ABSOLUTE TECHNIQUE FOR NEUTRON SOURCE CALIBRATION BY RADIATION INDUCED ACTIVITY

Transcription

1 ABSOLUTE TECHIQUE FO EUTO SOUCE CALIBATIO BY ADIATIO IDUCED ACTIVITY M.A. El-Klaly adiatin Prtectin Department, uclear esearch Center, Atmic Energy Authrity, Cair,. The neutrn yield frm a adium Beryllium neutrn surce has been determined experimentally by the induced -56 activity. The neutrn surce was placed in the center f a tank filled with aqueus manganese sulphate (SO 4 ) slutin. The irradiatin time usually lasted abut hurs in rder t secure saturatin. The average induced -56 activity within the SO 4 bath was then measured by the use f ai scintillatin detectr. This detectr was placed in a sealed aluminum jacket at the center f the tank. This detectr was cnnected with the necessary electrnic cunting system and was precalibrated against a 4 πβ-γ cincidence cunting system. The efficiency f the ai cunting system as a functin f SO 4 slutin density is investigated as well as the prper dimensin f the used tank fr the sake f calibratin purpses. The neutrn leakage within the SO 4 baths were als investigated fr different dimensins f tanks. The experimental errrs invlved in the cunting system were als cnsidered. The numerical value f neutrn yield frm the used radium beryllium neutrn surce was given with its crrespnding statistical errrs as ( ) x 10 6 neutrn per secnd. Keywrds: eutrn surce, calibratin, manganese sulphate, induced activity, neutrn leakage. ITODUCTIO umerus techniques are invlved fr evaluating neutrn yield frm neutrn surces [1-3], frm these methds: (1) Surces immersed in a large mderating vlume cntaining a slutin f manganese salt and the neutrn flux ut put was determined frm the -56 induced activity [4-7]. (2) Surces immersed in a large mderatr and the neutrn density are integrated by BF 3 cunters [8,9]. (3) Determinatin f the neutrn yield frm neutrn surces by induced activity f sme threshld detectrs [10-12]. (4) Surces are placed in the center f a mderatr as water r graphite with thermal neutrn detectr lcated at a large distance frm the surce [13-15]. The bject f this experimental wrk is t:

2 (1) Determine the ptimum cnditins fr calibratin f neutrn surces and t find ut the efficiency f measuring system. (2) Test experimentally the feasibility f using SO 4 baths f varius sizes fr the purpse f calibratin f neutrn surces. (3) Determine the abslute neutrn emissin rate frm radium beryllium ( a B e ) neutrn surce. EXPEIMETAL Variety f aluminum tanks f different sizes are used fr this calibratin. These are six cylindrical tanks painted frm inside and utside t prevent crrsin. The height f each tank is equal t its diameter. Their dimensins ( six tanks ) are 30, 35, 40, 45, 50 & 60 cm (fr the height and diameter.). These tanks were filled with SO 4 slutin. The cncentratin f the slutin was determined with high accuracy. The cunting system cnsists f ai (Tl) detectr cnnected with ultra-scaler. T calibrate this system (aqueus SO 4 slutin in the tank + cunting scaler), an auxiliary experiment is carried ut. Small quantity f slid SO 4 pwder is irradiated in the reactr. Its abslute activity is measured using 4 π β - γ cincident cunting technique. The active slid SO 4 pwder is then disslved in the slutin f the inactive SO 4 path and stirred thrughly t ensure hmgeneity. Then the cunt rate is determined by the ai (Tl) detectr which is placed at the center f the tank (taking int cnsideratin the decay time). This calibratin is dne fr each tank. Thus the efficiency f the cunting system culd be well defined using the earlier data f the cincident technique. The neutrn surce t be calibrated is then placed at the center f the tank cntaining the inactive SO 4 slutin (after cmplete decay). The time f irradiatin till saturatin is abut hurs. The surce is remved and the slutin is well stirred. The resulting activity was measured using the dip scintillatin cunting assembly. Thus the exact neutrn emissin rate f the neutrn surce can be derived after making the necessary experimental crrectins. Fr precise measurements the experimental steps were repeated several times t ensure its validity. Since the neutrn yield depends upn cncentratin, s the density f SO 4 slutin is determined befre and after each irradiatin. THEOETICAL COSIDEATIOS Fr calibratin f neutrn surces, the neutrn surce is placed in the middle f a cylindrical tank cntaining an aqueus slutin f SO 4. The neutrn irradiatin is cntinued till cmplete saturatin and the abslute activity f -56 (gamma emitter) is determined. Fr prductin f -56, it is knwn that the macrscpic neutrn scattering crss-sectin fr in an aqueus slutin is much larger than that due t absrptin crss-sectin (case f fast neutrn regin). This means that mst neutrns are slwed dwn t thermal energies ( capture crss-sectin is 13.2 barns). Als fr H σ c =0.3 b, Sσ c =0.05 b, and O σ c = b. The emissin rate (Q) f neutrns frm the neutrn surce is calculated frm the equatin [16], Q = + H + S + O + L + cnt. (1) where: = rate f neutrn capture in, H, S, O, leakage and in cntainer respectively. O and cnt can be neglected because f its small value

3 S O, this equatin can be simplified t read Q = + H + S + L (2) The rate f emissin f neutrns Q ( n/sec ) by sme numerical analysis can read [17]. (1 1 σ s H H Q = + + ) Q σ σ σ L (3) The factr takes int accunt the absrptin f epithermal neutrns. By substitutin the numerical values fr the crss-sectin in equatin (3) becmes: Q = ( X ) Q + L (4) H where X = the rati f cncentratin f H and atms in the bath ( i. e = ), Q = the activity f 56 in (dps) = the cunt rate at zer time after irradiatin. ε = the efficiency f the cunting system. S, equatin (4) can be written as Q = ε Q = ( X ) + L (5) ε T evaluate the neutrn yield (φ) frm the neutrn surce, the fllwing frmula can be applied [18]. Q mρφ (1 A where, m = mass f in SO 4 = Avgadr s number. ρ = natural abundance f 55 = 100%. σ = Absrptin crss-sectin f. φ = neutrn yield ( n / sec ). A = atmic weight f -55 t 1 = time f irradiatin. t 2 = time f decay. -λt e -λ ) e 1 t = 1 (6) By substituting fr the values f, σ and A in the last equatin, thus it will be [19]. Q = m φ ( 1 e -λt1 ) e -λt2 (7) The relatin between SO 4 density and its cncentratin can be estimated by empirical frmula [20]. C = ( 140 d ) ½ (8)

4 where: C = cncentratin f SO 4 slutin. D = its specific density at 25 C. ESULTS AD DISCUSSIO The abslute measurement f -56 activity was cunted by tw standard techniques. These are 4π β-γ cincidence and cmpared its results with 4π β prprtinal cunter. The assessment f the percentage errr invlved in the calibratin f the tw systems can be derived frm the relatin. cinc 4πβ % errr = ( ) cinc x 100 Five irradiated SO 4 slid samples have been measured using the tw standard measuring techniques. The calibratin result f the tw systems can be summarized in table (1). Table (1) Exp. n. 4 л Cinc Errr (%) errr x x x % x x x % x x x % x x x % x x x % It can be seen frm this table that fr five different experiments, the abslute cunt rates derived by the β - γ cincidence system des nt deviate by mre than (+3% ) frm the cunt rate derived by the standard 4лβ cunter. This satisfactry result give the reasn t prceed using the β, γ cincidence technique fr abslute cunting measurements. It is wrth while t mentin that by using the cincidence system, it culd be avid self absrptin f activated SO 4 samples. It shuld be mentined that six cylindrical tanks f different radii () ranging frm 15 t 30 cm are used. The characteristics f these tanks can be tabulated as in table (2) Tank n. adius (cm) Height H (cm) Table (2) Vl. (cm 3 ) Density (g/cm 3 ) Percentage cnc. C (%)

5 T find ut the efficiency f the cunting system (1.5 x 1.5 ai crystal + phtmultiplier + fast scaler) against the cincidence system the fllwing experiment was perfrmed. A slid SO 4 sample (0.3g) was irradiated in the reactr (2MW) fr 13 minutes in the thermal clumn at a distance 140 cm frm the reactr cre. The thermal neutrn flux at this pint was 5.1 x 10 9 n/cm 2 /sec. The btained -56 activity was abut 15 micr curies. The irradiated sample was abslutely measured using the standard 4лβ-γ cincidence cunting system (taking int accunt the decay time). Then this irradiated sample was disslved in the slutin f the first SO 4 tank and was measured by the cunting system (ai detectr) and crrected t zer time. On this basis the efficiency (ε) f this cunting system was derived frm the equatin: ε = where: ε = cunting system efficiency = cunt rate frm the ai (Tl) dectr = cunt rate frm the cincidence system x 100 The abslute errr f the efficiency was derived frm the relatin: ε = ( ) 2 + ( ) 2 This experiment has been repeated fr the six different SO 4 tanks. The result can be shwn in table (3). Table (3) Tank n. adius (cm) Efficiency ( ε ) % The variatin f detectr efficiency with variatin f SO 4 tank size (in terms f its radius ) are prvided in Fig. (1)

6 Figure 1. Variatin f the cunting system efficiency (ε) against tank radius (). Frm this figure it culd be cncluded that the efficiency f the cunting system decreases expnentially with the increase f tank radius. T find the neutrn leakage frm SO 4 bath, a standard a B e neutrn surced was lcated at the center f the bath. This surce was left irradiating the SO 4 slutin fr a gd time enugh t secure cmplete saturatin (16-18 hurs). The neutrn surce was then remved and the SO 4 slutin was stirred t guarantee hmgeneus distributin f the radiactive -56. The cunt rate f radiactivity frm -56 was determined by the pre- calibrated cunting system (i.e f knwn abslute efficiency). This cunting was repeated several times with intervals f 15 minutes and fr a perid equivalent t 3 T½ (abut 8 hurs). In each step the net cunt rate was derived and was crrected t zer time. The activity f -56 was evaluated. S, the number f neutrn absrptin was estimated. By the knwledge f neutrn yield frm the standard neutrn surce, the percentage f neutrn leakage can be estimated. The fractin f neutrn leakage as well as neutrn absrptin in SO 4 tanks f different sizes can be shwn in Table (4) Table (4) Tank n. adius (cm) Percentage absrptin (%) abs Percentage leakage (%) L These data can be illustrated in figure (2). It culd be cncluded that the relatinship between the percentage neutrn leakage and the radius f SO 4 tank seems t be expnential with negative slpe i.e. neutrn leakage decrease with increase f tank radius

7 Figure 2. elatin between percentage f neutrn leakage against tank radius () The density f SO 4 slutin is ne f the mst imprtant factr that affect the fractin f neutrn leakage. This factr was investigated by the use f SO 4 tank f diameter equals height = 30 cm with different cncentratins. The results are summarized in table (5) Table (5) Density (g/cm3) Cnc. (%) Q abs (%) Q L (%) Frm this table it culd be cncluded that the amunt f neutrn leakage frm the SO 4 slutin during neutrn surce calibratin was decreased by increase f slutin density. The btained data can be illustrated as shwn in figure (3)

8 Figure 3. elatin between neutrn absrptin in SO 4 slutin and its density. It culd be cncluded frm this figure that a linear relatinship exists between the neutrn absrptin and the density f SO 4 slutin with psitive slpe. This means that neutrn absrptin increases with increasing SO 4 cncentratin. Fr this reasn in case f weak surces, it is recmmended t use mre cncentrated slutin in rder t minimize the fractin f neutrn leakage. Als when using a cunter f pr efficiency it is advisable t use high cncentratin. It may be f great interest t find ut the imprtance f the size f the tank cntaining SO 4 slutin. Fr this reasn tanks f different sizes were used. The slutin in each tank was irradiated separately by the neutrn surce which was placed at the center f the tank till cmplete saturatin. The slutin was then stirred t ensure hmgeneity f -56 activity. The detectr was then placed instead f the surce. The rate f cunting f active manganese was taken every half an hur. This wrk has been repeated fr the six tanks. The results culd be illustrated in figure (4). Figure 4. elatin between the cunt rate f the detectr and decay time fr the different tanks f different sizes

9 It is bvius frm figure (5) that the cunt rate is high in case f small sizes than that f big sizes. This is in spite f high neutrn leakage in small tanks than that in big tanks. Fr this reasn it is recmmended t use small tank fr the calibratin especially if the detectr efficiency is small Figure 5. elatin between the tank radius and the cunting rate crrected t zer time. Calculatin f abslute neutrn yield frm radium beryllium neutrn surce was carried ut using SO 4 slutin in six tanks f different sizes. This depends n the previus experimental findings such as: neutrn leakage, detectr efficiency and effect f SO 4 cncentratin. These results can be summarized in table (6). Table (6) Tank n. adius (cm) Abslute neutrn Yield Q (n/sec) Abslute errr Q (n/sec) x x x x x x x x x x x x 10 6 Average x x 10 6 Frm this table it culd be cncluded that the numerical values f the neutrn yield did nt change significantly with the size f the manganese sulphate bath. Even the experimental errrs are mre r less the same. This data can be illustrated in figure (6)

10 Figure 6. elatin between the tank radius and the activity f the neutrn surces. It is shwn frm this figure that the neutrn emissin f the surce are equal regardless the cntainer dimensin s, the average neutrn yield frm the used ( a B e ) neutrn surce using the suggested SO 4 baths gives rise activity equal t ( ) x 10 6 neutrn per secnd. COCLUSIO Calibratin f radium beryllium neutrn surce with manganese sulphate baths culd lead t the fllwing cnclusins: (1) The efficiency f ai (Tl) cunting system was fund t decrease expnentially by increasing SO 4 bath dimensins. (2) The neutrn leakage within SO 4 tanks was fund t decrease expnentially by increasing tank dimensins. (3) The neutrn leakage was als fund t decrease linearly by increasing SO 4 density. (4) The nminal lsses f neutrn yield frm the neutrn surce can be attributed t: (a) fast neutrn capture in sulpher and Oxygen in SO 4 slutin. (b) thermal neutrn absrptin in the neutrn surce itself as well as its cntainer. (c) fast neutrn resnance capture in. (5) Manganese sulphat baths f relatively small dimensins are generally recmmended t be used fr calibratin purpses. This is because f: (a) relatively less cnsumed SO 4 salt necessary fr preparatin f saturated aqueus slutin. (b) Ease f mbility with cunting system. (c) elatively higher cunting rate. (d) elatively higher efficiency with the cunting system. (6) The neutrn yield frm the used ( a B e ) neutrn surce is equal t ( ) x 10 6 neutrn per secnd. This result des nt vary significantly frm the internatinal knwn neutrn surce f similar type. (7) This technique f calibratin can be used fr calibratin f neutrn surces f different neutrn spectrum

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