fission setup based on ppacs using a coincidence technique l. audouin, s. isaev, l. tassan-got, c....

Fission setup based on PPACs using a coincidence technique

L. Audouin, S. Isaev, L. Tassan-Got, C. StephanIPN d’Orsay

I. Durán, C. Paradela, D. Tarrío Univ. Santiago de Compostela

(on behalf of the n_TOF Collaboration)

PARADELA

I am going to present the work title ...developed in collaboration with the IPN d' Orsay and within the n_TOF experiment.

C. Paradela ANIMMA June 6-9, 2011 ICC-Ghent

Motivation

Fast reactors and ADS have renewed the interest on nuclear data, in particular those beyond 20 MeV.

A new generation of facilities allows studying nuclear reactions at high neutron energies.

For measuring fission, new devices are needed for discriminating fission from competing reactions ( spallation, multifragmentation,…)

PARADELA

The interes of this work is connected with the recent needs of nuclear data for developing the next generation of nuclear reactors: Fast reactors and ADS, both types present a harder neutron spectrum than current reactors and the neutron data bases present important discrepances or even holes in such energy range.These lacks can be explained by the difficulty for acceding to high energy neutron beams. During last decades, new facilities based on spallation targets have been built for example the n_TOF facility at CERN in which we perform our experiments, that reaches 1 GeV energies.If we want to measure fission for such high energies we need to distinguish it from other reactions that produces background signals in our detector.This is the motivation of the detection setup based on PPAC detectors that i will show.


Parallel Plate Avalanche Counter (PPAC)

Very thin detectors.

High detection efficiency for heavy ions (FF)

Fast timing with anode signal (0.5 ns resolution).

Fragment position from cathode signals.

PARADELA

AS fission detectors we have used parallel plate avalanche counters which are gas detectors extensively used in nuclear physics when you need to measure in strong radiation backgrounds, like monitoring an ion beam.They are efficient for detecting heavy ions against light particles or gammas.They can reach also good time resolucions, below 1 ns.The detector that we used is in fact a double ppac with one central anode providing the fast signal and two cathodes divided by strips which are connected to a delay line and that provides the position, each cathode one dimensional position and the strips are perpendicularly madeThe electrodes are made of 1.5 um mylar aluminised, very thin layer to minimise the interaction with the beam and the fragments.


Detection setup

10 PPACs and 9 targets (235U and 238U as references).

Gas flow inside a reaction chamber

Thin targets (300 µg/cm2) and backings (550 µg/cm2)

Both fragments detected by the closest PPACs. Trajectory reconstruction.

PARADELA

The full detection setup is composed by ten PPACs and 9 targets placed in between each two detectors as shown in the figure. Two of these targets are U235 and U238 which are used as references to extract the fission cross sections.The airtight chamber assures the gas flow inside. The gas used is C3F8 (octafluorpropane).One important characterist of this setup is the reduced amount of matter in beam. Not only the detectors are very thin but the targets and the backings. Therefore, the neutron beam traverse the setup with almost no reactions and both fission fragments can reach the two ppacs closer tot the target. this allows the trajectory reconstruction as shown in this schematic figure.


Coincidence technique

U-234: singles U-234: coincidences

Coincidence condition between anode signals rejects most of the background

Coincidence technique

PARADELA

because both fission fragments can traverse the target and backing, the detection of two signals in coincidence for two consecutive detectors identifies a fission event. This is very important to reject signals produced in other reaction like alpha emission or products of spallation reactions in the setup layers as it is shwon in these 2D plots. They represent the neutron energy producing the reaction against the PPAC amplitude signal. We can see that if we consider only one of the PPACs, there is an important background related with these alphas produced by the targets and the high energy reacions related signals for low amplitudes.When we ask for a coincidence on the complementary detector, almost all the background disappear remaining the fission signals.


Fission target identification

Correlation between PPAC time differences allows the unambigous target identification

T0 T1 T2T2T1 T2T0 T1 T2T0 T1 T2

PARADELA

This technique present some drawbacks: for ex. the angular efficiency reduction due to the absorption of the fission fragments emitted at large angles. Nevertheless this efficiency can be studied and corrected for obtaining an accurate cross section.Other problem is related with the multiple targets in the setup, because the ffision fragments can even traverse two detectors and produce missidentification of the fission event. However checking the times needed to traverse the distance between the detectors fired we can clearly separate the fission from different targets.For example a simplificated case with three detectors and two targets is shown here.The fissions coming from the target on the left produce signals with reduced spread in the time different between t0 and t1 and close to zero while this is found for detectors t1 and t2 when the target is emitted from the target on the right.


cathode active surface 200 mm

Tc1 Tc2

Tc1-Tc2

Cathode positioning

Diagonal condition:

(Tch1-Tanode)+(Tch2-Tanode)=DLT DLT: Total delay line length (~320 ns)

The time difference between both cathode ends provides the position of the signal.

PARADELA

ABout the position identification, both edges of the delay line are readed, so the the sum of both signals must be equal to the delay line. This allow us to reject wrong assignations. Then the difference between these cathode signals is related to the position in the delay line where the signal is created.As each cathode provides the horizontal and the vertical position, we can know where the FF hit the detector.


Cross Section: Energy Resolution

Very low background: lower yield between resonances than expected from evaluations

PARADELA

With this setup different information about the fission can be obtained.The more direct one is the fission yield that for low energies and small ranges where the flux is flat can be compared directly with the evaluated cross section, only scaling this XS. the 235U case is shown in the figure, and the good resolution obtained with the PPAC at n_TOF is evident. The resonance energies are perfectly reproduced and even in the dips between resonances ntof data is below showing the very low background of our measurement


Cross Section: Energy Range

Fission measured for neutron energies up to 1 GeV.

Cross sections have been measured for different isotopes: 234,233U, 237Np, 232Th, natPb, 209Bi.

[1] C. Paradela et al. Phys. Rev. C 82, 034601 (2010)

[2] L. Audouin et al. Proc. of ND2007, p. 421

[3] D. Tarrío et al. Phys. Rev. C 82, 044620 (2011)

PARADELA

With the fission yields we can obtain the cross sections by normalizing by the flux (obtained from U5), the target mass and the detection efficiency. The cross section measured with this setup in the range between 1 eV and 1 GeV. In the case of the cross section ratio of the two reference targets,is shown in the figure. The two sets of data above 20 MeV, lisowsky and shcherbakov are discrepant and our measurement is closer to the american one.However none of these measurments go reach 1 GeV so JENDL-HE estimatimation has been used to compare with our data.


U-234 cosine distribution for neutron energies near the fission threshold

Fission Fragment Angular Distribution

Cos ()Cos ()Cos ()

Log E =5.6 Log E =5.4

Log E =5.8

Log E =5.5

Log E =6.0 Log E =5.9

PARADELA

In addition to the fission yields and cross sections this setup provides also the trajectory of the fission fragments under certain assumptions, so we can also study the FF AD. For fast neutrons this distribution is not isotropic at all, but it changes quickly between forward and sideways emission for certain neutron energies as shown in the figure.Because of the reduced angular efficiency of our setup not all the angular distribution range in the cosine is available


Anisotropy extrapolation

B (

Ani

sotr

opy

p

ara

met

er)

Angular Distribution of fission fragments is described by W()1+Bcos2 Results obtained fitting in our reduced angular range Despite the constraints, present results are quite in agreement with previous data from Leachman (Phys. Rev. 137, B814 (1965)) and Tutin et al. (Nucl. Instrum. Meth. A 457 (2001) 646-652)

Neutron energy (MeV)

This workLeachmanTutin

PARADELA

so we need to assume certain hypotesis to extrapolate a value for the anisotropic parameter of the distribution which is the ratio between the counts at 0 degress and 90 degress.Despite these drawbacks we are able to estimate an anisotropic parameter that qualitatively reproduce the results obtained in the past for example for the U238 shown in the figure.


Conclusions

A fission detection setup based on PPACs has been implemented for the CERN n_TOF facility.

Both fission fragments are detected in coincidence and their trajectory is reconstructed.

Cross sections and angular distributions can be measured up to 1 GeV at the CERN n_TOF facility.

PARADELA

To sumarize this talk, I had presented a fission detection setup based on PPAC detectors that can measure up to 9 different targets simultaneously and that has been used at the CERN n_TOF facility.The fission identification is performed by the detection in coincidence of both fission fragments, and their trajectory can be reconstructed.It allow to obtain the fission cross sections in the wide energy range from 1 eV up to 1 GeV in the same experiment and the FFAD as well.

fission setup based on ppacs using a coincidence technique l. audouin, s. isaev, l. tassan-got, c....

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