neutron activation report

8/10/2019 Neutron Activation Report

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Neutron Activation Analysis

Ryan Ruston

Department of Physics & Astronomy,

University of Birmingham, BirminghamB15 2TT

Abstract

The purpose of this experiment was to understand neutron activation analysis by varying

the activation time and position of an activated sample in a water tank and measuring the

radioisotope decay with a Geiger counter. Trend lines were applied to the data by

minimising the chi-squared value and a Monte Carlo simulation of the experiment was run

using MCNP to analyse the neutron flux at different positions in the water tank. The results

found are scrutinised and suggestions for an improved experiment are given.


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Introduction

Neutron detection by activation

As well as determining neutron activity directly, neutron measurements can be carried out

indirectly by inducing radioactivity. This involves exposing a sample material to a flux of

neutrons so that the nuclei of stable elements become radioactive through neutron capture.

Many of these nuclei will then decay through beta and gamma emission which can be

measured. The measurements made can be used to deduce the number or energy

distribution of neutrons. Furthermore, from precise determination of the gamma energies,

it is usually possible to determine not which isotopes are present, but from the gamma

intensities it can be calculated how much of the original capturing isotope was present and

thus the amount of that element present in the sample [1].

Since neutron reaction cross-sections are highest at low neutron energies, activatedsamples are most commonly applied to slow neutrons. The geometry of activation samples

are usually thin foils or wires. This is because the mean free path of neutronsespecially in

materials with a large cross-sectionis quite small. The thickness of the sample is therefore

kept small to prevent disturbing the neutron flux under measurement, since the probability

of neutron interaction will be small.

Use of Neutron Activation Analysis (NAA)

The technique has a variety of applications: with a known cross-section, measuring activity

gives neutron flux, and thus gives a measure of neutron intensity. If neutron flux is known,

an unknown capture cross-section can be determined. Activation samples have the

advantage of small size, insensitivity to gamma radiation and low cost. They can tolerate

exposure to extreme environments where other detector might fail and require no electrical

connection to the outside world. It is also a non-destructive testing method, with only a

small amount of induced radioactivity being created and the sample unaffected by the

process in the long-term.

Activation samples are thus widely used for mapping spatial variation of steady-state

neutron fluxes in reactor cores, where extreme temperature, pressure and limited space

severely constrain the type of detector that may be used [2]. However since they are purely

integrating factors, they cannot provide any information about time variation of neutron

flux over course of exposure.

The most common application of NAA is to use cases with known flux and cross-section to

determine the target mass . After exposing an unknown sample to the neutron flux, manydifferent radiations can be observed from the radioactive decays of those isotopes

produced by neutron capture. Careful measurement of the gamma ray spectrum allows

determination of the isotopes that are present in the irradiated sample and in whatquantity. Multi-channel analysers (MCAs) can be constructed to automatically determine


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energies and identify peaks, so that a large number of samples can be counted fairly quickly.

This makes NAA a widely used technique that has important applications in environment

pollution research, archaeology, and forensic science.

Theory

The neutron source

Due to the fact that neutron emission is quite rare, practical neutron sources are not a

consequence of direct neutron emission. Instead, alpha emitting isotopes are surrounded

with a target material, such that reactions can be utilised. In this experiment an Am-Be neutron source is used. Americium-241 decays via alpha emission to Neptunium with a

half-life of 433 years. The resulting alpha particles can interact with the target materialBeryllium to produce a carbon atom and a neutron:

, (1)

, (2)

the value for this reaction is 5.71MeV. Using this method, approximately 82 free neutronscan be yielded per alpha decays [3].

Activation and decay

The rate at which activation occurs is

(3)

where is the neutron flux, is the activation cross-section averaged over the neutronspectrum and is the activation sample volume. The rate of activation per unit mass istherefore a direct indicator of neutron flux strength. As the sample irradiates, the

radionuclides formed undergo decay. The rate of change in over time is the differencebetween the rate of formation and the rate of decay:

(4)

This assumes that the rate is constant, which implies that the neutron flux does not varyduring exposure. It also neglects the burn up or decrease in the number of target nuclei

while the measurement is made. The solution for =0 at =0 is

( ). (5)

Since the activity

of the sample is

, this becomes


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( ). (6)

As can be seen in figure 1 below, the induced radioactivity builds up with time and

approaches an asymptote (saturated activity). For infinitely long irradiated times this is

given by

. (7)

An exposure time of three or four half-lives of induced activity is sufficient to bring the

samplesactivity to 6-12% of the saturated value. Combining equations 6 and 7 gives

( ), (8)

assuming the sample irradiates for time and is then removed with an activity. Afterexposure the sample can be transferred to an appropriate radiation counter for

measurement of activity. Because the activity is continuously decaying during this stage,

very precise measurements must be made of each of the times involved. If the counting is

carried out over an interval and , the number of counts will be

(9)

( ) , (10)

where is overall counting efficiency and is the number of background counts expected in

[4]. Substituting equation 8 into 10 gives

()() (11)


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Figure 1 shows the relation of the produced activated isotope and elapsed time. A1is the radioactivity at the

end of the activation time t1, A2is the radioactivity at initial measurement. Transport time and measuring

time are t2 and t3 respectively, and As is the saturation activity of the isotope (Image from Nuclear

Instruments and Methods in Physics Research, A. Murataka et al, 2009).

Silver activation materials

Naturally occurring silver has a composition almost equally split between the isotopes and . Therefore via neutron capture, the isotopes produced are predominantly and , which undergo the following beta minus decay:

, (12)

. (13)

These isotopes have a decay half-life of 2.37 minutes and 24.6 seconds respectively. Both

cadmium isotopes are stable and do not decay further. The figures 2 and 3 below show the

activation yield for the two isotopes of silver. They illustrate that almost all neutron capture

of the two natural isotopes leads to the silver isotopes and .


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Figure 2 shows the activation yield of Silver-108 from Silver-107 over different neutron energies (Image

taken from Janis nuclear data library).

Figure 3 shows the activation yield of Silver-110 from Silver-108 over different neutron energies (Image

taken from Janis nuclear data library).

The figures 4 and 5 below show the capture cross-sections for the two aforementioned

isotopes. It can be seen that they are large at low energies, making neutron activation more

likely with slow neutrons. The water in the water tank will act as a moderator to scatter

neutrons down to low energies. Therefore neutron capture by the two Silver isotopes isprobable.


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Figure 4 shows the capture cross-section of Silver-107 at different energies (Image taken from Janis nucleardata library).

Figure 5 shows the capture cross-section of Silver-109 at different energies (Image taken from Janis nuclear

data library).

The four figures above (figures 2 to 5) show what makes silver an ideal activation material: it

has a high and well understood isotope yield as well as a relatively high capture cross-

section. For these reasons silver will be the activation material used for this experiment.

Geiger-Mueller Counters

The Geiger-Mueller counter is one of the oldest types of radiation detector. It was

introduced by Geiger and Mueller in 1928, and have remained in use due to the fact they

are simple, low cost and easy to operate. Geiger-Mueller (G-M) counters are a class of gas-

filled detectors that are based on ionisation. Akin to other gas-filled detectors they employ

gas multiplication to increase the charge generated by the original ion pairs, but they do this

via another means.


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Gas multiplication is a result of increasing the electric field in a gas filled detector. If an

electron ion pair is created from incident radiation in the detector, the electron will be

accelerated to the anode. If these free electrons are accelerated by a voltage to a kinetic

energy greater than the ionisation energy of neutral atoms in the gas, a collision with a gas

molecule can cause another electron ion pair. This electron can also be accelerated, suchthat gas multiplication takes the form of a cascade known as a Townsend avalanche [5].

In G-M tubes, higher electric fields are produced that enhance the intensity of each

avalanche. At a critical value of the electric field, each avalanche can create on average one

more avalanche and thus sustain a chain reaction. Once Geiger discharge reaches a certain

size collective effects terminate the chain reaction, but since this limiting point is always

reached after approximately the same number of avalanches, all pulses from a Geiger tube

are the same amplitude regardless of the number of original ion pairs. Therefore G-M tubes

can only act as a simple counter of radiation induced events, and cannot provide anyinformation on the amount of energy deposited. This means G-M tubes are never used in

radiation spectroscopy.

A pulse from a G-M tube represents a large amount of collected charge when compared to

other detector types, with a typical number being about ion pairs. This highlevel signal eliminates the need for an external amplifier making the tubes themselves

relatively inexpensive. However, the dead time for a G-M tube is significantly greater than

for other detectors, which limits their use to fairly low count rate sources.

Figure 6 shows the mechanisms which trigger additional avalanches in a Geiger discharge (Image taken from

Radiation Detection and Measurement, G. Knoll, 2000)

Quenching

If a single gas is used in a G-M tube, after the primary Geiger discharge has terminated, thepositive ions will slowly drift from the anode to the cathode. Once reaching the cathode


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these ions can be neutralised by combining with electrons on the cathode surface, leading

to another free electron emerging from the cathode surface. If this electron then drifts

towards the anode and causes an avalanche, a second Geiger discharge will be produced.

This cycle may repeat to give a continuous output of pulses, as illustrated above in figure 6.

A way of preventing this is known as quenching.

Quenching consists of reducing the high voltage applied to the tube for a given time period

after each pulse to prevent further gas multiplication. This stops secondary avalanches

forming and even if a free electron is liberated at the cathode, is cannot cause a further

Geiger discharge. The voltage must be reduced for a time greater than the time needed for

a positive ion to drift from its original position to the cathode, which is usually a few

hundred microseconds.

The most common method of accomplishing this is by adding another gas, a quench gas to

the primary fill gas. This quench gas is chosen to have a lower ionisation potential and a

more complex molecular structure. This means that most positive ions formed by the

incident radiation are from the primary fill gas, and subsequently make many collisions with

neutral gas molecules as they drift towards the cathode. Any collisions that are made with

the quench gas will likely result in a transfer of positive charge to the quench gas, so that

the original primary gas molecule is neutralised. If the concentration of the quench gas is

correct (usually around 10%) then ions that arrive at the cathode will be that of the quench

gas.

When quench gas molecules are neutralised, the energy transferred goes to disassociationof the more complex molecule rather than the liberating of free electrons from the cathode

surface. The quench gas can therefore be called a sacrificial chargecarrier, which results

in no secondary avalanches. The most popular quench gases used are Ethyl Alcohol and

Ethyl Formate. Quench gases are gradually consumed during the lifetime of the G-M tube,

so must be replaced if the G-M tube is to remain operational.

Dead time

An accumulation of positive ion space charge that terminates the Geiger discharge also

results in a substantial time gap between the first and second Geiger discharge. Since the

electric field is reduced following a Geiger discharge, if another ionising event takes place

during this time a second pulse will not be seen because gas multiplication is suppressed.

This is the dead time for a Geiger tubeand is defined as the period between the initial

pulse and the second pulse, typically 50-100[6]. As already mentioned, the dead time fora Geiger Mueller counter is a significant factor that needs to be considered.

Geiger counting plateau

The operating point of a G-M tube is chosen by finding a plateau curve, where the radiationsource generates counts at a constant rate within the tube. To do this, the count rate is


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calculated as the voltage is raised from an initial low value. If the voltage is raised beyond a

threshold value, a maximum count rate will be found where increasing the voltage further

does not increase the number of recorded pulse, i.e. a plateau. Although is real cases, the

plateau actually has a small gradient, as shown below in figure 7. If the voltage is made very

high this plateau comes to an end because of the onset of continuous dischargemechanisms within the tube [7]. These discharge processes are potentially harmful and

should be avoided.

Figure 7 shows the change in count rate as voltage is increased (Image taken from

http://cistwiki.ufv.ca/bin/view/Phys382/IntroLab,21/03/2013)

G-M tube design

Figure 8 shows the basic design of a Geiger Mueller tube.

Cathode

Insulator

(gas tight)

Anode wireThin

end-window
http://cistwiki.ufv.ca/bin/view/Phys382/IntroLabhttp://cistwiki.ufv.ca/bin/view/Phys382/IntroLab


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Since gas and avalanche formation are required, like other proportional counters, a fine

anode wire is used to produce the electric field necessary to create the multiplying region.

Radiation enters the tube through the window as shown on figure 8 above. For recording

short range decays such as alpha particles, the window should be made as thin as possible.

Because no preamplifier is required, GM tubes really can be illustrated as simply as in figure8.

Figure 9 shows the electronics associated with a Geiger-Mueller tube.

The signal voltage for a Geiger Mueller tube is generated across the load resistance, i.e. the

series resistance between the high voltage supply and the anode in figure 9. Parallelcombination of with the capacitance of the tube and wiring determines the timeconstant of the charge collecting circuit. The coupling capacitor is needed to block thetubeshigh voltage but allow transmittance of the signal pulse at ground potential to the

following circuits. To prevent the pulse amplitude from attenuation, its value has to be great

enough so that is large compared with pulse period [8].

Method

The first task in the experiment was to find the Geiger counting plateau so that the

operating voltage could be chosen. Strontium ( )was used for this part of theexperiment because of its pure beta decay and suitable half-life. It was known that the

operating voltage of the Geiger-Mueller counter was between 350 and 500V, so recordings

were made at 10V intervals between these values. The number of counts for a given voltage

was recorded after 60 seconds and the count rate calculated. Two repeat measurements

High voltage

supply

CounterDiscriminatorGeiger-Mueller

tube


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were made and an average count rate found for each voltage interval. A graph of average

count rate against voltage was then plotted and can be seen below in figure11. By

comparing this plot to figure 7, a suitable operating voltage could be chosen.

Once the operating voltage had been chosen, a sample of Silver was selected for the

activation sample. The sample was confirmed to be silver using X-ray fluorescence. The

Silver sample was tied to string and carefully lowered into the water tank and exposed to

the neutron source (see figure 10) for varying times. The sample was then removed from

the water tank as quickly as possible and placed into the Geiger Mueller counter. This

procedure was repeated with Silver for activation times of 5, 10 and 15 minutes. The counts

were recorded for 5 minutes, and then left for approximately four decay times (minutes) to allow the sample to deactivate.

Figure 2 shows the experimental set up in the water tank.

Next, the variation of activated samplesdecay with axial distance from the neutron

source was investigated. The activation material was placed inside the water tank for 15

minutes at distances of 5, 10, 15 and 20cm. As before, the counts were recorded for 5

minutes and two repeats were made for each distance. A background count was found by

measuring the counts recorded without the presence of a source. This background count

was subtracted from the measurements made. The count rates were then calculated as well

Water

tank

Neutron

source

Activation

sample


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as the Poisson error. Plots were then produced for the two datasets and can be seen below

in figures 12 and 13.

Results and analysis

Figure 3 shows the change in recorded counts as the applied voltage is increased.

Figure 11 above shows the change in the count rate as the voltage is increased. The Geiger

plateau has been made easier to distinguish by plotting a straight line through the points;

this indicates the deviation from a linear relationship in the Geiger plateau region. The

Geiger plateau occurs from 430 and 460V. As a result, an operating voltage of 445V was

chosen for the rest of the experiment.

0

100

200

300

400

500

600

700

390 400 410 420 430 440 450 460 470 480 490 500 510

Activity(countspersecond)

Voltage (V)

Variation in activity with applied voltage


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Figure 4 shows the activated silver isotopes decay over time for different activation times. The equations of

the plotted trend lines for each activation time are shown on the right in the corresponding colour.

Figure 12 above shows that the activated isotopes follow the typical exponential decay,

displayed in the three equations to the right of the graph. Increasing activation time

increases the activation isotopes activity. This is because the longer the Silver activation

sample is exposed, the more likely it is to capture neutrons and become the isotopes thatsubsequently decay. This holds true until a saturation point is reached, where the rate of

neutron capture is equal to the rate of isotope decay.

0

0.5

1

1.5

2

2.5

3

3.5

4

0 50 100 150 200 250 300 350


Time (seconds)

Decay of activated isotopes for different activation times

5 min activation time



338 9

225 53


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Figure 5 shows the decay of the activated silver isotopes over time for different axial distances from the Am-

Be neutron source.

Figure 13 above shows the exponential decay of the activated isotopes when the position of

the sample in the water tank is altered. There is a much larger difference between the

activity seen between 5cm and 10cm and that seen between 10cm and 15cm. This is due to

the large fall off of neutron flux with distance from the source in the water tank, which canbe seen below in the MCNP simulation plot shown below in figure 14. The decays for 20, 15

and 10cm distances are very close to each other, and have a fairly low count rate. This is

probably due to the sample containing fairly few activated isotopes because of an

insufficient number of collisions with neutrons. This results in a less statistically reliable

dataset, particularly given the low activity of the sample even at close proximity to the

source and high activation times.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 50 100 150 200 250 300 350


Time (seconds)

Decay of activated isotopes at different axial distance in water

tank

5cm

10cm

15cm

20cm

298 96 63 85

234

34 45


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Figure 6 shows the simulated neutron flux variation at different axial distance from the Am-Be neutron

source.

Figure 14 above shows the change in neutron flux with distance from the source. This plot

was produced using a simulation ran with MCNP. The simulation was given the same

geometry, same source and was made from the same materials as in the real experiment.

The simulation helps to explain the large change in activity with distance seen in figure 13.

As the neutrons propagate radially from the Am-Be source, more and more of them are

scattered down to lower energies, with some of them being absorbed or escaping from the

water tank. This means that the neutron flux falls of rapidly with distance from the source.

Figure 7 shows the simulated neutron fluxes at different energies and positions from the neutron source.

Figure 15 above shows the energy distribution of the 5MeV Am-Be neutron source used in

the water tank. This was produced using the same MCNP simulation used for figure 14. It

0.00E+00

5.00E-03

1.00E-02

1.50E-02

2.00E-02

2.50E-02

5cm 10cm 15cm 20cm

Neutronflux

Axial distance from neutron source

Change in neutron flux with distance from Am-Be

source

Total neutron flux

0.00E+00

2.00E-03

4.00E-03

6.00E-03

8.00E-03

1.00E-02

1.20E-02

1.00E-10 1.00E-08 1.00E-06 1.00E-04 1.00E-02 1.00E+00

Neutron Flux

Energy (MeV)

Am-Be neutron flux at different energies

10cm

15cm

20cm

5cm


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demonstrates that most of the neutrons in all regions are at thermal neutron energies,

around 0.1eV. This is the characteristic thermal peak. The peak occurs at this point due to

the water in the tank acting as a moderator, and scattering the neutrons down to lower

energies.

Some neutrons remain in the highest energy region. These are fast neutrons, and have

avoided collisions with water molecules. There are far more fast neutrons at 5cm from the

source than at further distances, because as you move further from the source, the

probability of a given neutron colliding with a water molecules and scattering down to lower

energies becomes more and more likely.

The neutron fluxes decrease as the distance increases due to the effects of absorption and

leakage. It can be assumed that the neutrons present in the water tank are at the same

thermal energy as seen in the MCNP simulation. Looking back at figures 4 and 5, it can be

seen that the cross sections for the two Silver isotopes are high at 0.1eV. This makes the

capture of some neutrons by the Silver sample in the water tank is fairly likely.

Conclusions

The equations displayed in figures 12 and 13 were found using the solverfunction on excel,

where the four variables in the equations are varied to minimise the chi squared value. The

decay times of the two activated isotopes are 24.6 seconds and 2.37 minutes. Therefore

from the decay equation for two radioisotopes the count rate should be

. (14)

Hence for the activated Silver sample in this experiment, the trend lines should have the

form

. (15)

Yet the variables in the count rate equations displayed in the figures 12 and 13, found by

minimising the chi squared value, differ significantly from the form in equation 13. By

keeping the half-lives constant and only varying the constantsand to minimise chisquared, trend lines were produced that also fit the data very closely. However the

constantsand for these new trend lines vary significantly. This suggests that it would bevery difficult to ascertain which decays were occurring in this experimental setup from the

calculated half lives alone. This is because many different trend lines could be plotted for a

range of different half-lives, which could all closely match the decay. Of course, this is not a

problem for this experiment, since the decays taking place are known prior to the

experiment.


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References

[1] Krane, Introductory Nuclear Physics,Wiley, (1988), p.788.

[2] Krane, Introductory Nuclear Physics,Wiley, (1988), p.789.

[3] Knoll, G., Radiation Detection and Measurement, third edition, Wiley, (2000), p.22







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AppendixA

Shown below is the input file used for the MCNP simulation to find the energy spectrum of

the neutron flux at an axial distance of 5cm from the source.

MESSAGE:

C

C CELL CARDS

1 1 -1 -1 2 -3 #4 $ Water inside bucket

2 2 -7.92 1 -4 2 -3 #4 $ Steel in bucket cylinder

3 2 -7.92 -2 5 -4 #4 $ Steel bottom to bucket

C

C

99 0 #1 #2 #3 #4 $ Void outside bucket

C SURFACE CARDSC Inside of bucket

1 CZ 35.0 $ cylinder along Z, radius 35cm

2 PZ 0.0 $ bottom of bucket inside

3 PZ 80.0 $ top of bucket / water construct

C Outside of bucket

4 CZ 35.2 $ cylindrical outside of bucket, radius 35.2cm

5 PZ -0.2 $ bottom of bucket

C Activation sample

6 RPP 5 5.2 0.2 2 38 42

C MATERIAL CARDS

M1 1001.42c 0.6667 8016.42c 0.3333 $ Pure water

M2 26000.42c 0.74 24000.42c 0.18 28000.42c 0.08 $ Stainless steel

M3 26000.42c 1 $ Silver sample

C

C MODE CARD

MODE:N

IMP:N 1 1 1 1 0

C

C TALLY CARDS

C Flux at point inside silver sampleF15:N 5.1 1.1 40 4

C ENERGY BINS FOR TALLIES

E0 1e-9 1e-8 1e-7 1e-6 1e-5 1e-4 1e-3 1e-2 1e-1 1e0 1e1

C

C SOURCE DEFINITION

C 5.5MeV (average) AmBe neutron source, isotropic at origin

SDEF POS=0.0 0.0 40 ERG=5.50

C

C Number of histories to run

NPS 20000


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Appendix B

Table 1 below shows the change in total mean neutron flux as increasing particle histories.

The figure of can seem to converge, indicating an adequate number of particle histories

have been ran and the results are reliable. Table 2 shows the energy bins for the tally, andthe corresponding neutron flux and relative error.

Tally 15

nps mean error vov slope fom

1000 1.6000E-02 0.1586 0.1691 2.8 1157

2000 2.0317E-02 0.2304 0.7392 2.4 280

3000 1.9616E-02 0.1726 0.5399 2.4 336

4000 2.0775E-02 0.1407 0.3259 2.3 378

5000 1.9734E-02 0.1220 0.2909 2.3 404

6000 1.9432E-02 0.1076 0.2476 2.5 434

7000 1.8943E-02 0.0964 0.2306 2.7 4668000 1.9136E-02 0.0877 0.1910 2.5 492

9000 1.9207E-02 0.0796 0.1731 2.8 531

10000 1.9563E-02 0.0745 0.1415 2.9 546

11000 1.9223E-02 0.0698 0.1344 3.3 565

12000 2.0304E-02 0.0835 0.2347 3.1 363

13000 2.0102E-02 0.0785 0.2261 3.2 379

14000 1.9942E-02 0.0748 0.2114 3.0 389

15000 1.9969E-02 0.0709 0.1976 2.8 403

16000 1.9941E-02 0.0677 0.1853 2.7 415

17000 2.0041E-02 0.0643 0.1754 3.0 434

18000 1.9706E-02 0.0620 0.1722 3.0 44019000 1.9498E-02 0.0604 0.1616 3.0 440

20000 1.9134E-02 0.0587 0.1590 3.0 443Table 1 shows the change in the total mean flux with the number or particle histories, along with the relative error,

variance of the variance, slope and figure of merit.

1tally 15 nps = 20000

tally type 5 particle flux at a point detector. units 1/cm**2

tally for neutrons

detector located at x,y,z = 5.10000E+00 1.10000E+00 4.00000E+01

energy

1.0000E-09 1.65998E-07 0.3213

1.0000E-08 5.49643E-04 0.2784

1.0000E-07 9.61814E-03 0.1078

1.0000E-06 1.52330E-03 0.1201

1.0000E-05 4.38502E-04 0.1488

1.0000E-04 4.47610E-04 0.1558

1.0000E-03 6.76101E-04 0.1748

1.0000E-02 7.93731E-04 0.1426

1.0000E-01 6.90060E-04 0.1100

1.0000E+00 1.20449E-03 0.0566

1.0000E+01 3.19240E-03 0.0216

total 1.91341E-02 0.0587Table 2 shows the neutron flux at different energies along with the relative error,

neutron activation report

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