tagging
Technique, 1985 Summer Workshop, CEBAF, Newport News, Virginia
11) E. A. Arakelyan et al., Sov. J. Nucl. Phys. 49, 1022 (1989)
12) O. Hashimoto, H. Tamura, Progress in Particle and Nuclear Physics, 57, 564(2005)
13) Gert Andersson et al., Nucl. Instr. and Meth. 163, 165 (1979)
14) G. Anderson et al., Z. Physik A 293. 241 (1979)
15) Y. S. Kim et al., Phys. Rev. C 54, #5, 2469 (1996)
16) A. S. Iljinov et al., Nucl. Phys. A 543 517 (1992)
17) L. Isakson (spokesperson) et al. , Deeply bound pionic atom from ( γ, p) reaction, MAXlab experiment #12
18) A. T. Margaryan, Nucl. Instr. and Meth. A 357, 495 (1995)
19) K. Assamagan et. al., Nucl. Instr. and Meth. A 426, 405 (1999)
20) Y. Song et al., Kaon, pion and proton associated fission of Bi nuclei with electromagnetic
probes, to be published in PRC.
21) V. P. Likhachev et al., Phys. Rev. C 68, 014615 (2003)