New Experimental Results on the Cumulative Yields From Thermal Fission of $^{235} {\rm U}$ and $^{239}{\rm Pu}$ and From Photofission of $^{235} {\rm U}$ and $^{238} {\rm U}$ Induced

$Page 1: New Experimental Results on the Cumulative Yields From Thermal Fission of $^{235} {\rm U}$ and $^{239}{\rm Pu}$ and From Photofission of $^{235} {\rm U}$ and $^{238} {\rm U}$ Induced$
2064 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 58, NO. 4, AUGUST 2011

New Experimental Results on the Cumulative YieldsFrom Thermal Fission of �� and ��

�� andFrom Photofission of �� and ��

� Induced byBremsstrahlung

Frédérick Carrel, Mathieu Agelou, Mehdi Gmar, Frédéric Lainé, Joël Loridon, Jean-Luc Ma,Christian Passard, and Bénédicte Poumarède

Abstract—The yields of fission products are one of the maincharacteristics of the fission process. In the field of nuclear wastepackage characterization, using Photon Activation Analysis(PAA), these yields are needed in order to optimize a techniqueenabling the identification of actinides (��, ��

�, ��),

based on the detection of delayed gamma-rays. As the lack of datain the field of photofission is strongly penalizing for the tuning ofthis technique, we designed several measurement campaigns inorder to determine the yields of various photofission products. Theexperiments were based on the detection of delayed gamma-raysand delayed neutrons emitted during the same measurement. Thefeasibility of this technique was first verified in the context ofactive neutron interrogation, by comparing experimental resultsfor the thermal fission of ��

� and �� with reference values

provided by several recent databases (ENDFB 6.8, JEFF 3.1). Themethod was then applied to active photon interrogation, in orderto obtain the yields of nuclides formed by the photofission of ��and ��

�. This paper presents the experimental results obtainedwith these measurements.

Index Terms—Actinides, cumulative yields, delayed gamma-rays, delayed neutrons, fission, photofission.

I. INTRODUCTION

F ISSION belongs to the family of physical processes whichhave been extensively studied over the last sixty years, as a

result of the major role it plays in civil and military applications.This reaction can occur via three different mechanisms: sponta-neous fission, fission induced by an incident neutron, or fissioninduced by a high-energy photon (photofission). Whatever itsorigin, fission generally leads to the creation of two fission frag-ments emitting prompt gamma-rays and prompt neutrons. Thenuclides formed after these prompt emissions are called fission

Manuscript received September 08, 2010; revised January 25, 2011; acceptedApril 30, 2011. Date of publication June 30, 2011; date of current version Au-gust 17, 2011. This work was supported by AREVA NC and the CEA Directionof Objectives (DDIN).

F. Carrel, M. Agelou, M. Gmar, F. Lainé, and B. Poumarède are with theCEA, LIST, Gif-sur-Yvette F-91191, France (e-mail: [email protected];[email protected]; [email protected]; [email protected]; [email protected]).

J. Loridon, J.-L. Ma, and C. Passard are with the CEA, DEN, Cadarache, Nu-clear Measurement Laboratory, F-13108 Saint Paul lez Durance, France (e-mail:[email protected]; [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TNS.2011.2157169

products and their yields (and those of their daughters) are oneof the main characteristics of the fission process.

The yields of fission products are relatively well known forthe main actinides and are listed in severaldatabases (ENDFB 6.8, JEFF 3.1…) [1], [2]. On the otherhand, very little data has been published concerning photofis-sion yields. Most measurements of photofission yields werecarried out by a Belgian team, towards the end of the 1970’s.These results were obtained for the photofission of and

at different irradiation energies [3]–[5]. This lack of datawas recently highlighted by an American team, who were tryingto detect special nuclear material (SNM) enclosed inside cargocontainers, through the detection of delayed gamma-rays [6].

Since the beginning of the 1990’s, CEA has been workingon non-destructive active methods, based on the fission processand dedicated to the characterization of nuclear waste packages[7]–[9]. In the framework of this research, we have developeda technique enabling the identification of actinides, which arethe main emitters contained inside nuclear waste packages.This technique is based on the detection of delayed gamma-raysand the measurement of variations in fission product yields, ac-cording to the nature of the irradiated actinide [10], [11]. As itwas planned to apply this method using active photon interro-gation, the lack of data on photofission product yields turnedout to be extremely penalizing. To overcome this problem, a setof experiments was designed to obtain the yields of nuclidesformed by the photofission of and . During these ex-periments, the simultaneous detections of delayed neutrons anddelayed gamma-rays were combined. This approach enabled theuncertainties intrinsic to this type of measurement to be mini-mized. Initially, we carried out experiments in active neutron in-terrogation in order to verify the method’s feasibility. For this,experimental results for the thermal fission of andwere compared with the reference values provided by several re-cent databases (ENDFB 6.8, JEFF 3.1). This method was thenapplied to active photon interrogation, to derive the yields of nu-clides formed by the photofission of and .

The present paper deals firstly with the theoretical principle ofour method, based on the detection of delayed gamma-rays anddelayed neutrons. We emphasize the importance of accuratelydetermining the detection efficiencies of delayed neutrons anddelayed gamma-rays. For each type of interrogation, we thenpresent our experimental tools (irradiation facility, detectors,

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CARREL et al.: NEW EXPERIMENTAL RESULTS ON THE CUMULATIVE YIELDS FROM THERMAL FISSION OF AND 2065

analysis programs), describe the settings of the experimentalconfiguration and report on the yields obtained for each studiedactinide ( and in fission, and in photofis-sion).

II. THEORETICAL ASPECTS

Delayed gamma-rays are emitted by fission products duringtheir radioactive decay. Our experimental protocol is similar tothose described in [11] and can be divided into three steps, inorder to detect these fission products (see Fig. 1):

• a period of pulsed irradiation, used to create fission prod-ucts,

• a cooling period, needed to transfer the samples from theirradiation area to the counting area,

• a counting period, during which the delayed gamma-rayspectrum was acquired.

The detection of delayed gamma-rays produces a very richspectrum (see Fig. 2) with useful peaks at energies in excess of4 MeV.

Activities of fission products present in a radioactive decaychain can be calculated solving the Bateman equation [12]. Inour calculation, we have only taken the direct precursor (des-ignated as father nuclide) of each nuclide (designated asdaughter nuclide and emitter of the delayed gamma-ray of in-terest) into account and have neglected the other precursors inview of their short half-lives. Considering our experimental pro-tocol, the peak area at energy , due to the gammaemission of , is related to the irradiated mass of actinideby the following relation:

(1)

where

(2)

(3)

and , the mass of actinide , , the fission/photofissionrate (fission or photofission per second and per gram of actinide

), , Avogadro’s number, , the molar mass of actinide, , the minimum energy (in fission)

or the threshold energy (in photofission) of the probing parti-cles (MeV), , the maximum energy of the probing parti-cles (MeV), , the neutron or photon flux density (particles

), , the fission or photofission cross-sectionof actinide , , the absolute gamma efficiency at en-ergy , , the absolute intensity of the delayed gamma-ray,

, the independent yield of the daughter nuclide per fis-sion or photofission of actinide , , the cumulative yield ofthe father nuclide per fission or photofission of actinide . Theindependent yield of a fission product corresponds to the quan-tity of this nuclide created directly by the fission of actinide ,following prompt neutron emission but before delayed neutron

Fig. 1. Temporal protocol applied during fission and photofission experimentsfor the detection of delayed neutrons (DN detection) and delayed gamma-rays(DG detection).

Fig. 2. Delayed gamma-ray spectrum, obtained after the irradiation of a �

sample (photofission measurements). The energy range is restricted from800 keV to 1050 keV.

emission. The cumulative yield of a fission product is the sumof its independent yield, together with that of its precursor nu-clides, since fission products often belong to -decay chains.Finally, the terms and describing the temporal evolutionare given by:

(4)

(5)

with , the radioactive decay constant of the father nuclide, , the radioactive decay constant of the daughter

nuclide , , the pulse period (s), , the number of pulsesduring the irradiation period, , the pulse duration with

, , the cooling time (s) and , thecounting time (s).


By comparing the half-life of the daughter nuclide withthat of its father nuclide, the delayed gamma-ray emitters canbe separated into three different categories, as follows:

• the half-life of the daughter nuclide is much smallerthan that of its direct precursor . In this case,analysis of the peak area leads to the cumulative yield ofthe father nuclide;

• the half-life of the daughter nuclide is similar to thatof its direct precursor . The contributions of bothnuclides (father and daughter) must be taken into accountin the analysis of the peak area;

• the half-life of the daughter nuclide is considerablylonger than that of its direct precursor . As thisis the most common case, analysis of the peak area leadsto the cumulative yield of the daughter nuclide .

It is theoretically possible to obtain the yield of a given fissionproduct using direct analysis of the corresponding peak area.However, this quantity is difficult to evaluate, due to the exper-imental constraints of our irradiation facilities. In the case ofactive photon interrogation, the characteristics of the electronbeam (current intensity, mean energy of the electrons) are themain source of uncertainty. For this reason, we combine the de-tection of delayed gamma-rays and delayed neutrons emittedduring the same measurement, in order to reduce experimentaluncertainties. Delayed neutrons are detected between each irra-diation pulse, after a cooling period of duration and duringa counting period of duration . The number of delayedneutrons detected during the irradiation period of duration ,containing irradiation pulses, corresponds to the contributionof the 6 groups of precursor nuclides on each counting period.For an irradiated actinide , this is noted and can bewritten as:

(6)

with

(7)

(8)

(9)

and , the delayed neutron efficiency, , the frac-tion of delayed neutrons emitted by the group per fission orphotofission of actinide , , the radioactive decay constantassociated with the group and actinide .

The classification of delayed neutrons into six separategroups is available for all of the studied actinides ( and

in fission, and in photofission). For thecalculations and results shown here, we used data providedin the following references [13], [14]. If the irradiated samplecontains only one actinide , the detection of delayed neutronsand delayed gamma-rays emitted during the same measurementcan allow the determination of the parameters and to be

avoided. Unfortunately, as the samples used in photofission bothcontained and , a different approach was required.For a mixture containing both of these actinides (notations 5and 8 refer to and ), the ratio between the peak area

and the number of delayed neutrons can bewritten as follows:

(10)

with

(11)

(12)

(13)

(14)

and being the weight percentages of and in theirradiated mixture. For each peak at energy , the coefficients

and are determined by combining the resultsobtained with two different uranium samples. These samples,noted and , have different enrichments.

The parameter is obtained, by calculating for a given experi-mental protocol the ratio between the delayed neutrons detectedfor mixtures and . This ratio, noted , can be written inthe form:

(15)

with , , the respective masses of contained in sam-ples and and , , the respective masses of con-tained in samples and . For these measurements, we makethe assumption that the incident beam characteristics do not varysignificantly between the irradiation of samples and .

The accuracy of these results is very dependent on ourknowledge of the detection efficiencies. The delayed neutronefficiency was computed using different methods for the fissionand photofission experiments (see next section). The absolutegamma efficiencies were determined by using the irradiatedsamples directly. We firstly detected the gamma-ray emitted by

(granddaughter of ) at 1001.0 keV to obtain theabsolute gamma efficiency at this energy. Then, we determineda relative efficiency curve at 1001.0 keV, using fission productswhich have several peaks in their spectra, of which at leastone is close to 1001.0 keV. Fig. 3 shows an example of therelative efficiency curve at 1001.0 keV, defined over the range540.8 keV–2639.6 keV and determined during the photofissionexperiments.

By suitably combining the absolute gamma efficiency at1001.0 keV with the relative efficiency curve at 1001.0 keV,all the absolute gamma efficiencies can be determined over


Fig. 3. The relative efficiency curve at 1001.0 keV, defined over the range540.8 keV–2639.6 keV and obtained during the photofission experiments. Thedotted curve is used to fit the experimental data over the entire energy range.

the energy range of interest. In our calculations, the absoluteintensity at 1001.0 keV is equal to 0.835% [15]. We point outthat an erroneous value is given in the ENDFB 6.8 database

, which corresponds to a difference of 42%when compared to our value. The latter value is relativelyold and has been re-evaluated, in particular in the JEFF 3.1database .

In the following section, we present the experimental resultsobtained by active neutron interrogation (cumulative yields ofnuclides formed by the thermal fission of and ).These measurements were carried out to check the experimentalfeasibility of our method, based on the detection of delayedgamma-rays and delayed neutrons.

III. CUMULATIVE YIELDS FOR THE THERMAL FISSION OF

AND

The active neutron interrogation experiments were carried outin the PROMETHEE facility located at CEA Cadarache [16].This measurement cell is built around a d-t generator, deliv-ering 14 MeV neutron pulses. The neutrons are then moderatedby graphite blocks, to obtain a thermal flux near the sample.During our experiments, the pulse duration was set toand the irradiation frequency to 50 Hz. The total irradiation du-ration was equal to 30 min. For the detection of delayed neu-trons, the cooling time applied at the end of each pulse wasset to 15.47 ms and the counting time was equal to 4 ms.The duration between the end of the irradiation and thebeginning of each acquisition of the delayed gamma-ray spec-trum was equal to 60 s. The uranium and plutonium sampleswere in metallic form and were positioned behind a 4 mm thicklead protection to decrease their activity levels. The prompt neu-trons and the delayed neutrons were detected using the 88counters of the PROMETHEE measurement cell (the types andpositioning of counters can be found in [16]). The delayedgamma-rays were detected by a n-type HPGe detector (Can-berra GR 3018, relative efficiency= 30%, diameter= 52 mm,length= 46 mm) coupled with a digital gamma-ray spectrom-eter (ORTEC DSPECPro). A dynamic dead time correction wasapplied using the ’Zero Dead Time’ mode of this spectrometer

TABLE IWATT SPECTRUM PARAMETERS FOR �� , � AND ��[18],

SIMULATED PROMPT NEUTRON EFFICIENCIES AND COMPARISON

WITH EXPERIMENTAL RESULTS

[17]. The accuracy of the dead time correction has been pre-viously evaluated on the input count rate range of interest (theinput count rate at the beginning of the counting period was lessthan 60 kcps during all our experiments), in order to confirmthat it did not introduce any bias during the acquisition. Finally,the net peak areas were extracted using Matlab tools developedby the authors.

For these measurements, we did not have any referencesources for the experimental measurement of the delayedneutron efficiency. To determine this parameter, we combinedexperimental results and simulated values, using MCNPX[18], [19]. We firstly determined the prompt neutron efficiency,by means of neutron coincidence counting between promptneutrons. We then defined a correction factor corresponding tothe ratio between the experimental and simulated values for theprompt neutron efficiency. Finally, we obtained the parameter

, by weighting its simulated value by the previouslydefined correction factor. For each actinide, Table I gives theWatt spectrum parameters and compares experimental andsimulated values for the prompt neutron efficiency.

The agreement between experimental and simulated resultsis quite satisfying for these three nuclides. The simulated valuefor the delayed neutron efficiency is equal to (0.57%)for and .

In the case of delayed gamma-rays, the absolute efficiency at1001.0 keV is equal to . The relative statistical un-certainty (noted RU in the following of this article) on this valueis equal to 1.51%, at . The relative efficiency curve at 1001.0keV was determined over the range 540.8 keV–2639.6 keV. Theexperimental values were fitted using the following equation:

(16)

Tables II and III summarize the cumulative yields obtainedfor the thermal fission of and . Absolute intensi-ties of delayed gamma-rays have been taken from the ENDFB6.8 database (except when the contrary is stated). Two measure-ments were carried out for each type of sample. We also com-pared our cumulative yields with those provided by the ENDFB


TABLE IICUMULATIVE YIELDS FOR THE THERMAL FISSION OF � AND COMPARISON

WITH THE VALUES AVAILABLE IN THE ENDFB 6.8 (WHITE BACKGROUND) OR

JEFF 3.1 (GREY BACKGROUND) DATABASES

6.8 database (the cumulative yield values contained in the mostrecent evaluation, ENDFB 7.0, are the same for all the nuclides

TABLE IIICUMULATIVE YIELDS FOR THE THERMAL FISSION OF �� AND COMPARISON

WITH THE VALUES AVAILABLE IN THE ENDFB 6.8 (WHITE BACKGROUND) OR

JEFF 3.1 (GREY BACKGROUND) DATABASES

involved in this study). For some nuclides, significant discrepan-cies exist between the ENDFB 6.8 and JEFF 3.1 databases, forthe absolute emission intensities and cumulative yields ( ,

, , ). In these particular cases, we selected thedatabase presenting the best agreement with our experimentalresults (white background in the first column refers to data fromENDFB 6.8 and grey background refers to data from JEFF 3.1).

There are three types of uncertainty related to these measure-ments:

• The uncertainty on the delayed neutron efficiency: this wasset to 4%, corresponding to the greatest difference betweenthe experimental and simulated results for the prompt neu-tron efficiency.


• The uncertainty in the absolute gamma efficiencies: thiswas set to 8%, corresponding to the greatest difference be-tween our fit and the experimental data for the relative ef-ficiency curve at 1001.0 keV.

• The statistical uncertainty on the net peak areas.The uncertainties related to the detection efficiencies are more

significant than those arising from the statistics of the net peakareas. Finally, the global relative uncertainties in our cumulativeyield values lie in the range between 9% and 11%, for all ofthe studied nuclides (quadratic sum of the above three types ofuncertainty).

Roughly speaking, our experimental results are in goodagreement with the reference values found in the ENDFB 6.8database. However, some remarks should be made in somespecific cases:

• : our results are in better agreement with the valuesgiven in the JEFF 3.1 database, than with those found inENDFB 6.8. It is interesting to note that the absolute de-layed gamma-ray intensities for this nuclide were reeval-uated between ENDFB 6.8 and ENDFB 7.0 (the most re-cent evaluation, dating from 2006) and are now identical tothose found in JEFF 3.1.

• : the half-life of this nuclide was determined asduring our experiments. This value is in

good agreement with a recently published result [20] andhighlights a slight bias present in the reference data foundin all the databases . We used this valueto calculate the cumulative yield of for the thermalfission of and and for the photofission ofand .

• : the relative uncertainties in the cumulative yieldsof this nuclide are very high in the databases for and

(64%). Our experimental results are in good agree-ment with the values found in ENDFB 6.8, but our globalrelative uncertainty is much less significant.

• : our results are in better agreement with the valuesgiven in JEFF 3.1, than with those found in ENDFB 6.8.

The results obtained for the thermal fission of andvalidate the experimental feasibility of our method, based onthe detection of delayed gamma-rays and delayed neutrons. Wewere thus able to validly apply this technique to active photoninterrogation, in order to determine the cumulative yields of nu-clides formed by the photofission of and .

IV. CUMULATIVE YIELDS FOR THE PHOTOFISSION OF

AND

For the purposes of active photon interrogation, various ex-periments were carried out in the SAPHIR facility, located atCEA Saclay. SAPHIR houses a linear accelerator (LINAC) pro-viding a pulsed electron beam at a frequency of 25 Hz. High-en-ergy photons are produced by Bremsstrahlung radiation froma cylindrical tungsten target (5 cm diameter, 5 mm thickness).During our experiments, the mean energy of the electrons was16.3 MeV or 19.4 MeV. The pulse duration and peak currentwere respectively equal to and 100 mA. For these mea-surements, we used two reference samples, referred to inthe following as and . The sample was

Fig. 4. Experimental configuration of the delayed neutron detection duringphotofission experiments.

a hollow cylinder (internal diameter= 3.00 mm, external diam-eter= 6.97 mm, height= 20.03 mm), of global mass 6.50 g andwith a natural content of 0.7%. The was also cylin-drical (diameter= 5.46 mm, height= 10.14 mm) with a globalmass of 2.52 g and with a enrichment of 85.0%. Both sam-ples were transferred from the irradiation area to the countingarea, using a pneumatic rabbit, with a delay of only a few sec-onds. We were thus able to measure a delayed gamma-ray spec-trum in the absence of any background contribution. During ourexperiments, we adapted the irradiation durations, as well as thecooling and counting periods, according to the half-lives of thestudied photofission products.

The delayed neutrons were detected using two detectionblocks, positioned at 75 cm (noted d in Fig. 4) on both sides ofthe sample, each containing two counters (Canberra 150NH100). Fig. 4 illustrates this experimental configuration.

These blocks were made using cadmium (1 mm thickness),enabling the thermal neutron background signal to be sup-pressed and , needed to increase the interaction probabilityinside the counters. The dimensions of these blocks are thefollowing: 117 mm width (W), 262 mm length (L), 1500 mmheight. The distance between the entry of the block and thecentre of the tube is equal to 26 mm. After each pulse, weobserved a cooling time equal to 4.8 ms, before startingthe delayed neutron acquisition. The counting time wasequal to 33.6 ms between each irradiation pulse. The delayedneutron efficiency was determined using three AmLi sources(Amersham X3). Neutrons emitted by this type of source have amean energy of 451 keV [21], which is close to the mean energyof the delayed neutrons. In this way, it is possible to obtainan experimental value for this efficiency, equal to(RU= 2.36%).

Delayed gamma-rays were detected using a p-type HPGe de-tector (ORTEC GEM40P-PLUS, relative efficiency= 40%, di-ameter= 68.8 mm, length= 44.5 mm), coupled with a digitalgamma-ray spectrometer (ORTEC DSPECPro). The absoluteefficiency at 1001.0 keV is equal to (RU= 1.49%)


TABLE IVCUMULATIVE YIELDS FOR THE PHOTOFISSION OF � AND � (PART 1).

THE MEAN ENERGY OF THE ELECTRONS IS EQUAL TO 16.3 MEV

for measurements at 16.3 MeV and (RU= 0.20%)for measurements at 19.4 MeV. The relative efficiency curve at

TABLE VCUMULATIVE YIELDS FOR THE PHOTOFISSION OF � AND � (PART 2).

THE MEAN ENERGY OF THE ELECTRONS IS EQUAL TO 16.3 MEV

1001.0 keV was determined using and . The dif-ference between the fits obtained with both of these samples isless than 1%. Moreover, we designed two comparison methods,to check the accuracy of the relative efficiency curve. First of all,we compared the experimental results with the simulated valuesof relative efficiency obtained with MCNPX. We then used apechblende sample having similar dimensions to and de-termined a relative efficiency curve at 1001.0 keV by detectinggamma-rays emitted by Bi (this nuclide belongs to the decaychain of ). In the second case, twelve peaks (minimal in-tensity of 2%) have been used in order to cover an energy rangefrom 609.3 keV to 2204.1 keV. The relative efficiency curvesobtained using these methods have a maximum difference of3% over the energy range 540.8 keV–1435.9 keV and 8% over


TABLE VICUMULATIVE YIELDS OBTAINED FOR THE PHOTOFISSION OF � FOR A MEAN

ENERGY OF THE ELECTRONS EQUAL TO 19.4 MEV AND COMPARISON WITH

THE DATA OBTAINED FOR THE SAME NUCLIDES AT 16.3 MEV

the energy range 1435.9 keV–2639.6 keV, when compared withthe relative efficiency curve determined using photofission prod-ucts. Finally, the relative efficiency curve was derived for all ofour measurements, using , resulting in a fit described bythe equation:

(17)

The parameter , defined at the beginning of this article,was determined by combining the number of delayed neutronsemitted by both and samples, using the sameexperimental protocol. The final value, corresponding to theaverage of the results obtained with five different experimentalprotocols, is equal to 2.328 (RU= 0.90%).

Table IV and Table V summarize the cumulative yields ob-tained for the photofission of and , for a mean elec-tron energy equal to 16.3 MeV. The absolute intensities of thedelayed gamma-rays have been taken into the ENDFB 6.8 data-base (first column, white background) or into the JEFF 3.1 data-base (first column, grey background). For these measurements,the irradiation time was equal to 30 min and the coolingtime to 2 min (except when stated otherwise).

These experiments enabled several cumulative yields(22 values for the photofission of and ) to be mea-sured, covering a large range of nuclides in terms of half-lives(from 1 min to several hours). Moreover, the consistency be-tween the different values obtained for the same photofissionproduct (for instance ) is quite good in most cases. Oneother point of interest is the dependence of these cumulativeyields on the mean energy of the electrons. In Table VI, wepresent part of our results, obtained for the photofission ofwith a mean irradiation energy equal to 19.4 MeV.

The same measurements were carried out for the photofis-sion of and similar results were found for both irradiation

energies. This confirms the fact that the cumulative yields donot vary over this energy range, under the assumption that thenumber of delayed neutrons is the same between 16.3 MeV and19.4 MeV.

As for the case of fission, the global uncertainty on the cumu-lative yields combines the uncertainties on the net peak areas,the parameter and those related to the detection efficiencies,for delayed gamma-rays and delayed neutrons. The latter twoare always the most significant and are similar for measurementsat 16.3 MeV and 19.4 MeV. The global relative uncertainty onthe cumulative yields is equal to 4.7% for delayed gamma-raysover the range 540.8 keV –1435.9 keV and 8.8% for delayedgamma-rays with energies greater than 1435.9 keV.

V. CONCLUSIONS AND FUTURE DEVELOPMENTS

In this paper, we propose a method dedicated to the deter-mination of the yields of fission products emitting delayedgamma-rays. This technique is based on the detection ofdelayed gamma-rays and delayed neutrons, emitted duringthe same measurement. The experimental feasibility of thisprocess has been checked by active neutron interrogation, thuscomparing our results obtained for the thermal fission ofand (respectively 19 and 14 values for both actinides)with reference values available in the ENDFB 6.8 and JEFF3.1 databases. We then applied this technique to active photoninterrogation and were able to determine the cumulative yieldsfor the photofission of and (22 values for bothactinides). This fundamental data will be crucial for the de-velopment of a non-destructive active method, enabling theidentification of actinides contained in nuclear waste packages.These parameters can be also used in other fields of applica-tion (for example, the detection of SNM hidden inside cargocontainers). Finally, the yields of have recently beenmeasured using the same measurement technique and will bedescribed in a forthcoming publication.

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Documents

New Experimental Results on the Cumulative Yields From Thermal Fission of $^{235} {\rm U}$ and $^{239}{\rm Pu}$ and From Photofission of $^{235} {\rm U}$ and $^{238} {\rm U}$ Induced