热电离质谱法(TIMS)中铀、钚同位素的电离发射规律

Ionization Emission Law of Uranium and Plutonium Isotopes in Thermal Ionization Mass Spectrometry(TIMS)

铀、钚同位素的丰度测量结果可以为核科学发展和稀缺资源开发利用提供有价值的信息,而电离发射规律对丰度测量准确度具有决定性影响。采用热电离质谱法(TIMS),通过控制样品带、电离带电流和涂样量,对铀、钚的各同位素的电离发射规律进行研究。研究发现:当电离带电流及涂样量一致时,随着样品带电流的增加,铀同位素(^(234)U、^(235)U、^(236)U和^(238)U)和钚同位素(^(238)Pu、^(239)Pu、^(240)Pu、^(241)Pu、^(242)Pu)均表现出比较明显的质量分馏效应,当样品从样品带蒸发电离时,质量数小的同位素的比重比质量数大的同位素大。当涂样量较少时,^(235)U/^(238)U在TIMS同位素分析中表现出更高的精密度。在涂样量为3~5μg、稳定时间为25~30min、电信号为(1.2~2)×10^(-11)A的条件下进行测量,^(239)Pu、^(235)U的相对标准偏差分别为0.0030%、0.0015%,满足相关国家标准中相对标准偏差小于0.5%、0.05%的要求。

The abundance measurement results of uranium and plutonium isotopes can provide valuable information for the development of nuclear science and the exploitation and utilization of scarce resources,and the ionization emission law has a decisive influence on the accuracy of abundance measurement.Using thermal ionization mass spectrometry(TIMS),the ionization emission law of uranium and plutonium isotopes was studied by controlling the currents of sample zone and ionization zone as well as the amount of sample coating.Findings:when the current of ionization zone and the amount of sample coating are consistent,with the increase of the current of sample zone,uranium isotopes(^(234)U,^(235)U,^(236)U and ^(238)U)and plutonium isotopes(^(238)Pu,^(239)Pu,^(240)Pu,^(241)Pu and ^(242)Pu)all show obvious mass fractionation effect,and when the sample is vaporized and ionized from the sample zone,the ratio of the isotopes with small mass numbers is larger than that of the isotopes with large mass numbers.When the coating amount is small,^(235)U/^(238)U shows higher precision in the TIMS isotope analysis.The relative standard deviations of 239 Pu and 235U are 0.0030%and 0.0015%respectively under the conditions of 3~5μg of amount of sample coating,25~30 min of stabilization time and(1.2~2)×10^(-11) A of electrical signal,meeting the relative standard deviations of 0.5%and 0.05%in relevant national standards.