1. Measurement of Secondary Neutrons Produced from Thin Graphite stripper by 50 MeV/n U Beams
Nam-Suk Jung, Arim Lee, Hee-Seock Lee,
Leila Mokhtari Oranj, Mahdi Bakhtiari
Pohang Accelerator Laboratory / POSTECH, South Korea
Noriaki Nakao
Shimizu Corp. Japan
Yoshitomo Uwamino
RIKEN Nishina Center, Japan

2. Experiments at RIBF of RIKEN Nishina Center
Outline
Introduction
Experiments at RIBF of RIKEN Nishina Center
Results and Comparison with MC calculation
- PHITS, FLUKA
Summary 2

3. Introduction – RAON accelerator
Rare Isotope Science Project(RISP) of IBS has constructed “RAON” accelerator from 2012 to 2021.
One of the main beam: 200 MeV/n, 400 kW 238U77+ to 81+.
Beam
90° bending
section
Soil
Charge stripper
Low energy
experiment facility
Concrete
SCL2
180° bending section
Dipole
The structural drawing of RISP accelerator ( version)
238U33+, 34+ (17.5 MeV/n)
238U77+ to 81 +
(17.5 MeV/n)
Carbon stripper
(Φ 3 cm, t = 5.0 μm)
Dipole
magnet
Stripping and transport of high-charged 238U ions at RAON
Stripping reaction
Beam selection

4. Benchmarking data for U beams
O. Yordanov et. GSI, in 2005
: 1 GeV/n U + thick Fe
Y. Uwamino RIBF/RIKEN, in 2010
: 345 MeV/n U + thin Be, Pb, thick Fe
N. Nakao RIBF/RIKEN in 2014
: 345 MeV/n U + thin Be
Experimental data for benchmarking is rare !!
⇒ Energy of U beam is much higher than 17.5 MeV/n U.
⇒ We performed stripper section of RIBF/RIKEN. 4

5. Stripping section at RIBF of RIKEN
238U64+
(50 MeV/n)
238U86 +
Stripper
Dipole magnet
Stripping and transport of high-charged 238U ions at RIBF of RIKEN
Stripping reaction
Beam selection
Experiment Position
Faraday Cup
(FC-M11)
Stripper
Neutron area monitor
(FC-M04)
Stripping reaction of 238U with thin stripper was used for the experiment of secondary high-energy neutron measurement. 5

6. Stripper change
238U64+
(50 MeV/n)
238U86 +
Stripper
Dipole magnet
Stripping and transport of high-charged 238U ions at RIBF of RIKEN
Beryllium disk stripper (up to 2014)
Graphite disk stripper (since 2015)
15.6 mg/cm2 ⇒ 85 μm
7 mg/cm2 x 2 ea = 14 mg/cm2 ⇒ 70 μm
Damage issue (limit : ~ 20 days) 6

7. Experimental conditions & Analysis procedure
1st, in 2014
2nd, in 2015
Beam target
Beryllium (t = 85 μm)
Graphite (t = 70 μm)
Beam
50 MeV/n U-238
Activation samples
Bi, Al (Φ 2 cm, t = 2 mm),
Co (2.5 x 2.5 x 0.05 cm3)
Co (Φ 2 cm, t = 1 mm),
Samples position
Outside of stripper chamber
: 15°, 30°, 45° (Bi, Al, Co) and 90° (Bi, Al) respect to the beam axis
: 15°, 30°, 45°, 65.2° and 90° (Bi, Al, Co) respect to the beam axis
Irradiation time
~ 23 days
62 hr 30 min (~ 2.6 days)
Gamma spectrocopy results
Production rate of radionuclide in the samples
Neutron energy spectra
(By unfolding method, SAND-II)
Comparison
Calculated production rate of radionuclide by PHITS 2.64
Calculated neutron energy spectra by PHITS 2.64 and FLUKA c 7

8. For Graphite stripper (2015) For Graphite stripper (2015)
Activation sample position
For Graphite stripper (2015)
U beam
For Be stripper (2014)
U beam
65.2o
90o
30o 2 1 3 5 6 4
C stripper
45o
15o x
U beam
U beam 8
sample
For Graphite stripper (2015)

9. x Activation sample position (downstream samples) y x U-beam position
Reference point : shifted beam position
Co2
Bi2
Al2
Al1
Bi1
Co1
Bi4
Al4
Co4
Co3
Bi3
Al3
Co5
Bi5
Al5
Bi6
Al6
Co6
U-beam
position
by wire-scanner
: x = 0.43 mm,
y = mm . d
13°
18°
27°
30°
42°
44° x y
Sample
d [mm]
θ ± Δθ
θ_average
Co1
28.4
12.0° ± 4.1°
13°
(12.8°)
Bi1
29.2
12.3° ± 4.1°
Al1
33.1
13.9° ± 4.0°
Co2
43.9
18.2° ± 3.9°
18°
(17.5°)
Bi2
42.9
17.8° ± 3.9°
Al2
39.6
16.5° ± 3.9°
Co3
71.5
28.2° ± 3.3°
27°
(27.4°)
Bi3
67.0
26.7° ± 3.4°
Al3
69.2
27.4° ± 3.4°
Co4
74.5
29.2° ± 3.3°
30°
(29.9°)
Bi4
78.8
30.6° ± 3.2°
Al4
76.8
29.9° ± 3.2°
Co5
122.9
42.6° ± 2.3°
42°
(42.4°)
Bi5
120.7
42.1° ± 2.4°
Al5
121.7
42.4° ± 2.3°
Co6
129.3
44.1° ± 2.2°
44°
(44.3°)
Bi6
131.5
44.6° ± 2.2°
Al6
130.5
44.3° ± 2.2°
R=126.3mm
R=73mm
R=36mm x 2 1 3 5 6 4
C stripper z 9
sample

10. Beam current assumption
Irradiation time : 06 Dec. 18:30 ~ 09 Dec. 9:00 (62 hrs 30 mins)
U-beam current assumption : enA = nA (64+) = 1.77 x 1021 pps
Before stripper
: FC-M04 (Faraday cup),
PP-M04 (Beam profile monitor)
After dipole magnet
: FC-M11 (Faraday cup),
PP-H12 (Beam profile monitor)
Absolute beam current was measured using the faraday cup(systematic error: 20%).
The variation was recorded using beam profile monitor and monitored by neutron area monitor. 10

11. Interested γ - ray [keV]
Interested reaction, gamma and Cross-section
Reaction
Half-Life
Interested γ - ray [keV]
209Bi(n,3n)207Bi
209Bi(n,4n)206Bi
209Bi(n,5n)205Bi
209Bi(n,6n)204Bi
209Bi(n,7n)203Bi
209Bi(n,8n)202Bi
31.55 y
6.243 d
15.31 d
11.22 h
11.76 h
1.72 h
569.7, ,
343.5, 516.2, 803.1,
881.0, ,
703.5, 987.7,
374.8, 899.2, 984.0
820.2, 825.2,
422.1, 657.5, 960.7
27Al(n,α)24Na
14.96 h
1369.0
59Co(n,2n)58Co
59Co(n,3n)57Co
59Co(n,4n)56Co
59Co(n,5n)55Co
59Co(n,p)59Fe
59Co(n,α)56Mn
70.86 d
271.8 d
77.3 d
17.5 h
44.5 d
2.58 h
811.0
122.1, 136.5
1037.8, ,
931.3
1099.3,
846.8,
Neutron threshold energy of available Bi reaction is above 20 MeV. To extend the neutron spectra to lower energy region, we used Bi + Al.
We used Co for cross-check. Its minimum threshold energy is above 0.3 MeV in case of (n,p) reaction. 11

12. Gamma spectroscopy Spectroscopy system of RIKEN Gamma peak analysis
HPGe detector: CANBERRA GC2019
- Relative efficiency: 20%
- FWHM 1.33 MeV : 2.0 keV (from certification sheet) : 2.19 keV (from source measurement)
HyperGam software
- Developed at Seoul Nat’l Univ. in Korea
- Automatic analysis of complex gamma spectrum
- Analysis core routine : adopted from HYPERMET
791 keV Bi-204 (3.3%)
803 keV Bi-206 (99%)
816 keV Bi-203 (4%)
820 keV Bi-203 (29.7%)
825 keV Bi-203 (14.6%)
881 keV Bi-206 (66.2%) 12

13. Production rate
Production rate, R [sample atom-1 beam ion-1 ], was obtained with the following formula.
𝑅= 𝜆 𝑁 𝑝 𝑁 𝑠 𝛾𝜀 𝑒 −𝜆 𝑡 𝑐 (1− 𝑒 −𝜆 𝑡 𝑚 )( 6.242× 𝑄 ) 𝑞 𝑖 1− 𝑒 −𝜆 𝑡 𝑖 𝑒 −𝜆 𝑡 𝑐
𝜆 : Decay constant of the radionuclides
𝑁 𝑝 : Net area of gamma ray peak
𝑁 𝑠 : Number of atoms in the sample (parent nucleus)
𝛾 : Gamma emission probability
𝜀 : Absolute efficiency of HPGe detector at particular gamma-ray energy
(including the self-absorption effect)
𝑡 𝑖 : Irradiation time [sec]
𝑡 𝑐 : Cooling time [sec]
𝑡 𝑚 : Measurement time [sec]
𝑞 𝑖 : Beam current [ampere]
𝑄 : Charge state of ion 13

14. Measured production rates
Production rate of 209Bi(n, 4n)206Bi reaction at 13° angle is most high.
When the angle respect to the beam axis is increased,
⇒ Production rates of (n,xn) reaction decreased rapidly than (n,p) or (n,α) reaction.
⇒ The generation of high-energy neutron above 20 MeV decreased more. 14

15. PHITS calculation Conditions of PHITS calculation
Only Be stripper
Be stripper +
Around geometry
PHITS 2.64
Beam: 50 MeV/n 238U (7.92 mm shifted downward)
Physics model
→ Nucleus-Nucleus collisions: JQMD + GEM (above 10 MeV/n up to 100 GeV/n),
SPAR (below 10 MeV/n) → Nucleon-induced reaction:
INCL4.6 + GEM (from 20 MeV to 3 GeV), INC-ELF
→ JENDL-4.0 library (below 20 MeV)
Geometry: Graphite stripper, samples with around
geometry such as chamber, pipes
Estimation of particle fluence: [t-track] tally
Neutron spectra from thin Be (8.5 μm)
w or w/o around geometry, PHITS calculation
Production rate calculation
𝑅= 𝐸 𝑡ℎ𝑟𝑒𝑠 𝐸 𝑚𝑎𝑥 𝜎 𝐸 𝜙 𝐸 𝑑𝐸 𝑅
𝐸 𝑡ℎ𝑟𝑒𝑠
𝐸 𝑚𝑎𝑥
𝜎 𝐸
𝜙 𝐸
: Production rate [#/sample atom/238U]
: Threshold energy of reaction
: Maximum energy of reaction
: Neutron cross section of radionuclides [10-24 cm2]
: Neutron fluence [n/cm2/238U]
JENDL-4 (below 20 MeV)
JENDL-HE 2007 (above 20 MeV)
[t-track] tally
PHITS 15

16. Comparison of experiment & PHITS calculation (production rates)
Production rate from PHITS calculation overestimated experimental results at lower angles.
: ~ 4 times higher (maximum)
In case of samples which was installed at the side of the stripper chamber such as 65.2o and 90o, PHITS results were similar with experimental results. 16

17. Secondary neutron spectrum17

18. Secondary neutron spectrum
Below 45 degree chamber downstream)
: PHITS > FLUKA ≥ EXPERIMENT
Above 45 degree chamber sideward)
: PHITS ≒ EXPERIMENT ≥ FLUKA 18

19. Beryllium stripper vs. Graphite stripper
Beryllium disk stripper (t = 85 μm)
Graphite disk stripper (t = 70 μm)
Beryllium stripper generates more secondary neutrons than Graphite stripper because of thickness difference.
The shape of secondary neutron spectra is almost same between two cases. 19

20. Summary
Neutron production from thin graphite by 50 MeV/n 238U beam was observed successfully by activation analysis with several elements which have threshold reaction cross-section with high energy neutrons.
Production yields of radionuclides in the activation samples induced from each nuclear reaction were measured using the nuclear reactions of 209Bi(n,xn)210-xBi (x=4~8), 59Co(n,xn)60-xCo (x=2~5), 59Co(n,p)59Fe, 59Co(n, α)56Mn, 27Al(n, α)24Na.
Neutron spectra were obtained from measured production rates by unfolding with SAND-II code.
Experimental results were compared with calculation data by PHITS and FLUKA code. Some discrepancies between the calculations data and experimental results were found but might be acceptable. These results can compensate lacks of experimental data which are needed for benchmarking study. 20

21. Thank you for your attention !!
Summary
More MC comparison data will be introduced soon such as production rate using FLUKA, production rate and neutron energy spectrum using MARS15 and MCNPX.
Thank you for your attention !! 21

22. PHITS-2.64 vs. PHITS-2.82
Beryllium disk stripper (t = 8.5 μm, 1/10 thickness of real stripper) 22

23. Absolute efficiency of spectroscopy system
Mixed standard source
(point source)
Non irradiated Bi 1. 3.
Experimental setup for gamma ray point source above the non-irradiated Bi 2.
Absolute efficiency of HPGe detector, considering the self-absorption effect in the comparatively thick activation detectors cm from detector head)
1. Gamma ray point source only
2. Gamma ray point source
above the non irradiated activation sample
3. Volume source
Schematic drawing of 3 types system of gamma ray detection for absolute efficiency of HPGe detector using MCNPX code (F8 tally)
→ Reproduce experimental data within 10% 23

24. Production rate from each gamma-peak24