1. Maximum sample volume: 1.57 cm3 (20φ, 5 mm thickness)
Sample design and 𝜸-ray counting strategy of neutron activation system for triton burnup ratio measurements in KSTAR
Jungmin Joa, M. S. Cheonb, Kyoung-Jae Chunga* and Y. S. Hwanga
aDepartment of Energy System Engineering, Seoul National University, Seoul, Republic of KOREA
bITER KOREA, National Fusion Research Institute , Daejeon, Republic of KOREA
*Corresponding author:
Abstract
On the purpose of triton burnup ratio measurements in Korea Superconducting Tokamak Advanced Research (KSTAR) deuterium plasmas, appropriate neutron activation system (NAS) samples for 14.1 MeV d-t neutron measurements have been designed and γ-ray counting strategies are established. Neutronics calculations are performed with the MCNP neutron transport code for the KSTAR neutral beam injected deuterium plasma discharges. Based on those calculations, the activities induced by d-t neutrons are estimated with the FISPACT inventory code for candidate sample materials: Si, Cu, Al, Fe, Nb, Co, Ti and Ni. It is found that Si, Cu, Al, and Fe are suitable for the KSATR NAS in terms of the minimum detectable activity (MDA) calculated based on the standard deviation of blank measurements. Considering background γ-rays radiated from surrounding structures activated by thermalized 2.45 MeV d-d neutrons, appropriate γ-ray counting strategies for each selected sample are established.
Introduction
Triton burnup
Triton burnup ratio measurements in KSTAR
The properties of fast charged fusion reaction products in fusion devices are of interest since it directly related with ignition
Especially tritons with an energy of 1 MeV produced by d-d reactions have similar orbit to fusion α-particles, which makes good to simulate certain properties of fusion α-particle such as prompt losses.[1]
The tritons are generated and slowed down by the interactions with charged particles in a plasma. During the slowing down process, a fraction of the confined tritons interact with background deuterons, producing 14.1 MeV neutrons via d(t,n)4He reactions.
Thus, the measurement of triton burnup provides a quantitative measure of the triton confinement.
The cross-section for two different d-d reaction branches are nearly the same each other, the triton burnup can be easily calculated from the ratio of 14.1 MeV to 2.45 MeV neutron yields. [2,3]
KSTAR neutron activation system (NAS) [Fig.1 (a)] has been successfully operating with indium samples since 2011 KSTAR campaign and gives d-d neutron yields of KSTAR deuterium plasmas [Fig.1 (b)] [5].
d-d neutron yields are routinely monitored with a fission chamber and a He-3 proportional counter which are absolutely calibrated by NAS.
Therefore, triton burnup ratio can be easily determined in KSTAR, provided that the yield of 14.1 MeV d-t neutrons are measured with the NAS using appropriate samples.
However, not only relatively low expected d-t neutron yield but also the location of the NAS counting system seems to make it difficult to adopt the d-t neutron measurement. Because background γ-rays radiated from surrounding structures activated by thermalized 2.45 MeV d-d neutrons is expected to affect the measurement significantly.
Therefore, careful attention should be paid to the selection of samples and the strategy of γ-ray counting in the measurement of 14.1 MeV d-t neutrons with the KSTAR NAS
Fig. 1 (a) Layout of the KSTAR NAS [5]
(b) Average neutron production rate measured by the KSTAR NAS
Neutronics calculation and Sample designing
Candidate sample materials and neutron induced reactions
As shown in Fig. 1(b), the production rate of 2.45 MeV d-d neutrons in the KSTAR neutral beam heated deuterium plasma is around 5× 𝑛/𝑠
Thus, the expected 14.1 MeV d-t neutron production rate is estimated to be approximately 2.5× 𝑛/𝑠.
Neutron flux and its energy spectrum at the irradiation end are obtained by neutron transport calculations using a MCNP v.5 [Fig. 2] [5, 9]
Reaction
Isotopic
abundance (%)
Half-life
Eγ (keV)
Gamma
28Si(n,p)28Al
92.2
2.24 m
1779
100
63Cu(n,2n)62Cu
69.2
9.75 m
511
195
27Al(n,a)24Na
15.2 h
1368.6
56Fe(n,p)56Mn
91.7
2.58 h
846.8
93Nb(n,2n)92mNb
10.25 d
934.4
59Co(n,p)59Fe
44.6 d
1099.2
56.5
59Co(n,a)56Mn
846.7
48Ti(n,p)48Sc
73.8
43.7 h
983
58Ni(n,2n)57Ni
68.1
36 h
1377.6
Determination of maximum sample mass
To get high activity, large mass, i.e. large number of target nuclides are preferred.
However, the diameter of the sample is restricted by the diameter of a polyethylene capsule, thus, for the large amount of sample, the thickness has to be increased.
In thick sample case, neutron flux and spectrum can be changed due to the collisions inside the sample. Also it can be relatively easily affected by unwanted scattered neutron through the side surface. Due to this limitation, the maximum thickness of the sample is restricted to 5 mm which is similar to the indium sample for d-d neutron measurement.
Maximum sample volume: 1.57 cm3 (20φ, 5 mm thickness)
Irradiation end
Fig. 2 KSTAR MCNP model and irradiation end position [5]
Table 1. Candidate neutron induced reactions for 14.1 MeV neutron measurements.[6-8]
Reaction
Sample mass (g)
Activity (Bq)
28Si(n,p)28Al
3.64
63Cu(n,2n)62Cu
14.07
27Al(n,a)24Na
4.24
0.353
56Fe(n,p)56Mn
12.37
2.485
93Nb(n,2n)92mNb
13.46
0.074
59Co(n,p)59Fe
13.98
0.003
59Co(n,a)56Mn
0.778
48Ti(n,p)48Sc
7.08
0.042
58Ni(n,2n)57Ni
13.99
0.027
Activities induced by d-t neutrons for the candidate sample materials
Determination of neutron flux at the NAS irradiation end
As a rule of thumb, the energy criterion for the loss of fast ions in a given plasma condition [2]
The neutron irradiation time is set to 10 s. (Plasma discharge duration time for the typical KSTAR operation scenario is above 10 seconds)
𝐸 𝑙𝑜𝑠𝑠 ≡ 2 𝐼 𝑀𝐴 𝑍 𝑓 𝐴 𝑓 𝑅 0 𝑎(1+ 𝑎 𝑅 0
𝐼 𝑙𝑜𝑠𝑠 = 1 𝑍 𝑓 𝐴 𝑓 2 𝐸 𝑀𝑒𝑉 𝑎 𝑅 𝑎 𝑅 0
re-expressed in terms of required plasma current
With above conditions the activation of candidate materials has been calculated with a FISPACT inventory code [Table 2] [10]
For the KSTAR, Iloss is calculated to be 0.64 MA for 1 MeV triton.
Since this value is similar to the typical plasma current (~0.6 MA) of the KSTAR tokamak, the triton burnup ratio is expected to be ~0.5%, according to the previous measurement in other tokamak device [2].
Table 2. Activity of candidate samples with 1.57 cm3 volume (20 φ, 5 mm thickness) after 10 s irradiation.
Appropriate sample selection and 𝜸-ray counting strategy
Reaction
HPGe
Efficiency
MDA
(counts)
Activity
(Bq)
Minimum required
counting time
28Si(n,p)28Al
0.0016
12.622
35.41 s
63Cu(n,2n)62Cu
0.0056
88.479
78.83 s
27Al(n,a)24Na
0.0021
15.539
0.353
6.79 h
56Fe(n,p)56Mn
0.0034
72.394
2.485
3.79 h
93Nb(n,2n)92mNb
0.0031
31.182
0.074
39.5 h
59Co(n,p)59Fe
0.0026
51.238
0.003
Can’t be detected
59Co(n,a)56Mn
0.778
48Ti(n,p)48Sc
0.0029
26.060
0.042
183.2 h
58Ni(n,2n)57Ni
16.262
0.027
Appropriate sample selection
Background 𝜸-rays
Due to the thermalized d-d neutrons, certain background γ-ray peak signals occur right after the neutral beam injected deuterium plasma discharge.
The background γ-ray measurement result is shown in Fig. 3.
Unfortunately, some of these background γ-ray peak signals are exactly the same as the γ-ray energies of the selected samples: Si, Cu and Fe.
These three γ-rays are produced from activation of the surrounding aluminum structures, manganese in stainless steel structures, and certain material which decays with positron emission or radiates high energy γ-ray.
Minimum Detectable Activity (MDA) for each gamma ray
The true counts in HPGe detector are determined by the sample activity, half-life, counting time, gamma abundance and detection efficiency. The true counts should be exceed certain detection limit to distinguish them from background noises.
The widely used MDA criterion which first defined by Currie is [11]
Detection limit based on the minimum detectable activity (MDA) has been determined for each γ-ray energy region of interest [Table 3].
𝑀𝐷𝐴= 𝜎 𝐵
( 𝜎 𝐵 is the standard deviation of blank measurements)
𝜸-ray counting strategies for each selected samples
Table.3 MDA for each gamma ray energy region and the minimum required counting time
Figure 3. Background gamma ray measurement result
The activity irradiated from the surrounding materials changes shot by shot, depending on discharge scenarios.
Thus, the sample and background γ-ray counting have to be done within the same shot interval of 10 minutes for KSTAR.
Since gamma ray counting of the activated Fe sample takes time much longer than the usual shot interval, this reaction can only be used for the last plasma discharge of the day.
The use of the Al sample is also recommended for the last plasma discharge of the day, because not only the full γ-ray energy peak but also other reactions such as the Compton scattering can affect the γ-ray counting.
For the Si and Cu sample cases, it is possible to measure the sample and background γ-rays within the shot interval.
In copper case, two times of measurements with sufficient time interval are required due to the competing reactions (65Cu(n,2n)64Cu, 63Cu(n,γ)64Cu) [8]. Thus, in each measurement the background γ-ray should be determined.
Minimum required counting time to exceed MDA
Based on the MDA value and the activity of each sample, the minimum counting time for the required counts has been calculated [Table 3]. The maximum available counting time is restricted to 15 hours which is the time interval of discharge between successive days.
Due to their low activity, Co and Ni samples are abandoned. Also, it is hard to use Ti and Nb because they require over 15 hours to be measured.
Si, Cu, Al and Fe are appropriate to use in terms of counting time
Conclusion & Future work
REFERENCE
Appropriate samples are selected based on the neutronics calculation and MDA of candidate samples.
Background γ-rays which radiated from activated surrounding materials can disturb the true signal.
To compensate background γ-rays, appropriate γ-ray measurements strategies for each selected samples are established.
Among the selected materials, silicon is the most appropriate due to its short required counting time.
Copper takes more counting time and effort compare with silicon. It will be utilized for the purpose of cross-calibration
Due to its long required counting time aluminum and iron can be used cross-calibration purpose in the limited case only.
The expected d-t neutron yield can be much lower than previously estimated value and also, for further improvement of gamma counting statistics, enhanced γ-ray and neutron shielding in the NAS counting system are under preparation.
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