Measurements of the (n,xn)reactions cross sections using
new digital methods.
Habib KaramGroup GRACE
outlook• General context - importance of the (n,xn) reactions
- gamma prompt spectroscopy method
• Experiment at GELINA - digital cards
- work already achieved by the group GRACE
• PHD work - adaptation of the method used before
- tests at IReS(Strasbourg) and Geel (Belgium)
- results
3
General context• Increasing need of energy resources worldwide that enhances
the value of the nuclear energy.
• The problem of the radioactive waste is still lacking a good solution.
• The hybrid system could be the solution for the transmutation of the minor actinides.
• Need of nuclear data
• Imprecise cross sections
- Th/U neutron cycle
Above 6 MeV the 233U(n,2n)232U and the fission cross sections values are very close but they were never been measured.
Cross section values for the233U(n,2n)232U in three different
data bases
Method : prompt spectroscopy• Reactions studies using the characteristics of the gamma
rays emitted by the target while bombarded with a beam.
• Can be used with « white beams ». In this case the time of flight method should be applied.
• A very good energy and time resolution is required.
• Time of flight method : time windows corresponding to neutron energies in order to determine (n,xn) reactions.
HPGe detector (76 % eff. rel.) at 110°
Gamma rays spectra : 208Pb target
Experiment at GELINAExperiment at GELINA
GELINAGELINA
U target
((γ, f )γ, f ) ((γ,n)γ,n)
Enriched 235U target (93.17 %)
γ « Bremsstrahlung »
Planar detector
- t0 : Reference time
- t : Detection time
- Flight path : 30 m
- Flash is detected after 0.100 μs
- 20 MeV neutrons detected after 0.485 μs
- Frequency = 800Hz
Fligh
t path
t- t0 = time
of flight
(n,2n)
Time structure at 200 m
Time of flight (s)
En=
20
MeV
En=
6 M
eV
En=
1 M
eV
(n,n’) (n,) flash
Dead time with conventional electronics
Dead time with our digital electronics
Digital card :TNT 2 Digital card :TNT 2
Digital card developed at IReS, Strasbourg 14 bit coders, coding rate100 MHz – 4 inputs channels available. Operated by PC using a specialised software. Data Transfer on PC for storage/treatment.
• Work done by the group grace
• Measurement of the 207Pb et 208Pb (n,xn) cross sections successfully done using a flight path of 200 m and 60 MHz digital cards coding rates:
- 20 MeV neutrons could be detected even if there is
a flash gamma.
• Sample of 90 g enriched 207Pb borrowed from Oak Ridge
- relatively low activity
• Big CLOVER detectors
- gamma rays energies up to 2.614 MeV
PHD workPHD work Adaptation of the method for the 235U (n,xn) cross sections measurements.
- Digital cards : faster coding rate (100 MHz) - New code C to read and analyse data.
- High activity of the 235U target - Thinner targets less interactions shorter flight path should be used
- No neutrons detected after the flash gamma.
- Planar detectors - Low gamma energies.
Test of the TNT2 cardsTest of the TNT2 cards
Cobalt source
Plastic detector
Ge detector
Experimental setup at IReS to simulate Geel’s conditions.
-Testing the digital cards.- Finding the Ge planar time and energy resolution.
Scintillator
anodeCFD NIM input TNT 2NIM input TNT 2
Scintillator Neutron beam t0
Ge planar
detectorinput 2 and 4 TNT 2
Detection time t in the Ge
t – t0 time of flight
Time resolution
15,4
19,3
0
10
20
30
40
50
60
70
80
90
100
0 0,125 0,25 0,375 0,5 0,625 0,75 0,875 1
fraction
réso
lutio
n en
tem
ps
f it par PAW
moment d'ordre 2
Delay fixed at 80 ns
15,4
19,3
0
5
10
15
20
25
30
35
40
0 10 20 30 40 50 60 70 80 90
retard
réso
lutio
n en
tem
ps
f it par PAW
moment d'ordre 2
Fraction fixed at 1/8 ns
- Energy spectrum
13
32 K
eV
11
72 K
eV
FWHM @ 1172 = 1.89 KeVFWHM @ 1172 = 1.89 KeV
FWHM @ 1332 = 1.91 KeVFWHM @ 1332 = 1.91 KeV k=500 ns k=500 ns m=1000 nsm=1000 ns
Results• Code C for reading the files• Data analysis using PAW
Test run 11/2005 at Geel• purpose : testing the experimental setups and optimizing the acquisition parameters
2. Gamma flash rate
- The gamma flash rateThe gamma flash rate ( i.e gamma flash hits per second divided by the accelerator frequency) was 23 % (only was 23 % (only 9 % due to the target)9 % due to the target)
=> Beam’s collimation should be improved
Data analysisData analysis
Flash γ
γ Induced by neutrons
1. Time spectrum
11
1.2
11
4.7
} X
235 U
18
5.7
231T
h
Neutrons between 1 and 6 MeV
14
3.7
6 23
1 Th
16
3.3
231 T
h
3. Energy spectrum
235U activity
Neutrons
124.
5+12
5.2
(n,n
’) 23
5 U
129.
3 (n
,n’)
235
U
Time of flight between0.884 and 2.167 μs
15
2.7
(n
,2n
) 23
5 U
Neutrons between 6 and 20 MeV
235U activity
Neutrons
Time of flight between0.485 and 0.8842 μs
Energy spectrum
• Determine the neutron fluxset up a flux monitor (plastic scintillator)Optimization of the scintillator efficiency -> Geant 4 simulations -> test using a neutron source at IReS
Upcoming stepsUpcoming steps
• In beam tests - Adapt the TNT2 cards to fast detectors (scintillator) - Testing the scintillator (neutron flux) - Testing the new collimator ( gammas flash rate) - …
• Prise de données … … … …
On-line energy and time recording
En [MeV] tn @ 200 m [μs]
Δtn @ 200 m [μs]
tn @ 30 m [μs]
Δtn @ 30 m [μs]
Flash 0.662 - 0.1 -
20 3.26 2.60 0.485 0.385
15 3.75 3.09 0.559 0.459
6 5.89 5.23 0.88 0.78
1 14.4 13.7 2.167 2.067
0.3 26.2 25.5 3.957 3.857
Digital card parameters