Inelastic neutron scattering from carbon, iron, yttrium and lead
Cecilia Gustavsson, Jan Blomgren, Carl Hellesen, Angelica Öhrn, Stephan Pomp and the SCANDAL collaboration
Department of physics and astronomy, Uppsala University, Sweden
Cecilia Gustavsson, CNR11, Prague
Inelastic neutron scattering from carbon, iron, yttrium and lead
Outline
• Background
• Overview of the experiment and detector setup
• Analysis procedure
• Some preliminary results and comparison to various model predictions
• Comparisons to existing data for (n,n’x), (p,p’x), (n,p’x), reactions
• Summary and outlook
Cecilia Gustavsson, CNR11, Prague
Elastic scattering (n,n) has been reported for several nuclei using the SCANDAL setup installed at TSL in Uppsala.
Measuring (n,n) was the primary and successful objective of this setup...
...however, it was found that an extended analysis could extract (n,n’x) data from the same data sets!
Cecilia Gustavsson, CNR11, Prague
Experiment – neutron detection
SCANDAL – SCAttered Nucelon Detection AssembLy: The SCANDAL setup:
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Active plastic converter: CH2
• H(n,p)
• C(n,p) Q‐value: ‐12.6 MeV
• Overlap from around 20°.
Opening angle criterion 10°.
Proton track measured in DCH:s
Full proton energy measured in CsI:s
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For elastic scattering analysis, the analysis is straight forward. Always choose events with maximum energy.
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For inelastic scattering it cannot be determined event‐by‐event if energy was lost in the target or in conversion on 12C in the converter.
Cecilia Gustavsson, CNR11, Prague
Experiment – neutron facility
The Svedberg Laboratory (TSL) in Uppsala: Neutron facility @ TSL until 2004
• Neutron production: 7Li(p,n).
• Neutron energy: 20 ‐ (100) ‐ 175 MeV.
Neutrons @ experiment position:
• Beam size 9 cm in diameter.
• Neutron flux ≈ 4∙104 s‐1 cm‐2. Neutron monitors
Proton monitor
Cecilia Gustavsson, CNR11, Prague
Analysis procedure
Forward folding procedure:
1) Construct a physically reasonable parameterized trial spectrum. 2) Fold the trial spectrum with the detector response function. 3) Compare the folded trial spectrum with measured data.
4) Vary parameters of the trial spectrum iteratively to find the best fit to experimental data. 5) Calculate the ratio between input and output in the total forward folding procedure and use this ratio as a correction for the measured spectrum. Cecilia Gustavsson, CNR11, Prague
Analysis procedure
Components of the SCANDAL response function:
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Elastically scattered neutrons converted by the H(n,p) reaction form a peak .
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Neutrons converted by the 12C(n,p) reaction.
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Low energy tail from the neutron production using the 7Li(p,n) reaction
Also there is an effect of individual response to protons in individual CsIs of the detector.
From Klug et al., Nucl. Instr. Meth. A489 282 (2002)
Cecilia Gustavsson, CNR11, Prague
Analysis procedure
The TOF cut and the SCANDAL response function
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The SCANDAL response function can be well described if no TOF cut is applied to the incoming neutron spectrum.
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But without a TOF cut in the analysis, the low‐
energy neutrons from the tail will interfere with the (n,n’x) reaction that we want to measure.
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For data quality we need the TOF cut..
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...but it is difficult to describe such a cut analytically.
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Possible solution:
— Without TOF: Smaller uncertainty
in response function
........ With TOF: Smaller uncertainty in data/background
Experimentally measured response function!
From Klug et al., Nucl. Instr. Meth. A489 282 (2002)
Cecilia Gustavsson, CNR11, Prague
Analysis procedure
The following example is from the analysis of 56Fe(n,n’x) at an average angle of 36°.
The SCANDAL response function:
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The method used is to measure an experimental response function at a small angle (9°) during experiment and at full energy (96 MeV).
Contribution from inelastic scattering at 9° was calculated with the TALYS code and found to be negligible (< 1%) for all nuclei.
The measured proton energy spectrum and the corresponding excitation‐energy spectrum was taken as the response function of the detector.
At lower energies, the response had to be modelled. The same overall shape was assumed.
Cecilia Gustavsson, CNR11, Prague
Measured response function
at 9 degrees
Analysis procedure
The trial spectrum and its 2 components:
1) A gaussian representing the ground state (elastic scattering).
2) A continuum predicting the inelastic scattering.
Gaussian
For the continuum part, the PRECO code was used.
The method was tested by using other codes (as well as a flat distribution) to predict the inelastic spectrum. The relative strength between the gaussian and the continuum was also altered achieve the best fit the experimental data. Cecilia Gustavsson, CNR11, Prague
PRECO calculation
Analysis procedure
The iterative procedure applied to experimental data:
The trial spectrum was folded with the experimental response and compared to measured data. The difference between measured data and the result of the forward‐folding was used to calculate the input to the next forward‐folding.
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The red curve represents the final forward‐folded spectrum. The blue curve is the contribution from the inelastic part of the trial spectrum.
The ratio between the input and the output of the total forward‐folding was used to establish a correction with which the measured spectrum was multiplied bin‐by‐bin.
Cecilia Gustavsson, CNR11, Prague
Experimental data
at 36 degrees
Output from forwardfolding
Pre-equilibrium
contribution
Analysis procedure
Final result:
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The black curve represents the experimental data.
The green curve is the resulting spectrum, when all corrections have been done. Corrections are:
1) As calculated in the forward‐folding method.
2) Corrections for the energy dependence of the H(n,p) cross section which is closely related to the detection efficiency in SCANDAL.
3) Corrections for differences in the CsI energy efficiency.
Finally data were corrected for attenuation and multiple scattering in the target.
Experimental data
Final result
Cecilia Gustavsson, CNR11, Prague
Comparison to models
The results were compared to several model predictions of inelastic scattering:
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the nuclear reaction code TALYS developed by Koning et al.
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evaluated data of relevance for neutron and proton cancer therapy applications by Chadwick et al.; the ICRU63.
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the nuclear‐reaction code HMS‐ALICE developed by Blann.
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the PRECO code developed Kalbach for pre‐equilibrium reaction mechanisms.
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the quantum molecular dynamics code QMD. It is a microscopic method to calculate nuclear collision processes in the medium to high energy region. Calculations showed are performed by S. Chiba.
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the non‐elastic reaction code BRIEFF based on an intra‐nuclear cascade model, developed by Duarte.
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evaluations of the 12C(n,n’x) reaction performed by Watanabe et al. with the evaluated nuclear data library JENDL/HE‐2004.
Cecilia Gustavsson, CNR11, Prague
Preliminary results carbon
Energy excitation spectra for 12C
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Both panels show the same data compared to
different model predictions.
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Each data set consist of several angular bins; the
indicated angle is the average.
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Data cover 0-45 MeV excitation energy.
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Data and models at 39°, 48° and 58° have been
multiplied by 10-2, 10-4 and 10-6, respectively.
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In general, not a very good agreement between
data and models.
Cecilia Gustavsson, CNR11, Prague
Preliminary results carbon
Angular distribution spectra for 12C
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Angular distribution can be translated to dose
distribution and is of interest for medical
applications.
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Data are expected to be more forward-peaked at
high neutron energies (low excitation energies).
Cecilia Gustavsson, CNR11, Prague
Preliminary results iron
Energy excitation spectra for 56Fe
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Both panels show the same data compared to
different model predictions.
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Each data set consist of several angular bins; the
indicated angle is the average.
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Data cover 0-45 MeV excitation energy.
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Data and models at 36°, 44°, 52° and 65° have
been multiplied by 10-2, 10-4 , 10-6 and 10-8,
respectively.
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Better agreement between data and models.
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The models do not include a thorough treatment
of nuclear structure and are adapted for high
excitation energies.
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The giant resonance (isoscalar quadropole
resonance) structure at ≈15 MeV can not be seen
in any of the models (not included).
Cecilia Gustavsson, CNR11, Prague
Preliminary results iron
Angular distribution spectra for 56Fe
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Data are expected to be more forward-peaked at
high neutron energies (low excitation energies).
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Trends in in good agreement with the models.
Cecilia Gustavsson, CNR11, Prague
Comparision to (n,n’x) data at 65 MeV for iron and lead
Comparison to data at 65 MeV
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The present data for iron and lead have
been compared to (n,n’x) data at 65
MeV from University of California.
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Similar detector setups. Instead of
active converter, the UC experiment
used one plastic and one carbon
converter for carbon subtraction.
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Expected that the cross sections are
lower at 96 MeV.
Ref: E.L. Hjort, et al., Phys. Rev., C53, 237 (1996).
Cecilia Gustavsson, CNR11, Prague
Preliminary comparision to (p,p’x) and (n,p’x)
96 MeV, 26 degrees
Comparison of present data:
A = 56
A = 89
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56Fe(n,n’x)
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89Y(n,n’x)
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208Pb(n,n’x)
to 58Ni(p,p’x) and 56Fe(n,p’x)
to 90Zr(p,p’x)
to 209Bi(p,p’x) and 208Pb(n,p’x)
Similar comparison by Hjort et al., at 65 MeV
showed that the (n,n’x) channel gets more
intense relative the (p,p’x) channel at higher A.
(Ref: E.L. Hjort, et al., Phys. Rev., C53, 237 (1996).)
A = 208
Seen here: (n,n’x) becomes stronger relative
to (n,p’x) when A increases.
Cecilia Gustavsson, CNR11, Prague
Conclusions and outlook
Conclusions
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Data have been extracted for the (n,n’x) reaction up to 45 MeV excitation energy on carbon, iron, yttrium and lead for 96 MeV incident neutron energy.
Data have been compared to several model predictions and show a reasonably good agreement.
We have also compared data to data from the literature for (n,n’x), (n,p’x) and (p,p’x).
Outlook
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Some steps in the analysis could be improved or simulated e.g. the detector response to lower neutron energies and the model dependence in the forward‐folding procedure.
Publication ;)
Thank you for you attention!
Cecilia Gustavsson, CNR11, Prague