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