Given the task definition and input, reply with output. In this task, you're expected to write answers to questions involving multiple references to the same entity. The answer to the question should be unambiguous and a phrase in the paragraph. Most questions can have only one correct answer.

Passage: The reliability of the results can be improved by lengthening the testing time. For example, if counting beta decays for 250 minutes is enough to give an error of 80 years, with 68% confidence, then doubling the counting time to 500 minutes will allow a sample with only half as much 14C to be measured with the same error term of 80 years.Radiocarbon dating is generally limited to dating samples no more than 50,000 years old, as samples older than that have insufficient 14C to be measurable. Older dates have been obtained by using special sample preparation techniques, large samples, and very long measurement times. These techniques can allow measurement of dates up to 60,000 and in some cases up to 75,000 years before the present.Radiocarbon dates are generally presented with a range of one standard deviation (usually represented by the Greek letter sigma as 1) on either side of the mean. However, a date range of 1 represents only 68% confidence level, so the true age of the object being measured may lie outside the range of dates quoted. This was demonstrated in 1970 by an experiment run by the British Museum radiocarbon laboratory, in which weekly measurements were taken on the same sample for six months. The results varied widely (though consistently with a normal distribution of errors in the measurements), and included multiple date ranges (of 1 confidence) that did not overlap with each other. The measurements included one with a range from about 4250 to about 4390 years ago, and another with a range from about 4520 to about 4690.Errors in procedure can also lead to errors in the results. If 1% of the benzene in a modern reference sample accidentally evaporates, scintillation counting will give a radiocarbon age that is too young by about 80 years.
Question: What type of techniques can allow measurement of dates up to 60,000 and in some cases up to 75,000 years before the present?
special sample preparation techniques.